A theoretical analysis of the sum frequency generation spectrum of the water surface Original Research Article

The present paper provides a theoretical analysis of the sum frequency generation (SFG) spectrum of the water surface in the OH stretch mode frequency region based on ab initio molecular orbital theory and molecular dynamics simulation. The environmental effects on the normal modes of OH stretching vibration, their frequency shifts and hyperpolarizability are formulated and tested. The simulated SFG spectrum reproduces experimental results quite well. The surface susceptibilities of the dangling bond and hydrogen-bonded bands have opposite signs in their imaginary parts, which indicates opposite OH directions at the surface associated with these bands. The former band turns out to be sensitive only to the top monolayer, and the latter band to a few top monolayers. Further analysis reveals that those two bands arise from quite different types of molecular orientations. The assignment of the SFG spectrum is also analyzed in terms of the symmetric/antisymmetric character of the OH stretching modes and the degree of mode delocalization.