Final Translational Energy Research Articles

High-Dimensional Atomistic Neural Network Potential to Study the Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite.

A high dimensional and accurate atomistic neural network potential energy surface (ANN-PES) that describes the interaction between one O2 molecule and a highly oriented pyrolytic graphite (HOPG) surface has been constructed using the open-source package (aenet). The validation of the PES is performed by paying attention to static characteristics as well as by testing its performance in reproducing previous ab initio molecular dynamics simulation results. Subsequently, the ANN-PES is used to perform quasi-classical molecular dynamics calculations of the alignment-dependent scattering of O2 from HOPG. The results are obtained for 200 meV O2 molecules with different initial alignments impinging with a polar incidence angle with respect to the surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons to recent experimental results on this system. Our results show that the scattering of O2 from the HOPG surface is a rather direct process, that the angular distributions are alignment dependent, and that the final translational energy of end-on molecules is around 20% lower than that of side-on molecules. Upon collision with the surface, the molecules that are initially aligned perpendicular to the surface become highly rotationally excited, whereas a very small change in the rotational state of the scattered molecules is observed for the initial parallel alignments. The latter confirms the energy transfer dependence on the stereodynamics for the present system. The results of our simulations are in overall agreement with the experimental observations regarding the shape of the angular distributions and the alignment dependence of the in-plane reflected molecules.

Dynamics Studies of O2 Collision on Pt(111) Using a Global Potential Energy Surface

The dynamics of O2 collision on Pt(111) has been studied by molecular dynamics simulations using a global potential energy surface (PES), which was developed by fitting ∼4800 plane-wave density functional theory points using the permutation invariant polynomial-neural network method. The simulations were performed for the normal and off-normal incidences at a wide range of incident energies and surface temperatures. A generalized Langevin oscillator model was used to incorporate the molecule–surface coupling that plays an important role in this collision process. The trapping probability, final energy distribution, scattering angle distributions of scattered molecules, and the energy transfer to surface distribution were determined and analyzed. The PES shows an activated path to the chemisorption states, as the trapping probability directly increases with incident energy (larger than 0.1 eV) to reach a maximum and then decreases at high incident energy. The trapping probability obviously reduces by raising surface temperature at low incident energy because the initial kinetic energy of molecule is more likely to be dissipated into surface motions than molecular internal ro-vibrational modes. Additional parallel momentum of off-normal incidence leads to a strong suppression of the trapping probability, caused by the energetic corrugation of the lateral barrier heights. The scattering angular distributions are found to be quasispecular and the final translational energy slightly increases with scattering angle deviating significantly from the hard-cube model, indicating the nonconservation of parallel momentum during the collision on the corrugate PES.

Dynamics studies of diglycine scattering from highly oriented pyrolytic graphite

The dynamics of neutral diglycine collision with highly oriented pyrolytic graphite (HOPG) were studied by molecular dynamics simulations using a reactive force field. The simulations were performed at an initial incident energy of 481.5 kJ/mol for four different initial incident polar angles of 0°, 20°, 45°, and 70°, and a surface temperature of 677 K. The angular, translational and internal energy, and residence time distributions of the scattered products were determined and analyzed. As a polyatomic molecule, diglycine has several low frequency vibrational modes and shows a rather strong attraction to HOPG, which leads to a long residence time on the surface and facile energy loss, particularly along the normal surface. Since there is significant normal momentum lost while parallel momentum is partially conserved, the scattering angular distribution is found to be generally superspecular and the final translational energies are much lower than the values predicted by the so-called hard-cube model. This study extends our knowledge of collisional energy transfer during collisions of polypeptide molecules with HOPG, which is expected to help the design of a neutral-gas concentrator for the fly-by collection of such molecules in rarefied atmospheres.

Ab Initio Molecular Dynamics Study of Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite

We present ab initio molecular dynamics simulations based on density functional theory to study the alignment dependent scattering of O2 from highly oriented pyrolytic graphite (HOPG). Results are obtained for 200 meV O2 molecules with different alignments impinging with an angle of incidence measured from surface normal of 22.5○ on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made in order to perform comparisons with recent experimental results on this system. Our results show that a lower number of molecules initially aligned normal to the surface is reflected in the scattering plane than of molecules initially aligned parallel to the surface. This is not due to an increase of the trapping of end-on aligned molecules, but due to the fact that a large amount of them are scattered out of plane. Additionally, we find that the final translational energy of end-on molecules is around 10 % lower than that of side-on molecules. We show that this i...

Chemical Dynamics Simulations and Scattering Experiments for O2 Collisions with Graphite

Energy transfer in collisions of O2 with a graphite surface was studied by chemical dynamics simulations. The simulations were performed for three collision energies Ei of 2.1, 7.4, and 15 kcal/mol, with the initial incident angle fixed at θi = 45°. Simulations were performed for each Ei at a surface temperature Tsurf = 300 K. For the higher surface temperature of 1177 K, a simulation was only performed for Ei = 15 kcal/mol. The following properties were determined and analyzed for the O2 + graphite collisions: (1) translational energy distributions of the scattered O2; (2) distribution of the final polar and azimuthal angle for the scattered O2; and (3) number of bounces of O2 on the surface before scattering. The average energy transferred to the graphite surface and that remaining in O2 translation, i.e., ⟨ΔEsurf⟩ and ⟨Ef⟩, exhibit a linear dependence with the initial translational energy. For the O2 + graphite scattering, the physisorption/desorption residence time distribution decays exponentially, with an increase in residence time with a decrease in Ei. The rate at which the distribution decreases shows a near-linear dependence with an increase in Ei. For higher collisional energies of 7.4 and 15 kcal/mol, O2 scattering from the surface follows a nearly quasi-trapping desorption process. However, for the lowest collision energy, it mostly follows conventional physisorption/desorption. For all of the scattering conditions considered experimentally, the relationship between the average final translational energy and average scattering angle for the O2 molecules found from the simulations is in excellent agreement with the experimental results. This experimental validation of precise simulation outcomes is important as it indicates that collisional energy-transfer predictions for this system can be reliably used in assessing interfacial energy flow in a variety of technological applications, including high-performance flight systems.

Scattering Dynamics of Oxygen Atoms on Imidazolium Tetrafluoroborate Ionic Liquid Surfaces: Dependence on Alkyl Chain Length

The dynamics of oxygen-atom scattering from the surfaces of a series of room-temperature ionic liquids containing the 1-alkyl-3-methylimidazolium cation [Cnmim] and the tetrafluoroborate anion [BF4] were studied by reactive-atom scattering with mass spectrometric detection of products (RAS-MS). The length of the alkyl chain was varied (n = 4, 8, 12) in order to investigate the relationship between the scattering dynamics and the density of alkyl chains on the surface and their ordering. RAS-MS uses a beam-surface scattering technique with a hyperthermal O atom beam source and a rotatable mass spectrometer detector. Time-of-flight and angular distributions were collected for inelastically scattered O and reactively scattered OH and H2O, enabling product flux distributions to be obtained as a function of final translational energy and scattering angle, P(ET,θf). A new analysis technique was used to separate these distributions into three distinct components, corresponding to three dynamical pathways for sca...

Incidence energy dependent state-to-state time-of-flight measurements of NO(v = 3) collisions with Au(111): the fate of incidence vibrational and translational energy

We report measurements of translational energy distributions when scattering NO(vi = 3, Ji = 1.5) from a Au(111) surface into vibrational states vf = 1, 2, 3 and rotational states up to Jf = 32.5 for various incidence energies ranging from 0.11 eV to 0.98 eV. We observed that the vibration-to-translation as well as the translation-to-rotation coupling depend on translational incidence energy, EI. The vibration-to-translation coupling, i.e. the additional recoil energy observed for vibrationally inelastic (v = 3 → 2, 1) scattering, is seen to increase with increasing EI. The final translational energy decreases approximately linearly with increasing rotational excitation. At incidence energies EI > 0.5 eV, the slopes of these dependencies are constant and identical for the three vibrational channels. At lower incidence energies, the slopes gradually approach zero for the vibrationally elastic channel while they exhibit more abrupt transitions for the vibrationally inelastic channels. We discuss possible mechanisms for both effects within the context of nonadiabatic electron-hole pair mediated energy transfer and orientation effects.

Quasiclassical trajectory studies of 18O(3P) + NO2 isotope exchange and reaction to O2 + NO on D and D1 potentials

We report quasiclassical trajectory calculations for the bimolecular reaction (18)O((3)P) + NO2 on the recent potential energy surfaces of the ground (D0) and first excited (D1) states of NO3 [B. Fu, J. M. Bowman, H. Xiao, S. Maeda, and K. Morokuma, J. Chem. Theory. Comput. 9, 893 (2013)]. The branching ratio of isotope exchange versus O2 + NO formation, as well as the product angular distributions and energy and rovibrational state distributions are presented. The calculations are done at the collision energy of relevance to recent crossed beam experiments [K. A. Mar, A. L. Van Wyngarden, C.-W. Liang, Y. T. Lee, J. J. Lin, and K. A. Boering, J. Chem. Phys. 137, 044302 (2012)]. Very good agreement is achieved between the current calculations and these experiments for the branching ratio and final translational energy and angular distributions of isotope exchange products (16)O((3)P) + NO2 and O2 + NO formation products. The reactant (18)O atom results in (18)O(16)O but not N(18)O for the O2 + NO formation product channel, consistent with the experiment. In addition, the detailed vibrational and rotational state information of diatomic molecules calculated currently for the (34)O2 + NO formation channel on D0 and D1 states are in qualitative agreement with the previous experimental and theoretical results of the photodissociation of NO3 and are consistent with older thermal bimolecular kinetics measurements.

Dynamics of collisions of hydroxyl radicals with fluorinated self-assembled monolayers

We present a classical trajectory study of the dynamics of collisions between OH radicals and fluorinated self-assembled monolayers (F-SAMs). The gas/surface interaction potential required in the simulations has been derived from high-level ab initio calculations (focal-point-CCSD(T)/aug-cc-pVQZ) of various approaches of OH to a model fluorinated alkane. The two lowest-energy doublet potential energy surfaces considered in the electronic structure calculations have been averaged to produce a pairwise analytic potential. This analytic potential has been subsequently employed to propagate classical trajectories of collisions between OH and F-SAMs at initial conditions relevant to recent experiments on related systems. The calculated rotational distributions of the inelastically scattered OH agree well with the experiment, which serves to validate the accuracy of the simulations. Investigation of the dynamics of energy transfer for different initial rotational states of OH indicates that an increase in the initial rotation of OH results in increases in both the final average OH rotational and translational energy and in a slight decrease in the amount of energy transferred to the surface. Analysis of the dynamics as a function of the desorption angle of OH from the surface shows that while there is a correlation between the final scattering angle and OH’s amount of final translational energy, the amount of rotational energy in OH is largely independent of the desorption angle. The mechanism of the collisions is found to be mostly direct; in about 90% of most trajectories, OH only collides with the surface once before desorbing, which exemplifies the rigidity of fluorinated monolayer surfaces and their inability to efficiently accommodate gas species.

Three-State Trajectory Surface Hopping Studies of the Photodissociation Dynamics of Formaldehyde on ab Initio Potential Energy Surfaces

Full-dimensional, three-state, surface hopping calculations of the photodissociation dynamics of formaldehyde are reported on ab initio potential energy surfaces (PESs) for electronic states S(1), T(1), and S(0). This is the first such study initiated on S(1) with ab initio-calculated spin-orbit couplings among the three states. We employ previous PESs for S(0) and T(1), and a new PES for S(1), which we describe here, as well as new spin-orbit couplings. The time-dependent electronic state populations and the branching ratio of radical products produced from S(0) and T(1) states and that of total radical products and molecular products at three total energies are calculated. Details of the surface hopping dynamics are described, and a novel pathway for isomerization on T(1) via S(0) is reported. Final translational energy distributions of H + HCO products from S(0) and T(1) are also reported as well as the translational energy distribution and final rovibrational distributions of H(2) products from the molecular channel. The present results are compared to previous trajectory calculations initiated from the global minimum of S(0). The roaming pathway leading to low rotational distribution of CO and high vibrational population of H(2) is observed in the present calculations.

Modeling Reactive Scattering of F(2P) at a Liquid Squalane Interface: A Hybrid QM/MM Molecular Dynamics Study

To better understand the reactivity of gases with liquid surfaces, experimentalists have recently probed the reactive scattering of atomic fluorine at the surface of liquid squalane (C(30)H(62)). In this paper we further this research by simulating this scattering process at collision energies of 0.5 and 1.0 eV using a hybrid QM/MM molecular dynamics scheme. To model the structure of the liquid surface, classical molecular dynamics calculations were performed utilizing the OPLS-AA force field. During the F + squalane molecular dynamics simulation, QM/MM calculations are performed at every trajectory step by combining the MSINDO semiempirical Hamiltonian with OPLS-AA and using a dynamic partitioning of the atoms in the QM or MM regions via a "seed atom" method. This computational model provides a type of "on-the-fly" direct dynamics applicable to larger scale chemical processes that include the making/breaking of chemical bonds not available in standard force field models. Our results show that H abstraction is the only reactive scattering pathway and that most trajectories result in reactive scattering. Reaction statistics at the squalane surface are discussed, including variation of the results with incident angle and collision energy, and the probability of reaction as a function of carbon atom type, collision depth, and residence time. Product states, including angular distributions and final translational and rovibrational energies, are also considered and found to be significantly affected by the exothermic reaction energy for H abstraction. The vibrational distributions are in good agreement with recent experiments, but the rotational distributions are dominated by a nonthermal component while the experiments, which involve thermal incident energies, show comparable thermal and nonthermal contributions. Results for O + squalane at 1.0 eV, which we also present, show analogous comparisons with experiment, with OH vibrational distributions which are cold and match experiment, while the calculated rotational distributions are dominated by nonthermal behavior.

Experimental and theoretical study of CO collisions with CH3- and CF3-terminated self-assembled monolayers

We present an experimental and theoretical study of the dynamics of collisions of the CO molecule with organic surfaces. Experimentally, we scatter CO at 60 kJ mol(-1) and 30 degrees incident angle from regular (CH(3)-terminated) and omega-fluorinated (CF(3)-terminated) alkanethiol self-assembled monolayers (SAMs) and measure the time-of-flight distributions at the specular angle after collision. At a theoretical level, we carry out classical-trajectory simulations of the same scattering process using CO/SAM potential-energy surfaces derived from ab initio calculations. Agreement between measured and calculated final translational energy distributions justifies use of the calculations to examine dynamical behavior of the gas/surface system not available directly from the experiment. Calculated state-to-state energy-transfer properties indicate that the collisions are notably vibrationally adiabatic. Similarly, translational energy transfer from and to CO rotation is relatively weak. These trends are examined as a function of collision energy and incident angle to provide a deeper understanding of the factors governing state-to-state energy transfer in gas/organic-surface collisions.

Theoretical Study of the Stereodynamics of CO Collisions with CH3- and CF3-Terminated Alkanethiolate Self-Assembled Monolayers

We present a classical-trajectory study of CO collisions with regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) with a focus on analyzing the stereodynamics properties of the collision. The CO molecule is scattered with incident angles of either 30 degrees or 60 degrees with respect to the surface normal and with 60 kJ x mol(-1) collision energy, and we analyze final translational and rotational energy, mechanism of the collisions, and orientation and alignment of the rotational angular momentum. Analysis of the alignment of the final rotational angular momentum in collisions involving initially rotationally cold CO indicates a slight preference for "cartwheel" and "corkscrew" rotational motions. In contrast, collisions of initially excited CO slightly favor "helicopter" motion of the recoiling molecule. Moreover, studies of final orientation reveal that, while cartwheel "topspin" motion is favored for collisions in which initially cold CO becomes rotationally excited, no preferred handedness is observed when CO leaves the surfaces with "helicopter" motion. Analysis of trajectories involving initially rotationally excited CO in which the initial rotational angular momentum is aligned and/or oriented shows a non-negligible effect of the initial rotational motion on the dynamics of energy transfer. For instance, CO approaching the SAMs with helicopter motion retains a larger fraction of its initial rotation than molecules colliding with cartwheel-type motions. Conservation of the alignment and orientation of the initial rotational angular momentum vector is also enhanced with helicopter motion relative to cartwheel or random motions. The calculated trends in the stereodynamic properties for the two SAMs indicate that the CH3-SAM is effectively more corrugated than the CF3-SAM.

Collisions of Polar and Nonpolar Gases with Hydrogen Bonding and Hydrocarbon Self-Assembled Monolayers

Molecular beam scattering experiments are used to explore collisions of 60 kJ/mol Ne, CD4, ND3, and D2O with long-chain CH3-, NH2-, and OH-terminated self-assembled monolayers (SAMs) created via the chemisorption of alkanethiols on gold. Time-of-flight measurements for the scattered gases reveal the extent of energy exchange and the propensity for a gas to thermally accommodate with the surface during a collision. Of the four gases studied, Ne transfers the least amount of translational energy into the monolayers and D2O the most. Neon atoms recoil from the OH-SAM with an average of 14.4 kJ/mol of energy, while D2O retains only 6.4 kJ/mol of its 60 kJ/mol incident energy when scattering from the same surface. Overall, the trend in final translational energies follows the order Ne > CD4 > ND3 > D2O for scattering from all three SAMs. The observed trend in the energy exchange is correlated with the gas−surface attractive forces, as determined by ab initio calculations. The thermal accommodation efficiencies of the four gases follow the opposite trend. Thermalization for the Ne atoms is nearly negligible for all three monolayers, whereas D2O and ND3 approach near complete accommodation on all of the monolayers studied. The overall energy exchange and thermal accommodation efficiencies also depend markedly on the terminal group of the SAM. For Ne scattering, the trend for the overall energy transfer follows: CH3- > NH2- ≈ OH-SAMs. In contrast, the overall D2O energy transfer is greater when colliding with the OH-SAM than the nonpolar CH3-SAM. Together, the results show that the extent of energy transfer depends on a balance between the rigidity of the surface, as affected by intrasurface hydrogen bonding, and the strength of the gas−surface attractive forces, as determined by intermolecular interactions.

Preparation of ultralow atomic velocities by transforming bound states into tunneling resonances

A procedure is proposed to prepare the average and width of the velocity distribution of ultracold atoms. The atoms are set initially in the ground state of an optical trap formed by an inner red-detuned-laser well and an outer blue-detuned-laser barrier. Then the well and barrier parameters are changed until the ground state becomes a Breit-Wigner tunneling resonance. An optimal time dependence of the switching process, between the sudden and adiabatic limits, adjusts the final translational energies of the leaking atoms to the Lorentzian distribution of the resonance state.

Chemical Dynamics Simulations of CO2Scattering off a Fluorinated Self-Assembled Monolayer Surface

Chemical dynamics simulations at collision energies of 3.0, 10.6, and 20.0 kcal/mol were performed to study energy transfer in perpendicular (θi = 0°) collisions of carbon dioxide with a perfluorinated octanethiol self-assembled monolayer (F-SAM) surface. An accurate carbon dioxide + F-SAM intermolecular potential was developed from ab initio calculations extrapolated to the complete basis set limit. Three types of collision events are observed in the trajectories: direct scattering, physisorption on the top of the surface, and penetration of the surface. Energy transfer to carbon dioxide rotation and the final translational energy of carbon dioxide are both analyzed for each of these trajectory types. Penetration leads to near complete thermal accommodation with the F-SAM, whereas accommodation is not reached for either the direct or physisorption trajectories. The distributions of the translational energy and rotational angular momentum, P(Ef) and P(J), of the scattered CO2 molecules and for the differ...

Dynamics of Energy Transfer in Collisions of O(3P) Atoms with a 1-Decanethiol Self-Assembled Monolayer Surface

Chemical dynamics simulations are reported of energy transfer in collisions of O(3P) atoms with a 300 K 1-decanethiol self-assembled monolayer (H-SAM) surface. The simulations are performed with a nonreactive potential energy surface, developed from PMP2/aug-cc-pVTZ calculations of the O(3P) + H-SAM intermolecular potential, and the simulation results represent the energy transfer dynamics in the absence of O(3P) reaction. Collisions energies E(i) of 0.12, 2.30, 11.2, 75.0, and 120.5 kcal/mol and incident angles theta(i) of 15, 30, 45, 60, and 75 degrees were considered in the study (theta(i) = 0 degrees is the surface normal). The translational energy distribution of the scattered O(3P) atoms, P(E(f)), may be deconvoluted into Boltzmann and non-Boltzmann components, with the former fraction identified as f(B). The trajectories are also analyzed in terms of three types; that is, direct scattering from and physisorption on the top of the H-SAM and penetration of the H-SAM. There are three energy regimes in the scattering dynamics. For the low E(i) values of 0.12 and 2.30 kcal/mol, physisorption is important and both f(B) and the average final translational energy of the scattered O(3P) atom, E(f), are nearly independent of the incident angle. The dynamics is much different for hyperthermal energies of 75.0 and 120.5 kcal/mol, where penetration of the surface is important. For hyperthermal collisions, the penetration probability decreases as theta(i) is increased, with a significant transition between theta(i) of 60 and 75 degrees . Hyperthermal penetration occurs upon initial surface impact and is more probable if the impinging O(3P) atom may move down a channel between the chains. For E(i) = 120.5 kcal/mol, 90% of the trajectories penetrate at theta(i) = 15 degrees , while only 3% penetrate at theta(i) = 75 degrees. For the former theta(i), the energy transfer to the surface is efficient with E(f) = 4.04 kcal/mol, but for the latter theta(i), E(f) = 85.3 kcal/mol! Particularly interesting penetrating trajectories are those in which O(3P) is trapped in the H-SAM for times exceeding 60 ps, linger near the Au substrate, and strike the Au substrate and scatter directly. For E(i) = 11.2 kcal/mol, there is a transition between the scattering dynamics for the low and hyperthermal collision energies. Additional detail in the energy transfer dynamics is obtained from the final polar and azimuthal angles, the residence time on/in the H-SAM, the minimum height with respect to the Au substrate, and the number of inner turning points in the O-atom's velocity. Calculated values of E(f) vs the final polar angle, theta(f), are in qualitative agreement with experiment. The O(3P) + H-SAM nonreactive energy transfer dynamics, for E(i) of 11.2 kcal/mol and lower, are very similar to previously reported Ne + H-SAM simulations.

Energy transfer in hyperthermal Xe-graphite surface scattering

The scattering of a hyperthermal Xe from a graphite (0001) surface has been studied using a molecular beam-surface scattering technique and molecular dynamics (MD) simulations. The angular and velocity distributions of scattered Xe atoms were measured at incidence energies from 0.45 to 3.5 eV, three incidence angles of 15°, 35° and 60° and the surface temperatures of 300 K and 550 K. The observed time-of-flight spectra exhibit a sharp velocity distribution with only one velocity component, which is ascribed to the direct inelastic scattering process. The angle-resolved energy ratios of the mean final translational energy over the mean incidence energy Ef/Ei agree well with those predicted based on the assumption of the conservation of the momentum parallel to the surface. The Hard-Cube model, where the mass of the cube is approximately 310 u, has reproduced the angle-resolved flux distributions of scattered Xe atoms. In the Hard-Cube model almost 80% of the normal component of the incidence translational energy is found to be lost in collision. The MD simulations reproduce well the experimental results by using the Brenner potential for intralayer C atoms and a Lennard-Jones potential for interlayer C–C pair interactions.

Effects of projectile orientation and surface impact site on the efficiency of projectile excitation in surface-induced dissociation: Protonated diglycine collisions with diamond {1 1 1}

Classical trajectory simulations are performed for collisions between protonated diglycine (gly 2-H +) and a diamond {1 1 1} surface at incident angles θ i of 0° and 45°, with respect to the surface normal, and initial translational energies E i of 35 and 70 eV. The trajectories are analyzed to determine how the orientation angle of the peptide ion and the surface impact site affect the collisional energy transfer. There are two distinct impact points on the surface on which the peptide ion can collide, hydrogen and carbon atoms, denoted as H- and C-sites. While the impact point plays little to no role in determining energy transfer, for θ i = 0° the orientation angle of the peptide has a significant effect on energy transfer. When the peptide ion collides with its backbone vertical to the surface plane and, thus, with a C- or N-terminus approach, the internal energy change is a maximum and the final translational energy and surface internal energy change are at a minimum. When the peptide ion collides horizontally, the opposite occurs. In addition, for vertical collisions more energy is transferred to the peptide ion if the C-terminus first strikes the surface instead of the N-terminus. For non-perpendicular collisions, with θ i = 45°, the energy transfer efficiency is less sensitive to the peptide orientation. Peptide orientation becomes more important as E i is increased.

Dynamics of HCl Collisions with Hydroxyl- and Methyl-Terminated Self-Assembled Monolayers

Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.