Effect of Laser Energy and Atmosphere on the Emission Characteristics of Laser-Induced Plasmas

Effect of Laser Energy and Atmosphere on the Emission Characteristics of Laser-Induced Plasmas MIKIO KUZUYA,* HITOSHI MATSUMOTO, HIDEAKI TAKECHI, and ...
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Effect of Laser Energy and Atmosphere on the Emission Characteristics of Laser-Induced Plasmas MIKIO KUZUYA,* HITOSHI MATSUMOTO, HIDEAKI TAKECHI, and O S A M U M I K A M I Department o[ Electronic Engineering, College o[ Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai-shi, Aichi 487, Japan

The effects of laser energy and atmosphere on the emission characteristics of laser-induced plasmas were studied with the use of a Q-switched Nd:YAG laser over a laser energy range of 20 to 95 mJ. Argon, helium, and air were used as surrounding atmospheres, and the pressures were changed from atmospheric pressure to 1 Torr. The experimental results showed that the maximum spectral intensity was obtained in argon at around 200 Torr at a high laser energy of 95 m J, whereas the line-tobackground ratio was maximized in helium at around 40 Torr at a low energy of 20 m J . The results are discussed briefly on the basis of the temporal and spatial observations of the laser-induced plasmas. Index Headings: Laser-induced plasma; Laser energy; Atmosphere;

Emission intensity; Background; Line-to-background ratio.

INTRODUCTION The luminous plasma induced by a high-power laser focused on a sample has been used as the source for atomic emission spectrometry, l-3 It is well known that the atmosphere surrounding the sample strongly affects the emission characteristics of the laser-induced plasma. Therefore, a number of studies concerning the influence of the various ambient atmospheres have been carried out to find optimum atmospheric conditions for practical application to spectrochemical analysis. Kagawa e t al., 4 using a nitrogen laser (5 mJ, 5 ns duration), showed that a plasma emitting sharp atomic line spectra with negligibly low background signals can be generated in an argon atmosphere at around 1 Tort. A similar effect was observed with a TEA CO2 laser (500 mJ, 100 ns) in the atmosphere of air at I T o r t ) In contrast to this approach, Iida, 6 with the use of a Q-switched ruby laser (1.5 J, 20 ns), showed that moderate confinement of plasmas and a resultant increase of emission intensities are achieved in an atmosphere of argon at 50 Tort. Grant and Paul 7 also showed that the maximum spectral intensity and line-to-background ratio occur in argon at 50 Tort for the use of an excimer laser (40 mJ, 28 ns). Owens and Majidi, s using a 100-mJ Nd:YAG laser, showed that helium at superatmospheric pressure can enhance the emission intensity for the aluminum ionic line at 281.6 nm. We showed that an argon atmosphere at around 150 Tort is effective for the enhancement of emission signals and the elimination of self-absorption of spectra.9.1o These studies, however, were generally carried out at a fixed laser energy. But the effects of the ambient atmospheres on the emission characteristics of the laserReceived 6 J a n u a r y 1993; revision received 23 March 1993. * Author to whom correspondence should be sent.

Volume 47, Number 1 0, 1993

induced plasma are influenced by the laser energy used. Relatively few studies have been reported on the quantitative aspects of the effect of the laser energy. Piepmeier and Osten lz studied the atmospheric influences on the generation of plasma produced by a Q-switched Nd laser, with the laser energy changed from 10 to 100 mJ. However, they used only air as the atmosphere, and the effect of the laser energy on the atomic emission intensity was not discussed in detail. Treytl e t al. 12 investigated effects of atmosphere and laser energy on optical emission from the plasma produced by a Q-switched ruby laser operated in the energy range of about 1 to 8 mJ, using various gas media, i.e., argon, helium, nitrogen, oxygen, and air. Their experiments were carried out only at atmospheric pressure except for air, and the results did not exhibit systematic trends. Recently, Hwang e t al., 13 with the use of an excimer laser over the energy range 0 to 200 mJ, showed that an increase of laser energy increases the emission intensity of metal targets in the flowing atmospheres of argon and air at atmospheric pressure. However, laser energy effects on the atomic line spectra as well as the background signals were not discussed, because the authors observed the total emission intensity over a spectral range of 200 to 650 nm without using a spectrometer. Leis e t al. 14 reported that high laser energy causes a gas breakdown which leads to poor reproducibility of the observed signals, and hence they used a low laser energy of 5 mJ instead of the maximum energy of 250 mJ for atomic emission spectrometry of the Nd:YAG laser-produced plasmas. Very recently, Sdorra and Niemax, 15 using the Nd:YAG laser, studied the dependence of the spectral line intensity on laser energy over the range of 2 to 20 m J, only at a pressure of 105 Torr, however. The aim of the experiment reported in this paper was to study the effect of the laser energy and ambient atmosphere on the spectral line intensity as well as on the background intensity of the laser-induced plasma. The experiment was carried out with three different atmospheres (argon, helium, and air) from atmospheric pressure to 1 Torr, with the use of a Q-switched Nd:YAG laser over the energy range of 20 to 95 mJ. It is shown that an increase in the laser energy causes an increase in the intensity of the spectral line, but does not directly result in a higher line-to-background ratio, because the background intensity also increases with the laser energy. From the results obtained, the conditions with respect to the laser energy and the surrounding atmosphere for maximum spectral line intensity and line-to-background ratio are defined.

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© 1993Societyfor Applied Spectroscopy

APPLIED SPECTROSCOPY

1659

Nd:YAG LASER

Mirror

HY-400

M°n°chr°mat°r~ MTP-250 Monitor-TV

Objective lens

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Inlet[

Diglt.,al Osci I I oscope

Condenser lens

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Spectr°meter I 0p%ical fibe~[~ JSG-125

lezer Camera FRM2-RGB|

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I

Computer PC-9801

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chamber

,

Vacuum pump FIG. I.

PDA

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0p%ical Processing System

Schematic diagram of the experimental setup used for the measurementsof the laser-induced plasma.

EXPERIMENTAL Figure 1 shows a schematic diagram of the experimental arrangement used in this work. The laser used was a Nd:YAG laser (Lumonics HY-400) and operated in a Q-switched mode. The laser output has a pulse duration of 10 ns and a repetition rate of 10 Hz, and the pulse energy was varied from 20 to 95 mJ. The laser radiation was directed downward by a high-energy mirror and was focused onto the sample surface by an objective lens ([ = 50 mm) through the quartz glass window of a sample chamber. The radiation of the laser-produced plasma was observed at a right angle to the laser beam by imaging the plasma in a 4:1 ratio by a condenser lens (f = 40 mm) onto an entrance aperture (2 mm diameter) just in front of an optical fiber, whose exit end is bifurcated. One exit end of the fiber was placed against the entrance slit of a 1.25-m modified Czerny-Turner mounted spectrometer (JEOL JSG-125) having a grating of 1200 grooves/mm and a reciprocal linear dispersion of 0.62 nm/mm. The spectra were detected by a photodiode array (PDA) detector (Tracor Northern TN-6144; element size 25 #m x 2.5 ram, spectral range 11 nm) with a microchannel plate image intensifier. Spectral data acquisition and processing were carried out with an optical processing system (Seki Technotron SK-296). In this experiment, the PDA detector was operated in a nongated mode, with the intensifier actively intensifying incoming light during the exposure time. The exposure time was set to 0.5 s, and the time-integrated emission signal for five successive laser shots was accumulated for one measurement. The emission signal was corrected by subtraction of the dark current of the detector, which was 1660

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Volume47, Number10, 1993

separately measured during the same exposure time. The other exit was led to a monochromator (Nikon P-250, Czerny-Turner, focal length 250 mm, grating 1200 grooves/mm). As a detector, a photomultiplier (Hamamatsu Photonics R-306) was used, and the output signal was fed to a digital oscilloscope (Yokogawa DL-1200) to monitor the time profile of spectral emission. The sample was placed in a vacuum-tight sample chamber, which was designed in our laboratory for controlling the ambient atmosphere. The sample chamber is a rectangular box (8 cm x 8 cm x 8 cm) with optical windows in all four sides, and a window on top through which the laser beam enters. The sample chamber was mounted on a precise Z-axis transfer stage, and the sample stage was rotated at a speed of 2 rpm with the use of a stepping motor in order to present a fresh sample surface and to obtain the stable laser plasma during successive laser shots. A CCD video camera (SONY XC711) was used to monitor the laser-produced plasma. Through an image digitizer (Photron FRM2-RGB), the video signals were transferred to a personal computer (NEC PC-9801DA5), which provides the image processing and data storage. The sample chamber was evacuated by a rotary pump and then filled with the desired gas (argon, helium, or air). The pressure was varied in the range of 1 to 760 Torr and measured by a semiconductor pressure gauge (Okano Works, VA-2076S-2) calibrated with a mercury manometer. The samples used were nickel alloy (Ni, 60.5%; Cr, 23%; Fe, 14.1%; A1, 1.4%; other elements

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