************ The 3rd Lab Seminar ***************************************

Title : IR and Raman spectra of liquid water: Theory and interpretation
B. M. Auer and J. L. Skinner,
Journal of Chemical Physics 128, 224511 (2008).

Speaker : Yukichi KITAMURA

Date : Fri. June. 28th, 1:30 pm

Abstract :
IR and Raman (parallel- and perpendicular-polarized) spectra in the OH 
stretch region for liquid water were measured some years ago, but their 
interpretation is still controversial. In part, this is because theoreti-
cal calculation of such spectra for a neat liquid presents a formidable 
challenge due to the coupling between vibrational chromophores and the 
effects of motional narrowing. Recently we proposed an electronic 
structure/molecular dynamics method for calculating spectra of dilute 
HOD in liquid D2O, which relied on ab initio calculations on clusters 
to provide a map from nuclear coordinates of the molecules in the liquid 
to OH stretch frequencies, transition dipoles, and polarizabilities. 
Here, we extend this approach to the calculation of couplings between 
chromophores. From the trajectories of the fluctuating local-mode 
frequencies, transition moments, and coupling, we use our recently 
developed time-averaging approximation to calculate the line shapes. Our 
results are in good agreement with experiment for the IR and Raman line 
shapes, and capture the significant differences among them. Our analysis 
shows that while the coupling between chromophores is relatively modest, 
it nevertheless produces delocalization of the vibrational eignestates 
over up to 12 chromophores, which has a profound effect on the spectro-
scopy. In particular, our results demonstrate that the peak in the 
parallel-polarized Raman spectrum at about 3250 wavenumbers is collective 
in nature.


************************************************************************