Motional effect in surface sum-frequency vibrational spectroscopy

Fast orientational motion of molecules at a surface can affect the sum-freq uency vibrational spectra of the surface. Calculation shows that the effect is significant if the molecular orientation varies over a broad range with in the vibrational relaxation time. The stretch vibration of the free OH bo nds at the vapor/water interface ib used to illustrate the importance of th e effect.