Integration of Conventional Surface Science Techniques with Surface-Sensitive Azimuthal and Polarization Dependent Femtosecond-Resolved Sum Frequency Generation Spectroscopy

Experimental insight into the elementary processes underlying charge transfer across interfaces has blossomed with the wide-spread availability of ultra-high vacuum set-ups that allow the preparation and characterization of solid surfaces with well-defined molecular adsorbates over a wide ranges of temperatures. Thick layers of molecular adsorbates or heterostructures of 2D materials generally preclude the use of electrons or atoms as probes in such characterization. However with linear photon-in/photon-out techniques it is often challenging to assign the observed optical response to a particular portion of the interface. We and prior workers have demonstrated in work under ambient conditions that by full characterization of the symmetry of the second order nonlinear optical susceptibility, i.e. the $\chi^{(2)}$, in sum frequency generation (SFG) spectroscopy, this problem can be overcome. Here we describe an ultra-high vacuum system built to allow conventional UHV sample preparation and characterization, femtosecond and polarization resolved SFG spectroscopy, the azimuthal sample rotation necessary to fully describe $\chi^{(2)}$ symmetry and with sufficient stability to allow scanning SFG microscopy. We demonstrate these capabilities in proof-of-principle measurements on CO adsorbed on Pt(111) and of the clean Ag(111) surface. Because this set-up allows both full characterization of the nonlinear susceptibility and the temperature control and sample preparation/characterization of conventional UHV set-ups we expect it to be of great utility in investigation of both the basic physics and applications of solid, 2D material heterostructures.