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 ν_sNO₂ and ν_asNO₂ 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 ν_sNO₂ 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 ν_sNO₂ and ν_asNO₂ 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 ν_sNO₂ shift and width indicated that this vibration was the ∼480 cm^-1 nitro bend. The ν_sNO₂ temperature dependence was interpreted using a vibrational energy exchange mechanism between the nitro stretch and bend.