Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.