Inhomogeneity and Vibrational Coupling of Free OH Stretch at the Air/Water Interface

), revealing that the shoulder band appears due to vibrational coupling with the free OH stretch within a single interfacial water molecule at the topmost water layer. The relative amplitude of the cross-peak increases with decreasing pump frequency, accompanied by the redshift of the bleach of the free OH main peak. This indicates that the free OH stretch band is inhomogeneously broadened and that the magnitude of the vibrational coupling significantly varies among the subensembles of the free OH.