Sum frequency generation spectroscopy

To study selectively interfacial molecules, we use the nonlinear method sum frequency generation (SFG) spectroscopy. In this method no signal is generated from centrosymmetric media. At interfaces the bulk symmetry is broken, allowing probing the interface selectively. In SFG spectroscopy a visible and an infrared laser beam are overlapped in space and time at the surface of interest. If the infrared light is in resonance with a molecular vibration the signal is strongly enhanced. In this way the vibrational spectrum of interfacial molecules is obtained. The spectrum delivers not only information about which molecules are present, but also about their environment and orientation. In the recent years, the technique developed into phase, time, and multidimensional spectroscopy. With the phase resolved SFG method the orientation of molecules can be unraveled. Time resolved SFG delivers information about vibrational energy relaxation, rotational dynamics, and reaction pathways.

Transient absorption and 2D-IR spectroscopy

To study bulk dynamics, we are using transient absorption spectroscopy both in the visible and in the infrared region. Excitation pulses from the UV region to the IR region allow exciting electronic as well as vibrational transitions. The response of the system can either be monitored by probing electronic transitions with a white light probe or vibrational transitions with infrared light. By changing the infrared pump frequency two dimensional IR spectra could be obtained.