Orientational relaxation is one of the most fundamental dynamical characteristics of molecules at surfaces and interfaces. We present experimental data and a theoretical model of how the 2nd order nonlinear (surface-selective) vibrational sum frequency generation (SFG) spectroscopy can be used to measure orientational relaxation of small molecules at the air/water interface. The response function formalism for the polarization-selected SFG spectroscopic line shapes demonstrates that the orientational dynamics can be extracted when it proceeds on the time scale comparable to the vibrational dephasing. Using this methodology, the reorientation time for a small probe molecule, propiolic acid, is measured and compared with rotations of similar molecules in bulk water. The rotational diffusion at the air/water interface is nearly an order of magnitude faster than in bulk water, suggesting a significant difference in the dynamic properties between surface vs. bulk water.

Ultrafast orientational motion at the air/water interface probed by sum frequency generation spectroscopy

244th National Meeting (2012)

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