J. Phys. B: At. Mol. Opt. Phys. 32 (1999) 2583–2609. Printed in the UK

PII: S0953-4075(99)98645-5

A photoabsorption, photodissociation and photoelectron spectroscopy study of NH3 and ND3 D Edvardsson†, P Baltzer†, L Karlsson†, B Wannberg†, D M P Holland‡, D A Shaw‡ and E E Rennie§k † Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden ‡ Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK § Department of Chemistry, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK Received 23 October 1998 Abstract. The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of ammonia and deuterated ammonia have been measured from the ionization threshold to 25 eV using a double ion chamber and monochromated synchrotron radiation. The photoabsorption spectrum displays extensive vibrational progressions ˜ 2 A00 state. New associated with Rydberg series converging onto excited vibrational levels of the X 2 structure has been observed for ND3 in the 10.0–11.3 eV range, and vibrational progressions due ˜ ˜ ˜ to transitions into the E, F and G Rydberg states have been recorded with improved resolution. Features have been observed, for the first time, in the photoabsorption spectra of NH3 and ND3 ˜ 2 E ionization threshold, and interpretations for some due to Rydberg series converging onto the A of these features have been proposed based upon the corresponding photoelectron spectra. ˜ 2 A00 photoelectron band has been studied experimentally at a The He I excited NH3+ (1a200 )−1 X 2 resolution of 3 meV and two vibrational progressions, each involving excitation of the ν2+ mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This structure changes gradually in a way that can be explained by the variation of the H–N–H bond angle with the ν2+ mode. The effective bond angle has been found to be 120◦ for v2+ = 0, and similar to that of the neutral ground state near v2+ = 6. The second progression, of weak lines, has been interpreted tentatively as being due to nν2+ +ν4+ . The ν4+ mode is doubly degenerate and the excitation of a single quantum has been explained by vibronic coupling ˜ 2 E state. In addition, He II excitation has been used to record the entire valence shell with the A photoelectron spectrum.

1. Introduction Photoabsorption and photoelectron spectroscopy studies on ammonia have provided textbook examples of several molecular interaction phenomena, such as Jahn–Teller and Coriolis splitting, vibrational predissociation, inversion doubling, and non-planar to planar transitions. In the molecular ground state ammonia possesses a pyramidal geometry and its electronic orbital configuration may be written as (in C3v symmetry) ˜ 1 A1 . (1a1 )2 (2a1 )2 (1e)4 (3a1 )2 X However, owing to inversion doubling, the ground state is split into two components and these are normally designated as 1 A10 and 1 A200 in D3h symmetry. In this latter symmetry the ground state orbital configuration becomes ˜ 1 A10 (1a10 )2 (2a10 )2 (1e0 )4 (1a200 )2 X k Present address: Fritz-Haber Institute, Department of Surface Physics, Faradayweg 4-6, D-14195 Berlin, Germany. 0953-4075/99/112583+27$19.50

© 1999 IOP Publishing Ltd

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and the four vibrational modes may be denoted as: ν1 (a10 ) symmetric stretch, ν2 (a200 ) out-ofplane bend (umbrella mode), ν3 (e0 ) asymmetric stretch and ν4 (e0 ) asymmetric bend. Excitation of an electron from the outermost, lone-pair, orbital which is centred on the nitrogen atom and responsible for the pyramidal geometry in the ground state, results in Rydberg states possessing planar geometries. To the best of our knowledge, all the Rydberg states observed in previous investigations on ammonia have involved excitation from the outermost orbital and, as a consequence, the spectroscopic transitions have usually been described using D3h symmetry. Owing to the large geometrical change following excitation from the 1a200 orbital, absorption spectra encompassing the associated Rydberg states exhibit extended progressions in the ν20 vibrational mode. Early photographic and single-photon absorption studies (Benedict 1935, Duncan 1935a, b, 1936, Duncan and Harrison 1936, Tannenbaum et al 1953, Watanabe 1954, Sun and Weissler 1955, Walker and Weissler 1955, Watanabe and Mottl 1957, Douglas and Hollas 1961, Walsh and Warsop 1961, Douglas 1963, Thompson et al 1963, Metzger and Cook 1964, Watanabe and Sood 1965) investigated the Rydberg series converging onto the lowest ionization threshold, as well as obtaining absolute photoabsorption cross sections. Significant progress was achieved in 1980 by Glownia et al (1980a, b) who, in addition to measuring the single-photon VUV absorption spectrum, combined multiphoton ionization techniques with an expansion-cooled beam to record the two- and three-photon spectrum in the 3800–5000 Å region. By making use of the different selection rules applicable to single- and multi-photon absorption, together with the changes observed in the VUV and MPI spectra, Glownia et al were able to place the interpretation and assignment of the Rydberg states on a much firmer basis. More recently, laser techniques have been employed to study and characterize many of the Rydberg series converging onto the ionization threshold (Ashfold et al 1987, Vaida et al 1987, Miller et al 1988, Li and Vidal 1994, 1995, Cramb and Wallace 1994, Langford et al 1998), and single VUV absorption methods have been used to measure absolute photoabsorption cross sections (de Reilhac and Damany 1977, Suto and Lee 1983, Samson et al 1987, Xia et al 1991), photoionization yields (Dibeler et al 1966, McKulloh 1976, Kronebusch and Berkowitz 1976, Eland 1980, Locht et al 1991), fluorescence yields (Quinton and Simons 1982, Suto and Lee 1983, Apps et al 1994), and photoionization quantum efficiencies (Rebbert and Ausloos 1971). Complementary spectroscopic information has been obtained using electron impact (Wight and Brion 1974, Wight et al 1977, Brion et al 1977, Furlan et al 1985, Burton et al 1993a, Souza and Srivastava 1996). One of the aims of this investigation was to identify the structure observed in the photoabsorption spectrum of ND3 just below and above the ionization threshold. As far as we are aware the present data constitute the first absolute measurement of the photoabsorption cross section of ND3 . In addition, new structure has been observed in the photoabsorption ˜ 2 E ionization threshold. spectra of NH3 and of ND3 due to Rydberg series converging onto the A The single-photon excited photoelectron and threshold photoelectron spectra of NH3 and ND3 have been recorded using He I (Branton et al 1970, Weiss and Lawrence 1970, Turner et al 1970, Potts and Price 1972, Rabalais et al 1973, Kimura et al 1981, Ågren et al 1982), He II (Potts and Price 1972, Kronebusch and Berkowitz 1976, Bieri et al 1982), ZrMζ , YMζ and MgKα (Banna and Shirley 1975) and synchrotron radiation (Banna et al 1987, Piancastelli et al 1987, Locht et al 1992). These studies have shown that ionization from the 1a200 orbital gives rise to a photoelectron band displaying a prolonged vibrational progression in the ν2+ mode, whilst the photoelectron band associated with ionization from the 1e orbital exhibits a complex and diffuse vibronic structure due to Jahn–Teller interactions. Ionization from the 2a1 orbital results in a featureless band with a maximum at a binding energy of 27.7 eV. In this study, ˜ 2 E photoelectron bands have been recorded at a resolution higher than ˜ 2 A00 and the A the X 2

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achieved in previous single-photon investigations, thereby allowing the vibrational structure exhibited in these two bands to be examined in greater detail. Previous studies have shown that the NH+3 molecular ion has a planar equilibrium geometry ˜ 2 A00 ground state. Furthermore, since excitations of the ν + mode are very in the (1a200 )−1 X 2 1 weak, the N–H bond distance can be assumed to be essentially unchanged after the ionization. Classically, the nitrogen atom and the hydrogen atoms move back and forth through a plane that contains the centre of mass and that is parallel to the plane spanned by the three hydrogen atoms. The motion is such that the H–N–H bond angle varies, but the N–H distance changes very little. When the ion is planar the H–N–H angle is 120◦ , compared with the equilibrium bond angle of 107◦ in the neutral molecule. The geometry of polyatomic molecules is usually best determined by studying highresolution optical spectra. If the different rotational transitions can be observed, it is often a straightforward procedure to obtain both bond distances and bond angles of symmetric threeand four-atom molecules by isotopic substitution. In conventional UV-excited photoelectron spectroscopy the rotational structure is not normally resolved, so determinations of ionic state geometries from such spectra rely upon a vibrational analysis and the calculation of Franck– ˜ 2 A00 state of NH+3 several studies of this kind have been performed Condon factors. For the X 2 (Harshbarger 1970, Cederbaum and Domcke 1976, Ågren et al 1982) and these have enabled the ionic ground state potential energy curve to be deduced. Obviously it would be of interest to carry the analysis a step further by considering also the rotational degrees of freedom. To this end new photoelectron spectra have been recorded, at a resolution of about 3 meV, which show partially resolved rotational structure for all observed vibrational lines up to v2+ = 17. Much higher resolution can be obtained in ZEKE spectra, and in studies using this technique with a resolution down to 0.4 cm−1 the rotational structure has been resolved well for vibrational levels up to v2+ = 9 (Habenicht et al 1991, Reiser et al 1993). The ZEKE investigations have enabled the rotational constants to be deduced, and the adiabatic energies for the v2+ = 0, 1 and 2 levels to be determined. Very accurate energies for the ν3+ mode, as well as for the v + = 0 → v2+ = 1 and the v2+ = 1 → v2+ = 2 transitions, have been determined by laser spectroscopy (Bawendi et al 1989, Lee and Oka 1991). In recent studies using the REMPI method with (2 + 1) ionization (Dobber et al 1995) the energies of the ν1+ (a1 ) and the ν4+ (e) vibrational modes were determined to be 0.404 ± 0.007 eV and 0.197 ± 0.007 eV, respectively. ˜ 2 A00 state has been difficult to identify in previous The adiabatic transition in the X 2 investigations of the photoelectron spectrum because the vibrational lines are weak near the beginning of the band, and it is difficult to separate them from hot bands. In this paper, the more accurately determined vibrational spacings and inherent structure of the lines clearly identify the 0–0 band. The energy obtained agrees very well with that from the recent ZEKE spectra (Habenicht et al 1991, Reiser et al 1993). ˜ 2 E state photoelectron band the vibrational lines are quite broad and it For the (1e)−1 A does not seem possible to resolve them in greater detail than has already been achieved in the previous study of Rabalais et al (1973). However, the present spectrum has been recorded with a much better signal-to-background ratio and therefore gives some new information, particularly in regard to the location of the onset of the band.

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2. Experimental apparatus and procedure 2.1. Synchrotron radiation studies The photoabsorption cross section and the photoionization quantum efficiency were measured using a double ion chamber and synchrotron radiation emitted from the Daresbury Laboratory storage ring. The 5 m normal incidence monochromator (Holland et al 1989), and the experimental apparatus and procedure (Shaw et al 1992) have been described previously. The double ion chamber incorporated a set of plates, two of which were used to collect the photoions. At the rear of the chamber the transmitted radiation struck a sodium salicylate screen and the resulting fluorescence was detected with a photomultiplier. This signal was used to deduce the incident photon intensity. A lithium fluoride filter could be inserted into the photon beam between the monochromator exit slit and the entrance to the capillary to suppress higher-order radiation. A photoabsorption spectrum of ammonia was measured by scanning the monochromator over the desired wavelength range and recording two electrometer currents, a reading proportional to the photomultiplier signal, and the gas pressure. The entire procedure was then repeated using argon, xenon or nitric oxide. For the inert gases it may be assumed that the photoionization quantum efficiency is unity, whilst for nitric oxide the values reported by Watanabe et al (1967) were used. Hence the photoionization quantum efficiency of ammonia could be deduced.

2.2. High resolution He I and He II excited photoelectron studies The UV photoelectron spectra were recorded in Uppsala using a high-resolution spectrometer that has been described elsewhere (Baltzer et al 1991). In brief, the instrument is based on a hemispherical energy analyser with a mean radius of 144 mm. The photoionization takes place in a gas cell with the sample molecules in random motion. Plasma and contact potentials in the gas cell, which tend to broaden the lines, were actively removed by internal compensating electrodes. Energy drifts due to changing potentials, which represent another significant line broadening factor, were detected by determining the location of a reference line, and corrected by an offset voltage applied in the electron lens used to focus the photoelectrons onto the entrance slit of the analyser (Baltzer et al 1993). The detector employs a microchannel plate arrangement in combination with a CCD camera. The detector was connected on-line to a computer for data storage and handling. Resonance radiation from a discharge in helium was used for the ionization. The radiation source was a microwave-driven discharge taking place in a magnetic field under electron cyclotron resonance conditions. This arrangement guarantees a very high brilliance. The device was operated at such a low pressure that the linewidth was less than 1 meV, which is important in order to record high-resolution spectra. The sample gas of spectroscopic quality was obtained commercially and used without further purification. For the energy calibration, spectra were recorded of a gas mixture containing the sample gas and argon, using the Ar 3p3/2 line at 15.7596 eV (Moore 1971) as the energy reference. The absolute energy for well defined lines is determined with an accuracy of better than 2 meV whilst the line separations within individual photoelectron bands are determined with a much higher accuracy of around 0.5 meV. Some of the energies are therefore quoted to four decimal places.

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Figure 1. The absolute photoabsorption cross section and photoionization quantum efficiency of NH3 . The cross section is referenced to the left-hand scale and the efficiency to the right-hand scale. Key for photoionization quantum efficiency measurements: (——) present data, () Watanabe and Sood (1965), (4) Samson et al (1987).

3. Results and discussion 3.1. Photoabsorption spectra of NH3 and ND3 3.1.1. Overall spectrum. The absolute photoabsorption cross sections and the photoionization quantum efficiencies of NH3 and ND3 are plotted in figures 1 and 2, respectively. At photon energies above 12 eV the present absolute photoabsorption cross section for NH3 shows reasonable agreement with the results from the early investigations of Sun and Weissler (1955), Walker and Weissler (1955) and Watanabe and Sood (1965). However, the spectrum reported by Metzger and Cook (1964) is different in both overall shape and magnitude. The two most recent and detailed studies have been carried out by Samson et al (1987) and by Burton et al (1993a), and in most respects the present cross sections exhibit very good agreement with these investigations. For photon energies greater than 14 eV the present data lie slightly above those of Samson et al , and match closely the values given by Burton et al. However, in the region between 11.3 and 11.8 eV where the values of Samson et al are significantly greater than those measured by Burton et al, the present data strongly support the cross sections determined by Samson et al. Further support for a cross section value of about 25 Mb around 11.3 eV is provided by the photoabsorption study of Xia et al (1991). On the other hand, the earlier investigation by Suto and Lee (1983) resulted in a value even lower than that reported by Burton et al. 3.1.2. Photoabsorption spectrum of NH3 in the energy range 9.9–11.5 eV. Figure 3 shows the photoabsorption spectrum of NH3 as measured by the photomultiplier attached to the rear

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Figure 2. The absolute photoabsorption cross section and photoionization quantum efficiency of ND3 . The cross section is referenced to the left-hand scale and the efficiency to the right-hand scale.

of the double ion chamber. This spectrum has been used, instead of that recorded with the ion plates, because it extends below the ionization threshold. Vibrational progressions due to ˜ Rydberg states, together with two new unidentified progressions ˜ F˜ and G transitions into the E, (P1, P2) are marked in figure 3 and listed in table 1. The labelling of the previously established progressions has been taken from Watanabe and Sood (1965) and Li and Vidal (1995). All three of the known progressions, and also the new progressions, involve excitation from the 1a200 orbital. The assignments for the Rydberg states in ammonia have been discussed and summarized by Li and Vidal (1995) and by Langford et al (1998), and those states (E˜ 1 E0 and F˜ 1 E0 ) relevant to this paper have been attributed to the excitations 4de00 ← 1a200 and ˜ state has not been confirmed using 5de00 ← 1a200 , respectively. The identification of the G multiphoton techniques. The original assignment for this state, proposed by Watanabe and ˜ 1 A00 (7sa0 ← 1a00 ). The features discernible in the present spectrum are Sood (1965), is G 2 1 2 generally similar to those observed in the photoabsorption study by Xia et al (1991) and in the electron energy loss study by Furlan et al (1985). However, Xia et al assigned only a rather limited portion of their structure, and in the deconvoluted spectrum shown by Furlan et al a single vibrational progression has been associated with the combined contributions ˜ states. The higher resolution employed in this paper enables separate from both the F˜ and G ˜ Rydberg states. These assignments, progressions to be identified with each of the F˜ and G and the photon energy positions of the vibrational members in each progression, are in good agreement with those reported by Watanabe and Sood (1965). The vibrational envelope for the ˜ 2 A00 photoelectron band shows a maximum around v + = 6 (figure 11). This indicates that X 2 2 the anomalously high intensity in the peaks corresponding to the low vibrational members of ˜ state progression may be caused by additional contributions from the overlapping high the G ˜ state progression. vibrational members of the D

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Figure 3. An expanded view of the photoabsorption spectrum of NH3 showing structure due to ˜ 2 A00 state. The energies of the Rydberg series converging onto vibrationally excited levels of the X 2 ˜ 1 A00 Rydberg ˜ vibrational structure are given in table 1. The assignments for the E 1 E0 , F˜ 1 E0 and G 2 states have been taken from Li and Vidal (1995) and Watanabe and Sood (1965). Table 1. Vibrational progressions observed in the photoabsorption spectrum of NH3 in the photon energy range 10–11.5 eV. ˜ 1 A00 Energy F˜ 1 E0 Energy G Energy Progression Energy Progression Energy E˜ 1 E0 2 (4de00 ← 1a200 ) (eV) (5de00 ← 1a200 ) (eV) (7sa10 ← 1a200 ) (eV) P1 (eV) P2 (eV) v20 = 6 7 8 9 10 11 12 13 14 15

10.051 10.180 10.311 10.443 10.573 10.696 10.823 10.955 11.089 11.217

v20 = 3 4 5 6 7 8 9 10 11 12 13

9.988 10.113 10.239 10.375 10.500 10.629 10.760 10.888 11.023 11.159 11.288

v20 = 2 3 4 5 6 7 8 9 10

10.012 10.144 10.269 10.394 10.521 10.650 10.778 10.909 11.048

line 1 2 3

10.288 10.414 10.556

line 1 2 3 4 5

10.619 10.746 10.871 11.003 11.136

3.1.3. Photoabsorption spectrum of ND3 in the energy range 9.9–11.5 eV. To the best of our knowledge there have been no previous measurements of the absolute photoabsorption cross section of ND3 . Figure 4 shows the absorption spectrum, as recorded by the photomultiplier, between 9.9 and 11.5 eV. Three extended vibrational progressions (P3–P5) are indicated, ˜ Rydberg states. In ˜ F˜ and G and we tentatively associate these with transitions into the E, addition, two new unidentified progressions have been observed and are labelled P6 and P7. Interpretations for the features due to autoionizing Rydberg states occurring in this photon

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Figure 4. An expanded view of the photoabsorption spectrum of ND3 showing structure due to ˜ 2 A00 state. The energies of the Rydberg series converging onto vibrationally excited levels of the X 2 vibrational structure are given in table 2. See text for details of the proposed assignments.

energy range in ND3 have been put forward in two previous papers. In the first, Locht et al (1991) suggested that autoionization structure discernible in their photoionization efficiency curves could be classified as being due to vibrational progressions belonging to nsa1 and npe (n = 5, 6, 7) Rydberg series. In the second, Apps et al (1994) measured the fluorescence excitation spectrum of ND3 and observed features similar to those recorded in this paper. By comparing their fluorescence spectrum for ND3 with the corresponding absorption spectrum ˜ F˜ and for NH3 they associated three vibrational progressions with transitions into the E, ˜ Rydberg states. The higher resolution employed in the present study allows these three G vibrational progressions to be separated, whereas Apps et al used a deconvolution procedure. The similarity between the vibrational intensity distributions associated with transitions ˜ states in NH3 (figure 3) and the corresponding features in ND3 (figure 4) ˜ F˜ and G into the E, suggests that progressions P3–P5 should be assigned to these same transitions in ND3 . However, a conclusive assignment must await higher resolution studies using multiphoton techniques. The average vibrational separation for progressions P3–P5 is approximately 98 meV, confirming that the ν20 mode is being excited in the upper state. Locht et al (1991) identified three vibrational progressions, denoted A, B and C, in this region of the photoionization efficiency curve for ND3 . For the low vibrational members of progression A, which they assign to the 5sa1 Rydberg state, the energy positions are similar to those of the corresponding members of progression P3, which we tentatively assign to the E˜ 1 E0 state. The correlation between progressions B and C, and P4 and P5 is not so clear. Table 2 lists the energy locations and interpretations of the structure observed in the photoabsorption spectrum of ND3 in the photon energy range 10–11.5 eV.

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Table 2. Vibrational progressions observed in the photoabsorption spectrum of ND3 in the photon energy range 10–11.5 eV. Progression P3 Progression P4 Progression P5 ˜ 1 A00 E˜ 1 E0 Energy F˜ 1 E0 Energy G Energy Progression Energy Progression Energy 2 (4de00 ← 1a200 ) (eV) (5de00 ← 1a200 ) (eV) (7sa10 ← 1a200 ) (eV) P6 (eV) P7 (eV) line 1 2 3 4 5 6 7 8 9 10 11 12

10.002 10.095 10.191 10.287 10.384 10.481 10.581 10.681 10.781 10.883 10.986 11.089

line 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

9.959 10.054 10.152 10.250 10.345 10.438 10.534 10.634 11.733 11.825 11.921 11.022 11.125 11.223 11.322

line 1 2 3 4 5 6 7 8 9 10 11 12 13 14

9.980 10.078 10.176 10.272 10.368 10.462 10.560 10.658 10.755 10.853 10.953 11.056 11.157 11.258

line 1 2 3

10.016 10.112 10.210

line 1 2 3 4 5 6 7

10.496 10.598 10.700 10.801 10.900 10.999 11.095

3.1.4. Photoabsorption spectra of NH3 and ND3 in the energy range 13.2–17.7 eV. Figures 5 and 6 show the absolute photoabsorption cross sections of NH3 and ND3 , respectively, in the 13.2–17.7 eV region and in both cases structure is discernible which can be associated with ˜ 2 E state threshold. As far as we are aware this is the Rydberg series converging onto the A first time that such structure has been observed in the absorption spectrum. Also included ˜ 2 E state photoelectron band (Rabalais et al 1973), and in figure 5 is a recording of the A the strong resemblance between the vibronic structure observed in the absorption spectrum and that discernible in the corresponding photoelectron band is apparent. However, the exact correlation between features in the two spectra is not straightforward. In addition, it is not clear whether all the structure discernible in the photoabsorption spectrum should be attributed to one Rydberg state or more than one. The alignment of the photoelectron spectrum with the absorption spectrum, shown in figure 5, appears reasonable but other choices could be made and our positioning should be considered tentative. ˜ 2 E state The overall shape and complicated vibronic structure evident in the A photoelectron band is a consequence of Jahn–Teller interactions. In C3v symmetry these interactions allow a doubly degenerate (E) electronic state to be split by the doubly degenerate ˜ 2E (E) vibrational modes. Thus, both ν3 and ν4 become Jahn–Teller active vibrations in the A state. Early theoretical work by Lonquet-Higgins et al (1958) predicted that the photoelectron band due to a transition from a non-degenerate state to a degenerate state should display a double maximum. This prediction has been confirmed experimentally, where a separation of about 1 eV was observed between the peaks (Weiss and Lawrence 1970, Potts and Price 1972, ˜ 2 E photoelectron Rabalais et al 1973). The majority of the vibronic structure exhibited by the A band falls in two energy regions. Between 14.7 and 15.9 eV several irregularly spaced features are discernible, which Weiss and Lawrence suggest might involve ν1+ excitation. A second progression, with a vibrational spacing of about 165 meV (NH3 ) and 140 meV (ND3 ) occurs between 16.3 and 17 eV (Rabalais et al 1973, Weiss and Lawrence 1970). The vibrational mode responsible for the second progression could not be identified. ˜ 2 E state has been investigated Recently, the two-mode Jahn–Teller effect in the NH3+ A

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Figure 5. An expanded view of the photoabsorption spectrum of NH3 showing structure due to ˜ 2 E ionization threshold. The energies of the vibrational Rydberg series converging onto the A ˜ 2 E state photoelectron spectrum structure are given in table 3. The inset displays the NH+3 A recorded by Rabalais et al (1973).

theoretically by K¨oppel et al (1978) and by Haller et al (1980), and it was found that the inclusion of the interaction between the degenerate stretching and bending vibrational modes was essential to understand properly the complicated shape of the photoelectron band. The calculations showed that the vibrational structure due to the two-mode Jahn–Teller interaction was masked in the full spectrum by excitation of the totally symmetric modes, and this prevented individual progressions being resolved in the experimental spectrum. The calculation reproduced successfully the overall shape of the experimentally observed band although a detailed prediction of the vibrational structure was not feasible. If it is assumed that the photoabsorption and photoelectron spectra should be aligned as shown in figure 5 then, for example, the absorption feature occurring at 15.263 eV belongs to the Rydberg series converging onto the feature observed at 15.899 eV in the photoelectron spectrum. Application of the Rydberg formula results in a value of n∗ = 4.63 for this feature. Even if the proposed alignment of the photoabsorption and photoelectron spectra is not quite correct, it appears that the Rydberg state associated with the structure discernible in the photoabsorption spectrum between 13.2 and 17.7 eV cannot be assigned as the first, n = 3, member of a Ryd˜ 2 E ionization threshold. If this is the case, it is surprising that berg series converging onto the A prominent structure due to the n = 3 and n = 4 members is not observed at lower energy. Close examination of the photoabsorption spectrum between 13.5 and 15.5 eV suggests that some of the observed features may not be associated with the n = 5 Rydberg state. Thus it is conceivable that this additional structure might correspond to transitions into the n = 3 and n = 4 states. Table 3 lists the energy locations of the prominent features associated with Rydberg series

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Figure 6. An expanded view of the photoabsorption spectrum of ND3 showing structure due to ˜ 2 E ionization threshold. The energies of the vibrational Rydberg series converging onto the A structure are given in table 3.

˜ 2 E state thresholds. In accordance with the photoelectron converging onto the NH+3 and ND+3 A studies of Weiss and Lawrence (1970) the vibronic structure is more strongly developed for NH3 than for ND3 . However, inspection of the structure falling in the 15.1–17.2 eV region reveals that more peaks can be discerned in the ND3 spectrum than in the corresponding range of NH3 . In view of the results from the theoretical study of Haller et al (1980) it appears that the earlier interpretations of the higher-lying vibrational progression in NH3 in terms of a single mode were oversimplified. The average separation between the main peaks in NH3 (lines 18– 26) is about 168 meV, in agreement with the photoelectron studies (Weiss and Lawrence 1970, Rabalais et al 1973). This value does not match with any of the ionic vibrational energies determined using multiphoton ionization techniques (Dobber et al 1995). In the absorption spectrum of ND3 lines 11–17 are separated from one another by about 67 meV, although the spacing is somewhat irregular. The energy of the ν4+ vibrational mode has been measured as 141 meV (Dobber et al 1995). Thus, it is conceivable that lines 11–17 can be arranged into two overlapping vibrational progressions, both involving excitation of ν40 . The threshold photoelectron spectrum of NH3 has been measured by Locht et al (1992). ˜ 2 E state main band, associated with direct ionization, a broad and quasiIn addition to the A structureless shoulder was observed towards lower binding energies. This shoulder occurred between 13.26 and 14.6 eV and was not apparent in the He I excited photoelectron spectrum. It was suggested that this could be attributed to autoionization from a Rydberg state converging ˜ 2 E ionization threshold, and, based upon their derived term values, this state was onto the A assigned as 4s. Although the contribution from the shoulder in the threshold photoelectron spectrum occurs in an energy region which encompasses some of the absorption features observed in the present experiment, the term value and Rydberg state identification do not match

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D Edvardsson et al Table 3. Lines observed in the photoabsorption spectra of NH3 and ND3 in the photon energy range 13–17.7 eV Line

NH3 Energy (eV)

Line

ND3 Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

13.339 13.650 13.758 13.911 14.128 14.362 14.525 14.635 14.804 14.985 15.045 15.168 15.263 15.381 15.440 15.498 15.665 15.843 15.994 16.175 16.355 16.523 16.687 16.850 17.017 17.187

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

14.081 14.171 14.324 14.551 14.642 14.849 14.967 15.118 15.165 15.314 15.431 15.488 15.553 15.623 15.688 15.760 15.833 15.883 15.990 16.133 16.286 16.394 16.545 16.678 16.800 16.926

the present proposals. 3.2. Oscillator strength moments and sum rules The use of oscillator strength moments and sum rules to evaluate photoabsorption and photoionization cross sections has been reviewed by Berkowitz (1979). The generalized formula for the moments may be expressed as Z (4πa0 /α0 )p λ0 S(p) = σ (λ)λ−(p+2) dλ πα0 2 a0 0 where a0 is the Bohr radius and α0 is the fine structure constant. The integration extends from the absorption threshold λ0 . S(0) is equal to the number of electrons in the molecule. S(−2) is related to the electric dipole polarizability, αN , by αN = 4 a03 S(−2). In order to perform a sum rule analysis the absolute photoabsorption cross sections determined in this experiment need to be combined with complementary data covering the remaining wavelength range. Where possible, these data have been selected from similar photon absorption studies, rather than from high-energy electron impact work. For wavelengths shorter than 80 Å, the only absolute photoabsorption cross sections have been determined over a very limited range in the vicinity of the nitrogen K-shell edge (Akimov et al 1988). Consequently the atomic x-ray absorption data of Henke et al (1982) have had to be employed

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Figure 7. The absolute photoabsorption cross section of NH3 . Table 4. Results of sum rule analysis. Wavelength range (Å)

S(0)

S(−1)

S(−2)

Si (−1)

1.25–500 500–1217 1217–2182 Total

6.155 3.582 0.198 9.935

1.382 2.907 0.336 4.625

0.505 2.501 0.590 3.596

1.382 2.239 3.621

in several regions. A composite cross section has been compiled using the following data: Henke et al (1982) 1.25–29 Å, Akimov et al (1988) 29–31 Å, Henke et al 31–62 Å, Burton et al (1993a) 62–80 Å, Samson et al (1987) 80–500 Å, present data 500–1217 Å, Suto and Lee (1983) 1217–2058 Å, Burton et al 2058–2182 Å, and is shown in figure 7. The results of our sum rule analysis are given in table 4. The electric dipole polarizability can be calculated from S(−2), and yields αN = 2.13 × 10−24 cm3 in comparison with the direct experimental value (Bridge and Buckingham 1966) of 2.22 × 10−24 cm3 . Burton et al (1993b) have carried out a theoretical study of the dipole oscillator strength distributions for ammonia, and their recommended values for S(−1) and S(−2) are 4.945 and 3.64, respectively. Berkowitz (1979) has defined a parameter Si (−1) as the ionization component of S(−1). This parameter has been evaluated between 500 Å and the ionization threshold from the product of the photoionization quantum efficiency and the absolute photoabsorption cross section measured in this paper, and yields a value of 1.382. For shorter wavelengths we have assumed that the photoabsorption and photoionization cross sections are equivalent and use the photoabsorption cross section. We obtain Si (−1) = 3.62 for the wavelength range between 1.25 Å and threshold. Rieke and Prepejchal (1972) have reported a directly measured value of 3.58.

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3.3. Photoionization quantum efficiency When a molecule is excited into a neutral state lying above the ionization threshold, the photoionization quantum efficiency (γ ) defined as the ratio of the number of ions created to the number of photons absorbed, may be less than unity. Under such circumstances the excited state may decay either by fluorescence from the parent molecule or by dissociation into neutral fragments. The probability of fluorescence decay from the excited parent molecule is small enough to be neglected. However, neutral photodissociation cross sections often remain significant for several electron volts above the ionization threshold (Holland et al 1995, 1997), and may be composed of contributions from direct and indirect processes. The indirect processes are due to transitions into Rydberg states which subsequently predissociate to form neutral fragments. Such processes lead to the occurrence of prominent structure in the photodissociation cross section (Shaw et al 1995). Alternatively, the excited state may decay by autoionization resulting in the creation of an ion. The competition between predissociation and autoionization reveals itself directly as a variation in the photoionization quantum efficiency. Figures 1 and 2 show the photoionization quantum efficiencies for NH3 and ND3 , respectively. In both figures a step is observed at 11.8 eV, correlating with the cut-off in transmission of the lithium fluoride filter used to suppress higher-order radiation. For energies greater than 11.8 eV no filter was used. The general shape of the present photoionization efficiency measurements is in accordance with previous results in that γ increases gradually from the ionization threshold and passes through two broad minima around 11.8 and 14.6 eV before rising again to reach a value around unity at 17.7 eV. The results from previous photon (Watanabe and Sood 1965, Rebbert and Ausloos 1971, Samson et al 1987) and electron (Wight et al 1977, Brion et al 1977) impact studies confirm that for energies greater than 18 eV the efficiency remains approximately unity. The present data show very good agreement with those of Samson et al (1987) which encompass the 13.4–18.2 eV range. For energies less than 11.8 eV, where the lithium fluoride filter was used in this experiment, the photoionization efficiencies for NH3 and ND3 exhibit a larger than expected difference, with the results for ND3 showing a closer agreement with the data of Xia et al (1991). The efficiencies reported by Watanabe and Sood (1965), in the same photon energy region, lie significantly below both the present results and those of Xia et al (1991). Inspection of figures 1 and 2 in the threshold regions reveals that peaks in the absorption spectrum coincide with local minima in the photoionization quantum efficiency, indicating that predissociation into neutral products competes successfully with autoionization in the decay of these super-excited states. 3.4. Photoionization and photodissociation cross sections The absolute photoionization, and the absolute photodissociation, cross sections of NH3 and ND3 are plotted in figures 8 and 9, respectively. The photoionization cross section has been obtained from the present measurements from the product of the absolute photoabsorption cross section and the photoionization quantum efficiency. The absolute photodissociation cross section is then given by the difference between the photoabsorption and photoionization cross sections, assuming that fluorescence decay can be neglected. Relative photoionization yield curves of the ammonia parent and fragment ions have been measured previously, and the data indicate that for energies below 15.5 eV only the parent ion is formed (McCulloh 1976). Thus between the ionization threshold and 15.5 eV the NH+3 yield curves should be similar to the absolute photoionization cross section, and a comparison between the spectrum reported by Dibeler et al (1966) and the present data shows that this is the case. Several step-like features, associated with vibrational excitation, can be discerned close to threshold.

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Figure 8. The absolute photoabsorption cross section, referenced to the left-hand scale, and the absolute photodissociation cross section, referenced to the right-hand scale, of NH3 .

Figure 9. The absolute photoabsorption cross section, referenced to the left-hand scale, and the absolute photodissociation cross section, referenced to the right-hand scale, of ND3 .

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Figure 10. He II excited photoelectron spectrum of the NH3 molecule.

The photodissociation cross sections for NH3 and ND3 remain significant several electron volts above the ionization threshold and both exhibit a substantial peak around 15 eV. Suto and Lee (1983) have determined the threshold energy for the photodissociation of NH3 into ˜ 1 6 + ) as 4.17 eV, and Mordaunt et al (1996) have measured D0 (H–NH2 ) ˜ 3 6 − ) + H2 (X NH (X 0 as 4.60 eV. 3.5. Photoelectron spectra of NH3 Photoelectron spectra have been recorded using both He I and He II excitation, and figure 10 shows the He II excited spectrum in the binding energy range 10–27 eV. In the outer valence ˜ 2E ˜ 2 A00 and the A region the spectrum exhibits the two well known bands associated with the X 2 00 2 ˜ A ionic ionic states. In the inner valence region, the intense He I excited spectrum of the X 2 state is overlapping from 26.4 eV, thereby making observations of the He II excited spectrum beyond this limit very uncertain. The main inner valence band, associated primarily with the N 2s orbital, is known to be centred at 27.7 eV (Banna and Shirley 1975) and therefore ˜ 2 A1 state band is cannot be studied in detail. However, as these earlier studies reveal, the B very broad and can be expected to be observed at energies even lower than 26.4 eV. Owing to the low background in the present spectrum it is possible to detect very weak structures and figure 10 reveals that the band has an onset at 24.0 eV. Binding energies, relative intensities (branching ratios) and assignments referred to this spectrum, as well as some data from He I excited spectra, are summarized in table 5. The fine structure in the photoelectron bands has been obtained from the more highly resolved He I excited spectra, and figure 11 shows an overall recording of the band associated ˜ 2 A00 ionic state. The band resembles closely those observed in previous He I with the X 2 excited spectra and shows a long vibrational progression in the v2+ bending (umbrella) mode, accompanied by a weaker progression with similar spacings. In the following, attention will

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Table 5. Summary of the binding energies and relative intensities of the UV excited photoelectron spectrum of the ammonia molecule obtained in this study.

State

Adiabatic binding energy (eV)

Vertical binding energya (eV)

Branching ratiob

˜ 2 A00 X 2 ˜ 2E A ˜ 2 A1 B

10.186c 614.7d 24.0d

10.93 16.6

0.31 0.69

Vibrational (0–1) spacing (eV) 0.1108

a

Centroid of the photoelectron band. ˜ ionic states. ˜ and A The branching ratio includes only the X c Vibrational peak maximum. d Onset of the photoelectron band. b

Figure 11. A He I excited spectrum of the NH3 molecule showing the outermost photoelectron ˜ 2 A00 cationic state. Two progressions in the ν + bending (umbrella) mode band associated with the X 2 2 are indicated.

be focused first on the rotational structure discernible in the vibrational peaks associated with the main progression. After that, details of the weaker progression will be presented, together with a discussion of its origin. In earlier studies, based on less well resolved spectra, it was noted that the vibrational peaks of the main progression were very broad in comparison with the instrumental resolution. This was assumed to be due to unresolved inherent structure, and the spectrum shown in figure 11 verifies this assumption. At the beginning of the photoelectron band each vibrational peak is a doublet, consisting of a main component accompanied by a somewhat weaker structure on the high binding energy side. The separation between the two components is 14 meV in the 0–0 peak and decreases gradually with increasing vibrational quantum number. At the centre of the photoelectron band the peaks are single and essentially symmetric, whereas at higher energies an asymmetry starts to appear in the form of a tail on the low binding energy side. Above v2+ = 13 this tail appears as a separate structure.

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We ascribe this distortion of the line shape to rotational substructure associated with different rotational branches. The rotational energy Erot of a symmetric top molecule can be calculated using the expression (Herzberg 1966) Erot = BJ (J + 1) + (A − B)K 2 where K represents the component of the angular momentum about the principal rotational axis of the molecule and the two rotational constants A and B are defined as h A= 2 8π cIzz h B= 2 8π cIxx where Izz and Ixx (=Iyy ) are the two different moments of inertia of the molecule. For each value of K a number of transitions corresponding to different values of J can take place. Assuming that the dominating transitions involve K = 0 in both states, as observed in the ZEKE spectra (Habenicht et al 1991, Reiser et al 1993), and 1K = 0, the expression for the transition energy essentially reduces to that of a rigid diatomic rotator since the rotational constants are not very different in the neutral and ionic states (Reiser et al 1993). The fine structure of the lines may then be associated with rotational branches, in the first place corresponding to 1J = 0, ±1. A further investigation of this will be given below. Table 6 summarizes the energies (referring to peak maxima), relative intensities and assignments for the observed structures. Figure 12 shows the splitting between the main line and the additional component versus the vibrational energy. As can be seen, by connecting adjacent data points with a straight line, zero splitting is predicted at about 11 eV, that is, near the maximum intensity of the band, where the lines indeed are essentially symmetric. Table 6. Binding energies and relative intensities of the photoelectron band associated with the ˜ 2 A00 state of the NH+ molecular ion. Assignments involving the ν + mode are tentative. X 3 2 4 Binding energy (eV)

Relative intensity

Assignment

10.0717 10.090 10.1856 10.020 10.2964 10.312 10.4129 10.426 10.5325 10.544 10.6559 10.665 10.7819 10.788 10.8603 10.9115 10.9849 11.0420 11.1116 11.1723 11.2404

0.8 0.6 5.8 3.1 19.1 11.5 41.4 27.3 66.1 48.4 87.3 71.0 98.2 89.8 4.6 100 5.5 96.1 5.6 82.0 4.9

ν200 − 0 (1J = 0, −1) ν200 − 0 (1J = +1) 0–0 (1J = 0, −1) 0–0 (1J = +1) ν2+ (1J = 0, −1) ν2+ (1J = +1) 2ν2+ (1J = 0, −1) 2ν2+ (1J = +1) 3ν2+ (1J = 0, −1) 3ν2+ (1J = +1) 4ν2+ (1J = 0, −1) 4ν2+ (1J = +1) 5ν2+ (1J = 0, −1) 5ν2+ (1J = +1) ν4+ + 4ν2+ 6ν2+ ν4+ + 5ν2+ 7ν2+ ν4+ + 6ν2+ 8ν2+ ν4+ + 7ν2+

Binding energy (eV)

Relative intensity

Assignment

11.3047 11.3706 11.4382 11.5025 11.5724 11.6361 11.7084 11.7696 11.8450 11.9047 11.965 11.9828 12.0407 12.102 12.1217 12.1802 12.236 12.2596 12.373 12.3998

62.2 3.6 44.3 2.8 29.0 1.8 17.5 1.3 9.5 0.5 2.3 5.0 0.25 1.1 2.5 0.1 0.4 1.2 0.25 0.6

9ν2+ ν4+ + 8ν2+ 10ν2+ ν4+ + 9ν2+ 11ν2+ ν4+ + 10ν2+ 12ν2+ ν4+ + 11ν2+ 13ν2+ ν4+ + 12ν2+ 14ν2+ (1J 14ν2+ (1J ν4+ + 13+2 15ν2+ (1J 15ν2+ (1J ν4+ + 14ν2+ 16ν2+ (1J 16ν2+ (1J 17ν2+ (1J 17ν2+ (1J

= −1) = 0, +1) = −1) = 0, +1) = −1) = 0, +1) = −1) = 0, +1)

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Figure 12. Energy separation between the maxima in the doublet peak associated with the rotational branches 1J = +1 and 1J = 0, −1 versus the vibrational energy.

Figures 13–16 show the fine structure of the vibrational lines in greater detail. Figure 13 contains the first part of the band including the structure at 10.071 eV, the origin of which has been uncertain. In less well resolved spectra, this feature is separated from the next peak at 10.1856 eV with very nearly the same energy as the separation between the next two peaks, and the intensity is not unreasonably low to fit into the main vibrational progression. However, from the present spectrum it can be assigned rather safely as a hot band, corresponding to transitions to the vibrationless ionic ground state from neutral molecules thermally excited to the v200 = 1 level. The energy separation from the peak at 10.1856 eV is 115 meV, which is in close agreement with the energy of 115.6 meV for the ν2 mode in the neutral molecule (Benedict and Plyler 1957, Benedict et al 1960), whereas the next vibrational spacing measured is only 111 meV. This assignment agrees with the results of recent high-resolution ZEKE studies (Habenicht et al 1991, Reiser et al 1993). It may be noticed that the fine structure of the hot band is very similar to that of the 0–0 band, as would be expected for rotational excitations since the geometries of the states involved are similar in these cases. Vibrational adiabatic energies determined by ZEKE measurements have been reported for the v2+ = 0, 1, 2 levels and are 10.1864, 10.2984 and 10.4150 eV, respectively. These values correspond well with the energies of the main peak of the present lines but are shifted somewhat towards higher binding energies. Therefore, the main peak is most likely to be associated primarily with the superimposed 1J = 0 and 1J = −1 transitions (corresponding to Q and P branches of the rigid rotator) whilst the weaker peak at higher energies reflects the 1J = +1 transitions (R branch). In contrast, for the peaks with v2+ > 13 the additional structure at lower binding energies should be due to the P branch, whilst the main peak corresponds to the sum of the Q and R branches.

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˜ 2 A00 state photoelectron band involving the transitions 0–0, 21 , Figure 13. The first part of the X 2 0 3 2 20 , 20 and a hot band (h.b.) corresponding to the transition 210 .

˜ 2 A00 state photoelectron band involving the transitions 24 , 25 , Figure 14. The second part of the X 2 0 0 6 7 8 20 , 20 and 20 .

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˜ 2 A00 state photoelectron band involving the transitions 29 , 210 , Figure 15. The third part of the X 0 0 2 11 12 20 and 20 .

˜ 2 A00 state photoelectron band involving the transitions 213 , Figure 16. The fourth part of the X 2 0 2014 , 2015 , 2016 and 2017 . Each peak has a tail or a shoulder on the low binding energy side probably associated with the rotational branch 1J = −1.

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Figure 17. The v2+ = 3 line with peak maximum located at 10.5325 eV (data points) and a fitting of a theoretical curve (full curve) using the formula for a rigid rotator (see text).

A simulation of the line shape under the above approximation, that is, using the formula for a simple rigid diatomic rotator, has been made in order to test these assignments further. In these calculations the rotational constant B of the ionic state and the relative cross sections for the various branches have been used as parameters that could be adjusted to produce the best fit to the experimental spectrum. The result for the v2+ = 3 line is shown in figure 17. This fitting depends sensitively on all parameters, and the values obtained are at least qualitatively correct. Similar results were obtained for all vibrational lines and the results for the first four lines are given in table 7. The rotational constant B is clearly less accurate than that reported in the recent ZEKE measurements, and the value in the present study is systematically slightly lower, which may be due to the neglect of higher 1K-components in the calculations. As can be seen, the calculations strongly support the assignments in terms of different rotational branches and put the adiabatic energy on the sloping high energy side of the main peak, which is in better agreement with the ZEKE measurements. For the v2+ = 3 line in figure 17, the shift is 5 meV from the position of the peak maximum. Interestingly, the curve fittings show that the main part of the intensity comes from odd rotational branches, that is those with 1J = 1, 3, etc, while the even rotational branches with 1J = 0, 2, . . . give only minor contributions. This can be explained by assuming that the 1a200 orbital has essentially N 2p atomic character. For an atomic p-type orbital, the angular momentum quantum number l of the partial wave associated with the outgoing electron must

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Table 7. Weight factors for the different rotational branches and the rotational constant B obtained from curve fittings to the observed vibrational lines. The largest weight factor is normalized to one for each vibrational state. Weight factors Vibrational component v2+

N 1J = −3

O 1J = −2

P 1J = −1

Q 1J = 0

R 1J = 1

S 1J = 2

T 1J = 3

Rotational constant B

0 1 2 3

0.11 0.11 0.13 0.14

0.00 0.00 0.00 0.00

1.00 1.00 1.00 1.00

0.22 0.22 0.25 0.28

0.70 0.70 0.69 0.69

0.02 0.02 0.03 0.02

0.03 0.03 0.03 0.03

9.8 9.5 8.9 8.5

be even, whereas in the dipole approximation the incoming photon has an angular momentum of one unit. Since the total electronic angular momentum of the neutral molecule, as well as that of the ion, is zero, this implies, if the total angular momentum is to be conserved, that 1J must be odd, that is only odd rotational branches will be observed. The fact that the even branches do acquire some intensity suggests that the 1a200 orbital has some hydrogen character along with the N 2p. As can be seen from the weight factor for the Q branch (table 7), this influence increases successively with increasing vibrational energy. This behaviour is expected as a result of the accompanying increase in amplitude of the bending vibrational motion which promotes an increasing interaction with the hydrogen atoms. The origin of the progression of weak lines accompanying the main progression has been much discussed but an unambiguous assignment has not been obtained. In this study, the energies and relative intensities of these lines have been determined more accurately than previously (the results are included in table 6), which facilitate the interpretation. In figure 18 the energy separations between adjacent lines are plotted versus the vibrational energy, together with the corresponding data for the main progression. It is apparent that there are striking similarities between the curves. This supports the interpretation that progressively higher levels of the ν2+ mode are excited in the weaker progression, as well as in the stronger, and that the first few levels of the weaker lines are hidden under the strong lines. Thus, the weaker progression could reflect the ν2+ progression excited together with another mode. This interpretation is also consistent with the observed line shapes and the relative intensities which display a maximum around 11 eV. The only mode that fits reasonably well with an extrapolated energy for the onset of the weaker progression is ν4+ which, according to the ZEKE studies (Habenicht et al 1991, Reiser et al 1993), has an energy of 187 meV, whereas the extrapolated value would be around 200 meV. Interestingly, even the REMPI photoelectron spectra (Dobber et al 1995) show some weaker progressions in the ν2+ mode, along with an additional amount of vibrational energy. For one of these progressions the additional energy is 0.197 eV and this value was associated with a quantum of the ν4+ mode and a progression nν2+ + ν4+ . This energy is 10 meV higher than that obtained from the ZEKE spectra but is in very good agreement with our extrapolated value. We tentatively assign the lines accordingly in table 6. The ν4+ mode is doubly degenerate and excitation of a single quantum would therefore be ‘forbidden’ in photoelectron transitions. However, it could become allowed due to vibronic ˜ 2 E state. An angle resolved study of the vibrational structure could shed interaction with the A more light on this aspect. ˜ 2 E state, the structure reported in previous studies is essentially confirmed. For the A The energies and relative intensities of these features have been determined from the present spectra and the results are presented in table 8. Figure 10 illustrates that these structures cover

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˜ 2 A00 state photoelectron Figure 18. Energy separations between adjacent vibrational lines of the X 2 band versus the vibrational energy of the higher component. The long (upper) series of data points corresponds to the main lines and the shorter (lower) series corresponds to the progression of weaker lines in the spectrum.

essentially the entire band, except the highest energy part. In particular, the indications of line structure in the 15.9–16.3 eV range (Rabalais et al 1973) have been confirmed. The observed onset of the band is a rather diffuse and broad structure, with maximum intensity at 14.706 eV. The band seems to start at 14.6 eV but the background in this region is too high in the present spectra to warrant any definite conclusions. There could also be states at even lower energies which have too low a Franck–Condon factor to be observable. 4. Summary The absolute photoabsorption cross sections and the photoionization quantum efficiencies of NH3 and ND3 have been measured from the ionization threshold to 25 eV, and, for the deuterated sample, the present results constitute the first such measurements. New structure has been observed in the photoabsorption spectrum of ND3 in the 10.0–11.3 eV range, and vibrational ˜ Rydberg states have been recorded with ˜ F˜ and G progressions due to transitions into the E, improved resolution. Tentative assignments for some of this structure have been proposed based upon the corresponding photoelectron spectra. For the first time, features have been observed in the photoabsorption spectra of NH3 and ND3 due to Rydberg series converging onto ˜ 2 E ionization threshold. Again, the interpretation of these features has been guided by the A the corresponding photoelectron spectra which provide information concerning the vibrational

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Table 8. Binding energies and relative intensities of the photoelectron band associated with the ˜ 2 E state of the NH+ molecular ion. A 3 Binding energy (eV)

Relative intensity

14.706 14.919 15.185 15.249 15.376 15.447 15.542 15.655 15.899 15.990 16.055 16.117 16.179 16.370 16.572 16.74 16.89 17.08 17.25

2 6 27 26 57 51 61 85 93 91 93 96 97 97 100 95 81 68 51

energies and relative intensities. Absolute photodissociation and photoionization cross sections have been presented. A sum rule analysis has been performed by combining the present photoabsorption measurements with similar data covering the remaining wavelength regions. ˜ 2 A00 photoelectron band has been studied experimentally The He I excited NH3+ (1a200 )−1 X 2 at a resolution of 3 meV and two vibrational progressions, each involving excitation of the ν2+ mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This rotational structure has been modelled using energy expressions appropriate to a symmetric top molecule, together with the assumptions that K = 0 in both states, and 1K = 0. The simulated line shapes indicate that the strongest intensity contributions come from odd rotational branches, that is those with 1J = 1, 3, etc. The second progression has been interpreted tentatively as being due to nν2+ + ν4+ transitions, where the single quantum of the doubly degenerate ν4+ mode arises from ˜ 2 E state. The high resolution and low background achieved in vibronic coupling with the A ˜ 2 A00 state has enabled a longstanding controversy the present He I excited spectrum of the X 2 regarding hot-band excitation to be settled. Acknowledgments We are grateful for financial support and a CASE studentship (EER) from the Engineering and Physical Sciences Research Council. This work was also supported by the Swedish Natural Science Research Council. References Ågren H, Reineck I, Veenhuizen H, Maripuu R, Arneberg R and Karlsson L 1982 Mol. Phys. 45 477 Akimov V N, Vinogradov A S and Zhadenov A V 1988 Opt. Spektrosk. 65 349 Apps C J, Bramwell M J, Cooper J L, Whitehead J C and Winterbottom F 1994 Mol. Phys. 83 1265

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Ashfold M N R, Bennett C L and Stickland R J 1987 Comment. At. Mol. Phys. 19 181 Baltzer P, Karlsson L, Lundqvist M and Wannberg B 1993 Rev. Sci. Instrum. 64 2179 Baltzer P, Wannberg B and Carlsson G¨othe M 1991 Rev. Sci. Instrum. 62 643 Banna M S, Kossman H and Schmidt V 1987 Chem. Phys. 114 157 Banna M S and Shirley D A 1975 J. Chem. Phys. 63 4759 Bawendi M G, Rehfuss B D, Dinelli B M, Okumura M and Oka T 1989 J. Chem. Phys. 90 5910 Benedict W S 1935 Phys. Rev. 47 641 Benedict W S and Plyler E K 1957 Can. J. Phys. 35 1235 Benedict W S, Plyler E K and Tidwell E D 1960 J. Chem. Phys. 32 32 Berkowitz J 1979 Photoabsorption, Photoionization and Photoelectron Spectroscopy (New York: Academic) Bieri G, Åsbrink L and von Niessen W 1982 J. Electron Spectrosc. Relat. Phenom. 27 129 Branton G R, Frost D C, Makita T, McDowell C A and Stenhouse I A 1970 Phil. Trans. R. Soc. A 268 77 Bridge N J and Buckingham A D 1966 Proc. R. Soc. Lond. A 295 334 Brion C E, Hamnett A, Wight G R and van der Wiel M J 1977 J. Electron Spectrosc. Relat. Phenom. 12 323 Burton G R, Chan W F, Cooper G and Brion C E 1993a Chem. Phys. 177 217 Burton G R, Chan W F, Cooper G, Brion C E, Kumar A and Meath W J 1993b Can. J. Chem. 71 341 Cederbaum L S and Domcke W 1976 J. Chem. Phys. 64 603 Cramb D T and Wallace S C 1994 J. Chem. Phys. 101 6523 de Reilhac L and Damany N 1977 J. Quant. Spectrosc. Radiat. Trans. 18 121 Dibeler V H, Walker J A and Rosenstock H M 1966 J. Res. NBS A 70 459 Dobber M R, Buma W J and de Lange C A 1995 J. Phys. Chem. 99 1671 Douglas A E 1963 Disc. Faraday Soc. 35 158 Douglas A E and Hollas J M 1961 Can J. Phys. 39 479 Duncan A B F 1935a Phys. Rev. 47 822 ——1935b Phys. Rev. 47 886 ——1936 Phys. Rev. 50 700 Duncan A B F and Harrison G R 1936 Phys. Rev. 49 211 Eland J H D 1980 J. Chim. Phys. 77 613 Furlan M, Hubin-Franskin M-J, Delwiche J, Roy D and Collin J E 1985 J. Chem. Phys. 82 1797 Glownia J H, Riley S J, Colson S D and Nieman G C 1980a J. Chem. Phys. 72 5998 ——1980b J. Chem. Phys. 73 4926 Habenicht W, Reiser G and M¨uller-Dethlefs K 1991 J. Chem. Phys. 95 4809 Haller E, Cederbaum L S, Domcke W and K¨oppel H 1980 Chem. Phys. Lett. 72 427 Harshbarger W R 1970 J. Chem. Phys. 53 903 Henke B L, Lee P, Tanaka T J, Shimabukuro R L and Fujikawa B K 1982 At. Data Nucl. Data Tables 27 1 Herzberg G 1966 Molecular Spectra and Molecular Structure vol III Electronic Spectra and Electronic Structure of Polyatomic Molecules (New York: Van Nostrand Reinhold) Holland D M P, Shaw D A and Hayes M A 1995 Chem. Phys. 201 299 Holland D M P, Shaw D A, Hayes M A, Shpinkova L G, Rennie E E, Karlsson L, Baltzer P and Wannberg B 1997 Chem. Phys. 219 91 Holland D M P, West J B, MacDowell A A, Munro I H and Beckett A G 1989 Nucl. Instrum. Methods B 44 233 Kimura K, Katsumata S, Achiba Y, Yamazaki T and Iwata S 1981 Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules (Tokyo: Japan Scientific Societies Press) K¨oppel H, Cederbaum L S, Domcke W and von Niessen W 1978 Mol. Phys. 35 1283 Kronebusch P L and Berkowitz J 1976 Int. J. Mass Spectrom. Ion Phys. 22 283 Langford S R, Orr-Ewing A J, Morgon R A, Western C L, Ashfold M N R, R¨ykenberg A, Scheper C R, Buma W J and de Lange C A 1998 J. Chem. Phys. 108 6667 Lee S S and Oka T 1991 J. Chem. Phys. 94 1698 Li X and Vidal C R 1994 J. Chem. Phys. 101 5523 ——1995 J. Chem. Phys. 102 9167 Locht R, Hottmann K, Hagenow G, Denzer W and Baumg¨artel H 1992 Chem. Phys. Lett. 190 124 Locht R, Leyh B, Denzer W, Hagenow G and Baumg¨artel H 1991 Chem. Phys. 155 407 ¨ Lonquet-Higgins H C, Opik U, Pryce M H L and Sack R A 1958 Proc. R. Soc. A 244 1 McCulloh K E 1976 Int. J. Mass Spectrom. Ion Phys. 21 333 Metzger P H and Cook G R 1964 J. Chem. Phys. 41 642 Miller P J, Chupka W A and Eland J H D 1988 Chem. Phys. 122 395 Moore C E 1971 US National Bureau of Standards Circular No 467 (Washington, DC: US Govt Printing Office) Mordaunt D H, Dixon R N and Ashfold M N R 1996 J. Chem. Phys. 104 6472

A spectroscopy study of NH3 and ND3

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