Response of a ODT monolayer to rapid heating studied by vibrational sum frequency spectroscopy

A self-assembled monolayer of octadecanethiol adsorbed on a thin film of Au was flash-heated by illuminating the structure, further consisting of a glass substrate and a Ti contact layer, with a 19 ps laser pulse of 532 nm light from the backside, inducing a temperature increase by 113 K within 100 ps. The details of the temporal evolution of the temperature can be well modeled when taking into account the predominant light absorption in the Ti layer and the large thermal boundary resistance between the layers. The evolution of the molecular structure is time-resolved by vibrational sum frequency spectroscopy. While the molecules are initially tilted by 30° with respect to the surface normal, this order is rapidly lost, whereby the disordering lags 150 ps behind the temperature evolution. After 400 ps, the molecular order is dissolved.