H 2 Raman‐shifted YAG laser ultraviolet Raman spectrometer operating at wavelengths down to 184 nm

Apparatus is described for the acquisition of Raman spectra with excitation at wavelengths as short as 184 nm. The H 2 Raman‐shifted output of an Nd:YAG laser is directed on to a free‐flowing sample, and the Raman photons are collected and dispersed by a scanning 1.26 m single monochromator equipped with a 2400 groove holographic grating and a solar blind phototube with integrating electronics, or by a 0.5 m spectrograph with an 1800 groove holographic grating and a reticon multi‐channel detector. Data for air‐sensitive samples are obtained by enclosing the sample jet in an inert gas purged shroud equipped with a UV‐transparent optical window. Raman spectra are reported for phenylalanine at a series of excitation wavelengths down to 184 nm. They show a sharp transition between 200 and 192 nm from vibronic to Franck–Condon scattering. At 200 nm, the intensities of the vibronically induced ν 8a and ν 8b modes, at 1606 and 1586 cm −1 , are nearly the same as that of the 1000 cm −1 ν 1 ring breathing mode, whereas at 192 nm the relative intensities of the former bands are dramatically dimished. In contrast, the ν 8a and ν 8b modes of tyrosine continue to show enhancement comparable to the ring breathing modes at 192 nm, the maximum of the strong 1 B α, b tyrosine absorption. The spectrum of insulin at 192 nm shows comparable contributions from tyrosine, phenylalanine and amide vibrations, whereas at 200 and 218 nm excitation the aromatic contributions are dominant.