  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).
