Vibrational sum frequency generation spectroscopy of the air/water interface

Abstract

Vibrational sum frequency generation (SFG) spectroscopy is employed to investigate the water structure at an air/water interface. By obtaining the SFG intensity using different polarizations, it is reasonable to deconvolute the conventional SFG spectrum into five sub-bands, which are located at 3100, 3200, 3400, 3550 and 3700cm−1. In comparison with the Raman OH stretching bands of liquid water, we suggest that the five fitted sub-bands can be attributed to OH vibrations engaged in various local hydrogen-bonded networks, such as single donor–double acceptor (DAA), double donor–double acceptor (DDAA), single donor–single acceptor (DA), double donor–single acceptor (DDA), and free-OH vibrations, respectively. Owing to the truncations of hydrogen bonds at the air/water interface, obvious structural differences between interfacial water and bulk water can be expected. For ambient water, both DA and DDAA are the primary hydrogen-bonded networks. By contrast, DA hydrogen bonding can be regarded as the primary structural motif at the air/water interface. Additionally, we suggest that the loss of DDAA hydrogen bonding at the air/water interface provides the physical origin of surface tension, which can be applied to understand the formation of a water droplet.