Role of surface intramolecular dynamics in the efficiency of energy transfer in ne atom collisions with a n-hexylthiolate self-assembled monolayer

The energy-transfer dynamics associated with Ne atom collisions with a n-hexylthiolate self-assembled monolayer (SAM) surface at 20 kcal/mol collision energy are studied in chemical dynamics simulations by using both harmonic/separable and anharmonic/coupled surface models and by considering a SAM in both its classical potential energy minimum and containing a 293 K energy distribution. For the anharmonic surface, excitation of higher-energy potential energy minima arising from different intermolecular conformations of the alkyl chains and intramolecular vibrational energy redistribution (IVR) between the surface modes during the collision enhance the energy transfer from Ne atom translation to the surface. IVR is efficient for the alkyl chain's torsional modes and intramolecular chain-bending modes and occurs on the time scale of the Ne atom collisions. It does not occur during the collisions for other modes, such as the higher-frequency intrachain CCC bends. This IVR between surface modes, during the collision, increases the number of modes coupled to the Ne atom's translational motion and enhances energy transfer to the surface. Whether modes promoting or not promoting IVR are initially excited depends on the surface site at which the Ne atom collides. The simulation indicates that the presence of these different energy-transfer dynamics for different surface sites is the origin of the bimodal energy-transfer distribution function P(E-f). It is suggested that a large number of surface modes and chains coupled by IVR, for collisions at some surface sites, may create a sufficiently large bath to form a Boltzmann-like component in P(E-f).