Time-resolved vSFG of the water–air interface in an external field

In the present work we have studied the effect of an external electric field of strength ±0.01 V Å^-1 on the water molecules at the water-air interface using <i>ab initio</i> molecular dynamics. We calculated the vibrational sum-frequency generation spectra using surface-specific velocity-velocity correlation functions and used this to interpret the preferential orientation of interfacial water molecules in the presence of the field. Further, we use the time-averaged frequency distribution and frequency correlation functions to calculate the rate of vibrational correlation loss. The rate at which hydrogen bonds undergo breaking and reformation at the interface and in the presence of the field is explored using time-dependent vibrational sum-frequency generation spectroscopy. We find that OH modes show a faster rate of frequency correlation loss with a timescale of 4.7 ps in the presence of an external field, as compared to 5.7 ps under ambient conditions.