Erlangen 2018 – scientific programme

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MO: Fachverband Molekülphysik

MO 21: Advanced Time-Resolved Spectroscopy

MO 21.6: Talk

Thursday, March 8, 2018, 15:15–15:30, PA 2.150

Ultrafast Vibrational Relaxation of Water at the Ice - Air Interface — •Prerna Sudera, Jenée D Cyran, Alejandra Maria Sánchez, Mischa Bonn, and Ellen H. G. Backus — Max Planck Institute for Polymer Research, Mainz, Germany

For chemical reactions occurring on the ice surface - relevant for e.g. atmospheric reactions - the dynamics of the surface water molecules and energy flow pathways play an important role: following a chemical reaction, the rate of dissipation of excess energy determines the probability of a back-reaction occurring. Here we elucidate the relaxation dynamics of water molecules at the surface of single crystalline hexagonal ice after exciting the O-H stretch vibration and compare our results with the relaxation dynamics of the liquid water-air interface.

To study the vibrational dynamics we apply time-resolved sum frequency generation spectroscopy. With this method, we can vibrationally excite the surface molecules with a femtosecond infrared excitation pulse, and probe the dynamics of the interfacial molecules with the SFG process using a visible and an infrared pulse resonant with the O-H stretch vibrations. This detection scheme ensures that only the interfacial water molecules contribute to the signal; the use of ~50 fs laser pulses ensures high time resolution in the experiments.

Our results show that the relaxation dynamics of the O-H stretch mode after excitation at 3100 cm-1 at the basal plane of ice is around 70 fs - three times faster than for the water-air interface. We discuss the origins and implications of ultrafast energy dissipation at ice interfaces, compared to the liquid.