Liquid Filters for UV Resonance Raman Spectroscopy

Abstract

Solutions of organic compounds are proposed as viable high-pass, Rayleigh rejection filters for ultraviolet resonance Raman spectroscopy. The steep transmittance curves of these solutions effectively reject elastically scattered light in this region while passing Raman-shifted frequencies. The materials used in the filters are readily available and inexpensive, and the solutions are easily prepared. Filters for four lines in the range of 288 nm to 342 nm from a Raman-shifted 3rd and 4th harmonic of a Nd:YAG laser are presented, although the principle of preparing similar liquid filters can be applied to virtually any near-UV wavelength. The use of these filter solutions in conjunction with a single monochromator was found to significantly reduce levels of elastically scattered light without sacrifice of optical throughput; Raman scattering at frequency shifts within 200 cm−1 of the Rayleigh line could be observed, and the transmittance at shifts >1000 cm−1 was ≥80%. The Rayleighline rejection efficiencies for the filters in this study are modest (102–103) compared with those for filters employed in the visible region; but they can be easily boosted by increasing the chromophore concentration or filter pathlength with a trade-off of throughput for Raman scattering at small wavenumber shifts.

Dispersive Raman Spectrometers

“…Figure 8.20 shows absorption spectra for several organic solutions, which permit longpass operation between 290 and 350 nm. Figure 8.21 is a spectrum of CC14 obtained with a 299 nm laser, with and without an absorption filter (14). In this case, a Raman shift of 218 cm-' was observable, and transmission above 800 cm-' exceeded 30 per cent.…”

“…For example, a monochromator with a bandpass of 2 nm might be set for 642.8 nm, and a He-Ne laser (632.8 nm) is directed into the entrance slit. If benzene, single spectrograph, 5 14 Raman shift, em-' Raman spectrum of liquid benzene from a single spectrograph with and without a holographic laser rejection filter between the sample and the entrance slit. Intensity scales differ greatly between the two spectra.…”

“…Since light absorption is usually an inherent property of the filter material, one cannot choose from a wide range of rejection wavelengths as is the case for dielectric and holographic filters. Organic liquids and solutions have been reported as laser absorption filters for ultraviolet (UV) Raman in the 280 to 360 nm region (13,14). Figure 8.20 shows absorption spectra for several organic solutions, which permit longpass operation between 290 and 350 nm.…”

“…An often unappreciated advantage of a PMT is its large photosensitive area, about 2 x 8 mm. With a proper optical design, this large area can translate into a high An at the sample or an overall high A~f l (14). Although the smaller area of a CCD pixel is more than compensated by the multichannel advantage, this area difference does decrease the expected advantage based solely on an NL'2 prediction [as in Eq.…”

“…It would degrade the SNR if SA(, k >> S1I2 in Eq. (8)(9)(10)(11)(12)(13)(14)(15)(16), so one must reduce the noise in Sdark as much as possible. Ideally one would signal average Sdark for a very long time so that it does not contribute significantly to the overall noise.…”