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

The free OH moiety unique to the air/water interface is studied by interferometric two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The 2D spectrum immediately after vibrational excitation exhibits cross-peaks between the free OH main peak (∼3700 cm⁻¹) and its shoulder band (∼3650 cm⁻¹), 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.