Preresonance absolute differential Raman cross sections have been measured between 217 and 600 nm for the symmetric ring mode vibrations of imidazole and imidazolium and for the 1645-cm−1 bending vibration of water. For imidazole and imidazolium the Raman intensities observed with visible wavelength excitation derive mainly from states in the far-UV. The two 190-210 nm π → π* transitions in these species dominate the Raman intensities only for excitation below 300 nm. Both π → π* transitions appear to contribute equally to preresonance enhancement. The data project that selective imidazole ring enhancement from histidine residues in proteins will require excitation below 210 nm. The intensities of the 1645-cm−1 bending vibration of water derive from states in the far-UV. The Raman cross section of this vibration increases only slightly faster than the scattered frequency to the fourth power. This Raman band can now be used as an internal intensity standard for aqueous Raman studies. Preresonance Raman enhancement dominated by transitions which occur at extraordinarily high energies have now been observed for water, acetonitrile, acetone, sulfate, and perchlorate. This behavior may result because molecular valence transitions in the vacuum UV spectral region of condensed phase samples are strongly mixed with ionizing, charge transfer, and Rydberg transitions and have no separate and discrete existence.
Sanford A. Asher and James L. Murtaugh, "UV Raman Excitation Profiles of Imidazole, Imidazolium, and Water," Appl. Spectrosc. 42, 83-90 (1988)