Molecular stochastic process on gold surface observed in broadband-infrared, background-suppressed, sum frequency generation spectroscopy: picosecond heat transfer to self-assembled monolayers

The picosecond thermal response of a normal octadecanethiol self-assembled monolayer on a gold surface was studied using pump-probe, broadband-infrared, background-suppressed and vibrational sum frequency generation spectroscopy. The orientation fluctuations in response to flash heating were characterized from the intensity kinetics of the terminal methyl C-H stretching vibrations. An intensity decrease (5.7 +/- 1.3 ps) in the asymmetric modes indicated that stochastic processes of molecular conformations developed during multiple light-matter interactions. Including torsional diffusion into the rotation term of second order electrosusceptibility explained the experimental surface temperature increment two order-of-magnitudes smaller than the activation energy of the dihedral angles (~15 K).