18.2 Raman Instrumentation • Raman suffers from a small cross-section and a high resolution spectrum • the original instrumentation consisted of a double monochromator and photon counting • modern instruments use a single monochromator with an array detector, or an interferometer and a cooled Ge detector • Raman imaging is superior to infrared imaging, and Raman pump/probe spectroscopy can examine small volumes • trace analysis is possible in certain cases 18.2 : 1/8

Design Problems Statement of the Problem: The Raman cross-section is _______ orders of magnitude smaller than fluorescence and the spectral bands are ________. Instrumental Requirements: (problems in parentheses) • intense sources ♦ narrow-band lasers (need to reject unwanted emission lines) • processing of small signals ♦ photon counting (long time constants) ♦ array detection (stray light from Rayleigh line) • high spectral resolution ♦ double 1-meter monochromators (poor throughput) ♦ interferometers (Rayleigh line adds noise to spectrum) • Rayleigh scatter ♦ double monochromators reduce stray light (poor throughput) ♦ holographic rejection filters (can fluoresce!) • fluorescence background ♦ nanosecond temporal resolution (expensive and difficult) ♦ excitation in the near IR (1/λ4 dependence gives low signals) 18.2 : 2/8

Double Monochromator with Photon Counting Plusses: •argon-ion laser at 488 nm (20,491 cm-1) has several watts of power • use of two monochromators doubles the resolution to ~_______ nm mm-1 (2 cm-1 at 488 nm) • holographic gratings have low stray light (10-4)2 = 10-8, reducing Rayleigh • photon counting is combined with stepping the monochromator in 2 cm-1 increments

lens

slit

mirror

photon counting photomultiplier grating

mirror

mirror slit

laser beam is vertical to match the monochromator slits

sample

grating

lens

slit

mirror

Minuses: • high f/#; f/8 gathers ____% of the Raman signal • two gratings and four mirrors have a poor reflectivity; 0.94×0.72 = 0.32 • the large number of resolution elements dictates long signal processing times; (3,600-400 cm-1)/(2 cm-1) = 1,600 elements • photon counting for 10 s at each wavelength would require 16,000 s or ~4.5 hr • most of the spectrum is "______" making scanning a poor strategy 18.2 : 3/8

Single Monochromator with Array Detector CCD array detector

mirror

slit

mirror

Plusses: • laser diode at 830 nm reduces any laser beam is ____________ background vertical to match the • the holographic rejection filter virtually monochromator slits grating lens lens eliminates monochromator scatter from the ___________ line sample • a single 0.25 m monochromator holographic Rayleigh reduces cost and takes little space rejection filter • the array detector can examine 1024 wavelengths at one time, reducing 4.5 hr to 16 s!

Minuses: • the 830 nm excitation reduces the Raman intensity by ~_____ compared to excitation at 488 nm • to keep the laser diode from drifting in wavelength, it has to have an external cavity or involve multi-quantum wells (both expensive) • unless they are cooled to 77 K or combined with image intensifiers, array detectors are not as sensitive as photomultipliers 18.2 : 4/8

Fourier Transform Raman Plusses: • Nd:YAG laser at 1.06 μm virtually eliminates any fluorescence background • interferometers have a low ____ and high ___________ • the holographic rejection filter keeps the Rayleigh line from spreading its photon noise across the spectrum • the germanium detector can "see" individual Raman photons • the interferometer examines all of the Raman bands at the same time Minuses: • 1.06 μm excitation reduces the Raman intensity by ~______ compared to excitation at 488 nm • the Ge detector has to be cooled to 77 K 18.2 : 5/8

mirror piston with air bearing

sample holder lens

laser beam

parabolic mirror with a small hole for the laser beam

liquid N2 cooled Ge detector

spatial filter

holographic Rayleigh rejection filters

movable mirror beam splitter

fixed mirror

Vibrational Microscopy Consider imaging a sample to obtain the spatial distribution of vibrational frequencies. The spatial resolution is determined by the focal spot size, which is given by,

where ρ is the focal radius, λ is the wavelength, r is the beam radius, and f/# is the lens f-number. To compare the resolution of infrared and Raman detection, consider a vibration at 1,000 cm-1, which is 10 μm. In infrared spectroscopy the minimum image spot size will be 12.2 μm × f/#. If the Raman spectrum is obtained with 0.488 μm excitation, the vibration will appear at 0.513 μm. The minimum image spot size would then be 0.625 μm × f/#. This is ~______ times smaller, which is why Raman imaging has become so popular. The ratio of Stokes to anti-Stokes emission can be used to monitor microscopic variations in ___________! 18.2 : 6/8

Volume Resolution One variant of Raman spectroscopy is called ____________ detection. It involves two laser beams, one at a fixed wavelength, the other tunable. Consider excitation at 488 nm and a 1,000 cm-1 vibration. The Raman band will appear at 513 nm. An argon-ion laser @488 nm, is intensity modulated at 10 kHz. A dye laser travels through the same solution volume as the argon-ion laser. The dye laser is scanned from 500 nm to 530 nm. When the dye laser output reaches 513 nm, energy will be transferred from the 488 nm beam to the 513 nm beam. Since the transferred light is modulated, it is detected as a 10 kHz signal added to the 513 nm beam. If the two lasers intersect at a sharp angle, only the volume irradiated by both beams will contribute to the signal. This volume can easily reach ___________! 18.2 : 7/8

488 nm 20,491 cm -1

1000 cm-1

513 nm 19,491 cm -1

Trace Analysis Ordinary Raman has a detection limit near millimolar. Resonance enhancement can increase a Raman signal by ______ ________. This yields detection limits from 10-5 to 10-9 M. The exact value depends upon the molar absorptivity and the amount the normal mode polarizability is affected by the optical transition. The downside of resonance Raman is simplification of the spectrum. Surface enhanced Raman spectroscopy (SERS) can enhance the signal by ___________. The increase is due to high electric fields produced by light interacting with nanoparticles. Unfortunately, only Cu, Ag, and Au surfaces give significant enhancement. Also, the particle shape dependence is not well understood. Resonance excitation has been combined with SERS to give enhancements as large as ________ (cross-section comparable to fluorescence). This has permitted obtaining a Raman spectrum from a single molecule. The technique has not yet become routine because the theory is not well understood. 18.2 : 8/8