Gigahertz modulation of femtosecond time-resolved surface sum-frequeny generation due to acoustic strain pulses

We study the properties of aqueous solution/air interfaces with time-resolved surface sum-frequency generation (SFG) spectroscopy. Using this technique, we monitor the change in SFG signal following the excitation of the water hydroxyl stretch vibrations with a strong ultrashort infrared pump pulse. We observe that, in addition to the excitation-induced changes in the signal resulting from vibrational excitation and relaxation on a time scale of picoseconds, the infrared excitation also induces a modulation of the SFG signal with a period of ∼180 ps. This modulation is caused by the interference between the SFG field directly radiated away from the air/liquid interface and the SFG field reflected or generated at the acoustic strain wavefront that is created by the infrared excitation pulse. The modulation is observed to be much stronger for aqueous salt solutions than for pure water, and the phase of the oscillations depends on the nature of the dissolved salt. This dependence of the phase can be explained from the differences in surface propensities of the ions of the different salts, giving rise to different net orientation of the interfacial water species.