Temperature-Dependent Dynamic Response to Flash Heating of Molecular Monolayers on Metal Surfaces: Vibrational Energy Exchange

An ultrafast nonlinear coherent laser spectroscopy termed vibrational sum-frequency generation (SFG) was used to monitor vibrational transitions of a self-assembled monolayer (SAM) of 4-nitrobenzenethiolate (NBT) on Au after Au flash heating. Ultrafast thermoreflectance measurements showed the surface temperature jumps ΔT were in the 35–250 K range. The NBT symmetric and antisymmetric nitro stretches νsNO2 and νasNO2 and a phenyl ring stretch νCC were probed. Flash heating caused these transitions to lose intensity, shift, and broaden. The time dependences all had overshoot–decay–plateau structures. In the long-lived plateau, the SAM was in thermal equilibrium with the hot Au surface. The SFG plateau intensity losses of νsNO2 and νCC, two vibrations with parallel transition moments, were identical, indicating that the SFG intensity loss was caused by thermally induced SAM orientational disorder. The T-jump-induced frequency shifts of νsNO2 and νasNO2 were identical and opposite in sign. The rise times of the shifts were identical and equal to the ∼3.5 ps time constant for the rise of Au surface temperature, which indicates that both shifts were caused by anharmonic coupling to the same lower-energy vibration. The temperature dependence of the νsNO2 shift and width indicated that this vibration was the ∼480 cm–1 nitro bend. The νsNO2 temperature dependence was interpreted using a vibrational energy exchange mechanism between the nitro stretch and bend.