Pre-resonance Raman excitation profile of the 3400 cm-1 mode of liquid water

The absolute differential Raman scattering cross-section of the 3400 cm(-1) symmetric vibrational mode of liquid water was measured for the excitation wavelength range 450-200 run. Available data on the Raman cross-section are critically reviewed. Although the values obtained by most authors at 488 run excitation are in good agreement, the values are widely different for a few other reported excitation wavelengths. Direct comparison of these results with those measured in this work is not strictly valid owing to the large differences in experimental procedures and methods of data analysis. The excitation wavelength dependence of the Raman cross-section (excitation profile) was fitted to a single-term Albrecht A-term pre-resonance approximation formula (AATPA). A best fit was obtained for the value of the resonant state, v(e) = 89 100 cm(-1) and the coupling constant, K = 39.2 X 10(-27) cm(2) (integrated). The data show consistent lower (than predicted) values for wavelengths down to about 220 nm and higher values below this. The imponderable errors in the implementation of corrections at very short wavelengths could well have given rise to a systematic error in this excitation wavelength region. Copyright (C) 2001 John Wiley & Sons, Ltd.