A theoretical analysis of the sum frequency generation spectrum of the water surface

The present paper provides a theoretical analysis of the sum frequency gene ration (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 an d 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 opposi te OH directions at the surface associated with these bands. The former ban d 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 aris e from quite different types of molecular orientations. The assignment of t he SFG spectrum is also analyzed in terms of the symmetric/antisymmetric ch aracter of the OH stretching modes and the degree of mode delocalization. ( C) 2000 Elsevier Science B.V. All rights reserved.