Inelastic Scattering Dynamics of Ar from a Perfluorinated Self-Assembled Monolayer Surface ² Saulo A. Va ´ zquez, ‡ John R. Morris, £ Asif Rahaman, § Oleg A. Mazyar, § Grigoriy Vayner, § Srirangam V. Addepalli, # William L. Hase,* ,§ and Emilio Martı ´nez-Nu ´ n ˜ ez* ,‡ Departamento de Quı ´mica Fı ´sica, UniVersidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain, Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409, and High Performance Computing Center, Texas Tech UniVersity, Lubbock, Texas 79409 ReceiVed: August 10, 2007; In Final Form: October 9, 2007 Dynamics of Ar atom collisions with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) surface on gold were investigated by classical trajectory simulations and atomic beam scattering techniques. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface; in the UA model, the CF 3 and CF 2 units are represented as single pseudoatoms. Additionally the nonbonded interactions in both models are different. The simulations show the three limiting mechanisms expected for collisions of rare gas atoms (or small molecules) with SAMs, that is, direct scattering, physisorption, and penetration. Surface penetration results in a translational energy distribution, P(E f ), that can be approximately fit to the Boltzmann for thermal desorption, suggesting that surface accommodation is attained to a large extent. Fluorination of the alkanethiol monolayer leads to less energy transfer in Ar collisions. This results from a denser and stiffer surface structure in comparison with that of the alkanethiol SAM, which introduces constraints for conformational changes which play a significant role in the energy-transfer process. The trajectory simulations predict P(E f ) distributions in quite good agreement with those observed in the experiments. The results obtained with the EA and UA models are in reasonably good agreement, although there are some differences. I. Introduction Self-assembled monolayers (SAMs) of thiolates on metals are widely used in nanoscience and nanotechnology. 1 They are also very valuable materials for exploring the dynamics of collisions of gases with organic surfaces because their highly ordered and well-characterized structures simplify the elucida- tion of the microscopic mechanisms of energy transfer. The first gas-surface scattering study involving SAMs was reported by Cohen et al., 2 who measured the fraction of translational energy transferred in collisions of monatomic and diatomic gases with long-chain, amphiphilic monolayers. They found a correlation between the extent of energy transfer and the rigidity of the chains and the gas/surface mass ratio. In addition, they suggested that the internal rotation of the terminal methyl groups and the concerted waving motion of chains perpendicular to the carbon skeleton play a significant role in the energy transfer. Since the publication of the above study by Cohen et al., 2 numerous experimental and theoretical investigations have explored energy transfer in collisions of gas-phase species with SAM surfaces 3-25 and liquid surfaces such as perfluorinated polyether. 26-31 These investigations show that three limiting types of events, direct impulsive scattering, physisorption, and penetration, can take place upon collision. Direct impulsive scattering refers to the process in which the projectile rebounds directly from the surface after a single encounter. 32-35 Phys- isorption occurs when the gas species is adsorbed on the surface for a substantial period of time. Physisorption together with penetration into the monolayer have often been classified as trapping desorption (TD). 35-37 Because of the inherent difficulty to directly observe trapping desorption, the common experimental practice 2-6,32-38 is to equate the fraction of TD to the fraction of the translational energy distribution of the scattered species, P(E f ), that can be fit to a Maxwell-Boltzmann distribution for thermal desorp- tion, 39 that is where k B is Boltzmann’s constant, E f is the final translational energy of the scattered gas particle, and T s is the surface temperature. The remaining higher-energy component of the distribution is then assigned to inelastic scattering. However, there are uncertainties in this approach since classical trajectory simulations of Ne scattering off SAMs adsorbed on Au{1,1,1} have shown that a Boltzmann component in P(E f ) does not necessarily arise from a trapping desorption intermediate. 10-13 A substantial number of trajectories associated with this component come from single-bounce encounters. Moreover, the low-energy component of the P(E f ) distribution may be fit by an effective surface temperature that differs from the actual temperature of the surface. In many instances, this effective temperature is significantly higher than T s , which may be interpreted in terms of thermalization with a subset of surface ² Part of the “Giacinto Scoles Festschrift”. * To whom correspondence should be addressed. E-mail: bill.hase@ttu.edu (W.L.H.); qfemilio@usc.es (E.M.-N.). ‡ Universidad de Santiago de Compostela. £ Virginia Tech. § Department of Chemistry and Biochemistry, Texas Tech University. # High Performance Computing Center, Texas Tech University. P(E f ) ) (k B T s ) -2 E f exp(-E f /k B T s ) (1) 12785 J. Phys. Chem. A 2007, 111, 12785-12794 10.1021/jp076431m CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007