Chapter 34 - On ultrafast IR spectroscopy in water

Abstract

Modern ultrafast spectroscopic methods have opened a new experimental window on the molecular dynamics of water, examining the OH-stretch dynamics for the experimentally convenient aqueous system of HOD diluted in D2O using ultrafast infrared (IR), IR-Raman, or photon echo spectroscopy. One major focus of these experiments has been the excited OH vibration population lifetime. This is interpreted as being dominated by either vibrational pre-dissociation, that is, the breaking of the hydrogen bond of an excited OH stretch, or by vibrational relaxation of the excited OH without such IR-induced bond breaking. Recently the focus has been on the theoretical interpretation of spectral diffusion, i.e., the OH frequency’'s dynamical modulation. Experimental interpretations have considered that the vibrational frequency of an OH involved in a hydrogen bond is a measure of the length (OO distance) of that bond, and often involve the image of overdamped OO motion. Molecular Dynamics (MD) simulations were performed to test this picture, and to study the spectral diffusion in the OH stretch associated with a hole burned in the ground state. Some major results of those studies are recounted in this chapter.