Liquid Filters for UV Resonance Raman Spectroscopy

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.