Ultrafast Excitation of Molecular Adsorbates on Flash-Heated Gold Surfaces

Abstract

An ultrafast flash-thermal conductance technique is used to study energy transfer from a flash-heated polycrystalline Au(111) surface to adsorbed thiolate self-assembled monolayers (SAMs). The focus is on understanding energy transfer processes to parts of SAM molecules situated within a few carbon atoms of the Au surface, by probing specific SAM functional groups with vibrational sum-frequency generation (SFG) spectroscopy. The SFG intensity drop after flash-heating for benzenethiol (BT) CH-stretch transitions shows a substantial overshoot lasting several tens of picoseconds before BT and Au equilibrate at a higher temperature estimated at 600 degrees C. The thermal redshift of BT CH-stretch transitions also shows an overshoot. Other aromatic molecules and aliphatic molecules such as cyclohexanethiol (CHT) and hexanethiol (C6) have an overshoot as well. A model is proposed where the overshoot is primarily the result of hot surface electrons existing only during the flash-heating pulses. The intensity overshoot is caused by electron excitation of the probed vibrations and the redshift overshoot is caused by electron excitation of lower-energy vibrations anharmonically coupled to the probed vibration. Although electron excitation causes a substantial perturbation, up to 50% in some cases, of the SFG signal, the total amount of energy deposited into SAMs by electrons is much smaller than the heat transferred by Au surface phonons. Studies of a variety of molecular structures including substituted benzenes, biphenyl and terphenyl, and benzene rings connected to the Au surface by alkane linkers show that the likelihood of electron excitation becomes small for distances of 4-5 carbon atoms above the surface.