Response of a 4-nitrothiophenol monolayer to rapid heating studied by vibrational sum frequency spectroscopy

A monolayer of 4-nitrothiophenol adsorbed on an Au substrate was heated by illuminating the substrate with a 19 ps laser pulse of 532 nm wavelength. Within 91 ps, the temperature of the sample increased from room temperature by 113 K. Vibrational sum frequency spectroscopy was used to characterize the adsorption geometry of the molecules in the ordered domains in the monolayer film. Upon heating, the initially ordered monolayer largely lost its structure. While the molecules are initially tilted by about 50° with respect to the surface normal, the analysis indicates that the mean tilt angle increased to 80° with a spread for individual molecules of up to a tilt angle of 40° upon heating. The evolution of this loss of order lagged about 100 ps behind the temperature rise of the substrate.