Ultrafast Nonlinear Coherent Vibrational Sum-Frequency Spectroscopy Methods To Study Thermal Conductance of Molecules at Interfaces (2009)

It is difficult to study molecules at surfaces or interfaces because the total number of molecules is small, and this is especially problematic in studies of interfacial molecular dynamics with high time resolution. Vibrational sum-frequency generation (SFG) spectroscopy, where infrared (IR) and visible pulses are combined at an interface, has emerged as a powerful method to probe interfacial molecular dynamics. The nonlinear coherent nature of SFG helps overcome the sensitivity issues, especially when femtosecond IR pulses are used. With femtosecond pulses, a range of vibrational transitions can be probed simultaneously and high time resolution can be achieved. Ultrafast SFG experiments use three pulses, a pump pulse to generate nonequilibrium conditions with a pair of probe pulses, and two time delay parameters. Mapping SFG intensity as a function of the two time delays creates a two-dimensional surface, where one axis (t(1)) provides information about molecular dynamics driven by the pump pulses, and the other axis (t(2)) about the dynamics of the SFG probing process. We present examples of ultrafast SFG measurements drawn from our studies of heat transport through interfacial molecules that are models for molecular wires in electronic circuits. In these flash-heating experiments, a self-assembled monolayer (SAM) of long-chain molecules adsorbed on a metal surface is subjected to a large amplitude (up to 800 K) temperature jump. Specific vibrational reporter groups on the SAM molecules probed by SFG serve as tiny ultrafast thermometers approximately 1.5 A thick with a approximately 1 ps response time. These SFG thermometers can monitor ultrafast heat transport through the SAM molecules. By varying the lengths of the molecular wires we can tell if the heat is propagating ballistically along the chains, at constant speed, or diffusively. In our analysis of 2D SFG methods, we first describe a simpler situation where the visible probe pulse is effectively infinite in duration. This is the usual way time-resolved SFG measurements are made, and the SFG experiment then becomes a function of a single time delay, the pump-IR probe delay t(1). Unfortunately, in this case the SFG signals have a large contribution from the nonresonant (NR) background generated by the metal surface, which adds a great deal of noise to the data, and the time resolution is limited by the molecule's vibrational dephasing time constant T(2), which is often 1 ps or more. We have recently shown that the NR background can be suppressed using a time delay t(2) between IR and visible probe pulses. In this now 2D SFG method, one would expect that information about the molecular response to the pump pulses would be contained in slices along the t(1) axis, but by simulating the experiment we show that the t(1) and t(2) parameters interact. Changing t(2) to suppress the NR background causes t(1) slices to shift in time. We also show how to improve the time resolution of ultrafast SFG experiments while maintaining NR suppression using femtosecond visible pulses at appropriate t(2) delay values.