Solvation Dynamics at the Air/Water Interface with Time-Resolved Sum-Frequency Generation

The dynamics of molecular solvation at the air/water interface has been monitored with femtosecond time-resolved pump-sum frequency generation (TR-SFG), a technique that has been shown to be feasible in the study of ultrafast rotional motions. In the work reported here, the solvation process was monitored by femtosecond photoexcitation of interfacial coumarin 314 (C314) molecules. In these experiments, the SFG signal is brought into a vibrational resonance with the carbonyl symmetric stretch of C314 by tuning the IR pulse to the carbonyl frequency by using a pump-TR-SFG probe. Two solvation time constants were obtained, 230 ± 40 fs and 2.17 ± 0.3 ps. These results are the same within experimental error as those measured in time-resolved second-harmonic generation (TR-SHG) experiments. This suggests that the solvent response is due to solvation-induced shifts of the electronic-state energies in the SFG hyperpolarizability and not significantly to solvation effects on the energy of the carbonyl vibration nor to the strength of the carbonyl vibrational transition moment. In addition, an explanation of the similar solvation dynamics of a newly created ion at the air/water interface and in bulk water, which is based on molecular-dynamics simulations (Benjamin, I. J. Chem. Phys. 1991, 95 ( 5 ), 3698−3709), could explain the similar solvation dynamics we found for C314. The physical description is that the first solvation shell is essentially the same in bulk water and at the air/water interface.