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

TL;DR

This study investigates the ultrafast thermal response and molecular conformational dynamics of a self-assembled monolayer on gold using advanced broadband infrared spectroscopy, revealing stochastic processes at picosecond timescales.

Contribution

It introduces a novel spectroscopic approach to observe molecular stochastic processes and heat transfer in self-assembled monolayers on metal surfaces.

Findings

Molecular conformations fluctuate within 5.7 ps after heating.

Surface temperature increase is much smaller than expected from activation energy.

Torsional diffusion significantly influences the molecular response.

Abstract

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).