Dynamics of Energy Transfer in Collisions of O (3P) Atoms with a 1

J. Phys. Chem. B 2006, 110, 11863-11877

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Dynamics of Energy Transfer in Collisions of O(3P) Atoms with a 1-Decanethiol Self-Assembled Monolayer Surface Urosˇ S. Tasic´ ,† Tianying Yan,‡ and William L. Hase*,† Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409-1061, and Institute of New Energy Material Chemistry and Institute of Scientific Computing, Nankai UniVersity, China ReceiVed: February 21, 2006; In Final Form: April 4, 2006

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 Ei of 0.12, 2.30, 11.2, 75.0, and 120.5 kcal/mol and incident angles θi of 15, 30, 45, 60, and 75° were considered in the study (θi ) 0° is the surface normal). The translational energy distribution of the scattered O(3P) atoms, P(Ef), may be deconvoluted into Boltzmann and non-Boltzmann components, with the former fraction identified as fB. 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 Ei values of 0.12 and 2.30 kcal/mol, physisorption is important and both fB and the average final translational energy of the scattered O(3P) atom, 〈Ef〉, 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 θi is increased, with a significant transition between θi of 60 and 75°. 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 Ei ) 120.5 kcal/mol, 90% of the trajectories penetrate at θi ) 15°, while only 3% penetrate at θi ) 75°. For the former θi, the energy transfer to the surface is efficient with 〈Ef〉 ) 4.04 kcal/mol, but for the latter θi, 〈Ef〉 ) 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 Ei ) 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 〈Ef〉 vs the final polar angle, θf, are in qualitative agreement with experiment. The O(3P) + H-SAM nonreactive energy transfer dynamics, for Ei of 11.2 kcal/mol and lower, are very similar to previously reported Ne + H-SAM simulations.

I. Introduction Interest in collisions of O(3P) atoms with hydrocarbons has been motivated by the erosion of polymeric coatings on surfaces of spacecrafts in low Earth orbit (LEO)1. Ground-state oxygen atoms are the most abundant species available in LEO2 and are suspected to be a major culprit in degradation of spacecraft coatings, either directly or through synergistic interactions with VUV/UV radiation and/or other reactive species in the LEO environment.3 A spacecraft in LEO travels at a velocity of about 8 km/s, giving rise to a relative translational energy of ∼5 eV for O(3P) striking it.4 Under such harsh oxidizing conditions, there is considerable erosion of the spacecraft’s surface. A number of experiments have probed the dynamics and kinetics of O(3P) atoms with hydrocarbon surfaces.5-10 Particularly relevant are those by Minton and co-workers, who pioneered development of a molecular beam technique for * To whom correspondence should be addressed. E-mail: [email protected]. † Texas Tech University. ‡ Nankai University.

studying the reactions of O(3P) with hydrocarbons at high collision energies relevant to LEO.5-7 In their experiments, the energy distributions and scattering angles of the unreactive colliding O-atoms are measured, as well as those of the OH and H2O reaction products. Reactions between O(3P) and hydrocarbon molecules have been used as model systems for complex chemistry associated with hyperthermal O(3P) and polymeric hydrocarbon surfaces.11,12 At the hyperthermal energies of LEO, electronic structure calculations have predicted O(3P) + alkane C-C and C-H bond rupture pathways, in addition to the H-atom abstraction pathway accessible at low energies.12-19 These bond rupture pathways have been observed in hyperthermal crossed-beams experiments of O(3P) reactions with CH4, C2H10, and C3H8.11,12 The experiments also found that triplet-singlet intersystem crossing is negligible at hyperthermal collision energies. 12 Theoretical and computational studies have led to a greater understanding of the dynamics of O(3P) + alkane molecule reactions at hyperthermal energies.11-17 Electronic structure calculations have provided structures, vibrational frequencies,

10.1021/jp0611065 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/02/2006

11864 J. Phys. Chem. B, Vol. 110, No. 24, 2006

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and energies for the C-C and C-H bond rupture pathways,12-17 that is, the pathways forming CH3OH + CH3 and C2H5O + H from O(3P) + C2H6, and for additional secondary and unimolecular reactions which may occur when O(3P) atoms react with alkane molecules.17 Direct dynamics classical trajectory simulations have been performed to determine reaction cross-sections, product scattering angles, and product energy partitioning distributions for O(3P) + CH4 and O(3P) + C2H6.11-15,17 The electronic structure theoretical methods used in these simulations include MSINDO, PM3, DFT (B3LYP/6-31G*), and the MSIDO, PM3 semiempirical methods with specific reaction parameters (SRPs), that is, PM3-SRP and MSINDO-SRP. Overall, good agreement has been found between the experimental and simulation results for O(3P) with both CH4 and C2H6. Direct dynamics simulation methods, used to study O(3P) + alkane reactions, have been extended to simulate collisions of O(3P) with alkanethiolate self-assembled monolayer (SAM) surfaces, that is, H-SAMs.18,19 These simulations have employed quantum mechanics/molecular mechanics (QM/MM) potential models, with the QM potential represented by either PM3-SRP or MSINDO. At low collision energies, the observed products are OH and H2O.18 At hyperthermal collision energies, both the C-H and C-C bond rupture pathways are also observed.19 Although the results are not expected to be quantitative, they have provided important fundamental details concerning the dynamics of O(3P) reactions with alkane surfaces. Considerable additional work needs to be done to develop a QM semiempirical model which accurately represents the many possible primary, secondary, and unimolecular pathways for O(3P) + alkane reactions at hyperthermal conditions.17 One of the limitations of the above simulations of O(3P) collisions with the H-SAM surfaces is that the MSINDO and PM3-SRP models have only approximate O(3P) + H-SAM intermolecular potentials. Thus, these simulations may not give an accurate representation of the scattering dynamics for the unreactive O(3P) atoms. This is a significant shortcoming, since the angular and velocity distributions have been measured for O(3P) atoms that scatter inelastically off alkane surfaces.5-7 Nonreactive, inelastic collisions have been identified as the dominant events for both low and high collision energies.7 In the work reported here, an analytic function is used for the O(3P) + H-SAM potential energy surface, which incorporates an accurate intermolecular potential for nonreactive interactions17 and does not include a potential for the reactive channels. Thus, this model potential will give the correct inelastic scattering dynamics in the absence of chemical reactions with the surface. This will be an important result. The effect of chemical reaction on the inelastic scattering may be obtained when an accurate semiempirical model is developed for the complex chemistry associated with these systems at hyperthermal conditions. The O(3P) + H-SAM inelastic scattering dynamics in the absence of chemical reaction may also be compared with computational and experimental studies of rare gas + H-SAM inelastic scattering.20-32 II. Computational Procedure A. H-SAM/Au{111} Potential Energy Function. The HSAM modeled in the simulations is 1-decanethiol on Au{111}. The potential energy function for O(3P) + H-SAM inelastic collisions is written as

V ) Vsurf + VO,surf

(1)

where Vsurf is the potential for the 1-decanethiol SAM on Au-

{111} and VO,surface is the intermolecular potential between O(3P) and the H-SAM. An explicit-atom model, described previously,33 was used for the SAM potential. It was developed by Mar and Klein,34 incorporating previous work by Williams35 and Jorgensen,36 to represent important properties of the SAM’s experimental structure.37,38 The specific model used in the current simulations includes periodic boundary conditions for the two dimensions parallel to the Au plane. The primary cell consists of 36 1-decanethiol chains uniformly spaced on a oneatomic thick plane of 120 Au atoms arranged in a rhombus shape, representing the Au{111} surface. Each CH3(CH2)9S chain is absorbed on the Au{111} surface by bonding the sulfur atom to a 3-fold hollow site on the surface. For the 0 K, optimized surface of the H-SAM, each alkanethiol chain has an all-trans (-C-)10 backbone, that is, all C-atoms are coplanar. The chain backbone plane forms a ψ ) 65° rotation angle with respect to the Au plane. Each chain backbone is straight and forms a θ ) 34.1° tilt angle with respect to the surface normal, in accord with experiment.38 In the energy minimized structure, any two S atoms at the roots of adjacent chains are separated by a distance, D ) 4.99 Å, and any two C10 atoms at the tops of adjacent chains are also separated by distance D because the chains are mutually parallel. The empty space between each “rectangular group” of four neighboring chains forms a “channel” that may accept a small atom, such as O(3P), resulting in surface penetration. The packing of the alkanethiolate chains on the Au{111} base constitutes a (x3 × x3)R30° molecular level lattice structure, which is the basic periodicity of alkyl thiosurfaces found from experiments.38 The c(4x3 × 2x3)R30° superlattice, revealed from diffraction37,39 and STM experiments,40,41 is based on the (x3 × x3)R30° structure. The model for the Au{111} surface consists of one atomic layer of 120 rigid Au-atoms. Previous work23 has shown that this model, a nonrigid model with standard Au-Au forces and the gold layer coupled to a 293 K thermal bath, and a twolayer nonrigid model not coupled to a thermal bath all give the same energy transfer dynamics for Ne + H-SAM collisions. The Vsurf potential has bonding terms for each chain and terms for nonbonded interactions in the same and in different chains. The former consists of stretches, bends, and torsions. It is a molecular mechanics (MM) potential,42 and its analytic form has been given previously.33 The nonbonded potential is comprised of Lennard-Jones 6-12 and Buckingham terms. Harmonic stretch terms are used to bond each S-atom to the three Au-atoms at a hollow site. The parameters43-48 for the Vsurf bonding terms are listed in Table 1. Also listed are the 0 K, optimized values of the stretch, bend, and torsion coordinates of the H-SAM. Table 2 lists the parameters34 for the nonbonded potential terms. For the H-SAM’s 0 K structure, each chain has six neighboring chains and only the intermolecular interactions for these chains are included in the potential. An atom on a chain has intermolecular interactions with each atom of the six nearest-neighbor chains. However, intermolecular interactions are only included between CH3, CH2, and S moieties on an individual chain which are separated by four or more positions. B. O(3P) + H-SAM Intermolecular Potential Energy Function. The intermolecular potential between O(3P) and the surface is represented as a sum of two-body interactions. The O‚‚‚H and O‚‚‚C interactions are assumed to be the same as those for O(3P) + CH4, which were determined from previously reported ab initio calculations of the O(3P)‚‚‚CH4 intermolecular potential.17 In this work, the spin-projected PMP2/aug-cc-pVTZ level of theory, including basis set superposition error (BSSE)

Collisions of O3(P) Atoms with 1-Decanethiol

J. Phys. Chem. B, Vol. 110, No. 24, 2006 11865

TABLE 1: Parameters for the H-SAM/Au{111} Bonding Potentialsa potential type

group

harmonic stretch Au-Sb S-Cb,c C-Cb C-Hd,e harmonic bend S-C-Ce,f S-C-Hg C-C-Ce C-C-He,h H-C-He,h torsion C-C-C-Hi C-C-C-Ce and (S-C-C-C)e

potential parameters

opt. coor.j

ro ) 2.55, k ) 2.80 2.55 ro ) 1.82, k ) 5.70 1.84 ro ) 1.53, k ) 4.86 1.54 ro ) 1.08, k ) 4.05 1.08 θo ) 114.7, k ) 0.68 116.4 θo ) 109.5, k ) 0.54 111.8 θo ) 112.4, k ) 0.85 113.2 θo ) 109.5, k ) 0.54 108.7 θo ) 109.5, k ) 0.54 109.0 Vo ) 3.39 φ1 ) 0.0, V1 ) 3.705 175.5 φ2 ) 180.0, V2 ) -0.135 φ3 ) 0.0, V3 ) 1.571 (179.4)

a Stretching and bending force constants have units of mdyn/Å and mdyn‚Å/rad2, respectively. Angles are in degrees, and torsion potentials are in kcal/mol. b Parameters from ref 20. c Parameters from ref 43. d Parameters from ref 44. e Parameters from ref 33. f Parameters from ref 45. g The S-C-H potential is assumed to be the same as the C-C-H potential, and the S-C-C-C potential is assumed to be the same as the C-C-C-C potential. h Parameters from ref 46. i This potential has a 3-fold symmetry, and the internal rotational barrier is taken from ref 34. j The 0 K optimized values for the internal coordinates.

TABLE 2: Parameters for the H-SAM/Au{111} Nonbonded Potentialsa interaction

parametersb

S‚‚‚S S‚‚‚C S‚‚‚H C‚‚‚C C‚‚‚H H‚‚‚H

Buckingham A ) 79 937 B ) 3.18 A ) 81 763 B ) 3.39 A ) 14 565 B ) 3.46 A ) 83 630 B ) 3.60 A ) 8 766 B ) 3.67 A ) 2 654 B ) 3.74

Au‚‚‚H Au‚‚‚C

Lennard-Jonesc A ) 121 746 B ) -141.8 A ) 1 062 338 B ) -521.0

C ) -2 002 C ) -1 066 C ) -233.8 C ) -568.0 C ) -125.0 C ) -27.3

a The Buckingham parameters are taken from ref 34. The LennardJones parameters are taken from ref 48. b A, B, and C have units of kcal/mol, Å, and kcal‚Å6/mol, respectively. c A and B have units of kcal‚Å12/mol and kcal‚Å6/mol, respectively.

Figure 1. PMP2/cc-pVTZ potential energy curves for O(3P) + CH4 ([) and its analytic function fit (s) compared with the potential energy curves for Ne + CH4 (‚‚‚) calculated at the MP4/aug-cc-pVTZ level. Three representative approaches are shown: face, edge, and vertex. R is the O‚‚‚C or Ne‚‚‚C distance in angstroms.

TABLE 3: Positions and Depths of Minima for the O(3P) + CH4 and Ne + CH4 Potential Energy Curvesa O(3P) + CH4b

O(3P)‚‚‚CH

correction, was used to calculate the 4 potential energy curves for face, edge, and vertex orientations, with CH4 frozen in its equilibrium configuration. The energies for these three curves were fit simultaneously, with a sum of Buckingham terms

V(r) ) Ae-Br + C/r6

(2)

to determine the values of the A, B, and C parameters for the O‚‚‚H and O‚‚‚C interactions. The fits to the ab initio potential energy curves are shown in Figure 1. The units for A, B, and C are kcal/mol, Å, and kcal‚Å6/mol, respectively. The O‚‚‚H values are A ) 1 070.7, B ) 3.194, and C ) -6.56. For O‚‚‚C, they are A ) 25 106.4, B ) 3.324, and C ) -1 044.36. The global potential energy minimum lies on the curve for the face configuration. The positions and depths of the minima for the three curves are listed in Table 3. Also shown in Figure 1, for comparison, are the ab initio intermolecular potential energy curves calculated for the Ne‚‚‚CH4 system, using the MP4/aug-cc-pVTZ level of theory with BSSE correction. An MP4 calculation with a basis set identified as [6s4p] contracted from [14s9p] for C, [8s5p] contracted from [14s10p] for Ne, [5s] contracted from [10s]

ab initio curve

Vo

Ro

Ne + CH4c

fit Vo

fitd

ab initio Ro

Vo

Ro

Vo

Ro

face -0.270 3.36 -0.276 3.50 -0.146 3.47 -0.166 3.43 edge -0.180 3.59 -0.228 3.63 -0.117 3.69 -0.131 3.58 vertex -0.055 4.26 -0.127 4.08 -0.091 4.05 -0.122 4.04 a V is in kcal/mol and R in angstroms. R is the O‚‚‚C or Ne‚‚‚C o o o distance. b Results of PMP2/aug-cc-pVTZ calculations with BSSE correction. c Results of MP4/aug-cc-pVTZ calculations with BSSE correction, reported in Table 1 of ref 23. d Values based on the fit NeIV to MP4/aug-cc-pVTZ in ref 23.

for H, and extended with different polarization functions49 yields nearly identical curves. Overall, the O(3P) + CH4 and Ne + CH4 potential energy curves are in very good agreement. The minima for the two sets of curves are similar, as shown in Table 3. The O(3P) + CH4 potential is only slightly deeper in the face and edge orientations and slightly shallower in the vertex orientation. In the absence of any calculated values, the O‚‚‚S interaction potential is assumed to be equivalent to that for O‚‚‚C and the O‚‚‚Au interaction is assumed to be purely repulsive, with parameters given by the exponential term of the O‚‚‚C interac-

11866 J. Phys. Chem. B, Vol. 110, No. 24, 2006 tion. Although only approximate, inclusion of these model O‚‚‚S and O‚‚‚Au interactions is essential, because the oxygen atoms penetrate down to the gold substrate in a significant fraction of high collision energy trajectories. These interactions prevent oxygen atoms from unphysical penetration through the Au surface, while not affecting the fraction of penetrating trajectories. However, the approximate nature of these potentials may affect the O-atom’s residence time near the Au surface. C. Trajectory Simulations. The classical trajectory simulations were performed with the general chemical dynamics computer program VENUS.50 The algorithms for choosing the initial conditions for the trajectories are standard options in VENUS, and initial coordinates for a beam of O(3P) atoms were chosen according to the sampling method described previously.20,21 Initial translational energies Ei of 0.12, 2.3, 11.2, 71.0, and 120.5 kcal/mol were considered for the O(3P) beam to investigate scattering from hypothermal to hyperthermal conditions and compare with experiments by Minton and co-workers5 for O(3P) + liquid squalane scattering. The initial incident polar angle θi was set at 15°, 30°, 45°, 60°, or 75 ° with respect to the surface normal. The azimuthal angle, χi, the projection of the incident beam onto the SAM surface, was chosen randomly between 0° and 360° to represent collisions with different domains of growth and, thus, different chain orientations on the surface.20 It is possible to grow an ordered H-SAM surface, with large growth domains.26 To simulate scattering off such a surface, the azimuthal angle is not chosen randomly, since it is fixed by the experiment. For Ar scattering off an ordered H-SAM surface, the scattering dynamics depend on the azimuthal angle.26 For each trajectory, random velocities were initially assigned to the surface atoms by sampling their 300 K MaxwellBoltzmann distributions. Standard classical molecular dynamics,51 with equilibration and annealing, was then performed so that the surface was in an equilibrium state for a 300 K classical Boltzmann distribution. The trajectories were integrated with combined fourth-order Runge-Kutta-Gill and sixth-order Adams-Moulton algorithms, a standard option in VENUS. A fixed time step of 0.2 fs was used. Each trajectory was initiated with the O(3P) atom ∼25 Å above the terminal carbon atoms of the H-SAM and terminated when the scattered O(3P) atom exceeded this height or the total integration time reached 60 ps. Typically, 500-1000 trajectories were run for each set of initial conditions (Ei, θi). The individual trajectories were analyzed to study the dynamics of O(3P) + H-SAM collisions. During the course of a trajectory, the position of O(3P), with respect to the atoms of the H-SAM, was monitored to identify the trajectory type. Other pieces of information collected for the O(3P) atom are its final translational energy Ef, its final polar and azimuthal angles of θf and χf, its residence time τres on/in the surface, the minimum height, hmin, it obtains with respect to the Au substrate, and its number of hops, Nhop. A trajectory with only one inner turning point in the O(3P) + Au surface relative motion does not hop, a trajectory with two inner points has one hop, and so forth. The residence time is the length of time the O(3P) atom is within 5 Å above the terminal C-atom layer of the H-SAM. In previous related simulations, this distance was taken as 3.95 and 5.3 Å.21,25 For a small number of trajectories, the O(3P) atom does not desorb within the 60 ps integration time. The final velocity and scattering angles are undetermined in these cases, and it is assumed that O(3P) would ultimately desorb fully thermalized. Thus, for these trajectories, the final velocity is randomly

Tasic´ et al.

Figure 2. Plots of the average fraction of O(3P) collision energy transferred, for different Ei, as a function of θi.

assigned from a 300 K Boltzmann distribution, θf is randomly sampled from the cosine distribution, and χf is sampled uniformly within the 0°-360° range. To eliminate the possibility of biasing the results toward a Boltzmann distribution, the incomplete trajectories are flagged and may be eliminated from all analyses that involve Ef, θf, or χf values. The presence of the incomplete trajectories means that the τres and Nhop values are underestimates of the actual values. In practice, they both may be very large, with Nhop > 100. Nevertheless, under most conditions, the averages 〈Nhop〉 and 〈τres〉 are not significantly affected by the incomplete trajectories. III. Trajectory Results The O(3P) + H-SAM scattering dynamics was simulated for incident collision energies Ei of 0.12, 2.30, 11.2, 71.0, and 120.5 kcal/mol and incident angles θi of 15, 30, 45, 60, and 75°, with respect to the surface normal. The results for each Ei and θi simulation were not differentiated by the final polar angle θf or azimuthal angle χf. Instead, the results for all θf and χf were combined to form distribution functions and make analyses. The results of the simulations are described below. A. Translational Energy Distribution. For the low Ei values of 0.12 and 2.30 kcal/mol, physisorption on the H-SAM surface is very important and P(Ef) is independent of θi. However, for the higher Ei values, the efficiency of energy transfer depends on θi and decreases as θi is increased. This is illustrated in Table 5 and Figure 2, where the average final translational energy 〈Ef〉 and energy transfer fraction are given for the different simulations. Representative P(Ef) distributions are shown in Figure 3. As was done in previous work,20-22 the low energy portion of each P(Ef) curve was fit to the Boltzmann distribution function for thermal desorption from the surface

PB(Ef) )

Ef R TB2 2

exp(-Ef/RTB)

(3)

where TB is the temperature for this distribution. For fit 1, TB was fixed at 300 K, the temperature of the H-SAM surface. For fit 2, TB was allowed to vary.21,22 The fraction of the Boltzmann component in the fit, fB, is the area under the PB(Ef) curve divided by the area under the total P(Ef) curve. The fitted values for fB and TB are listed in Table 4. There are a number of important findings from these fits. Allowing TB to vary leads to a larger fB and a value for TB > 300 K. For Ei ) 2.30 kcal/mol, fB appears insensitive to the incident polar angle θi, while for the larger Ei, fB decreases as θi is increased. For the highest θi of 75° and Ei g 11.2 kcal/mol, there is no



Figure 3. Representative histograms of the final O(3P) translational energy distributions, P(Ef), for different initial values of Ei and θi. The curves are the Boltzmann components for the TB ) 300 K fit.

TABLE 4: Boltzmann Component in Fits of P(Ef) to a Bimodal Distributiona Incident Energy (kcal/mol) incident angle θi 15° 30° 45° 60° 75°

fB fit 1 fit 2 fit 1 fit 2 fit 1 fit 2 fit 1 fit 2 fit 1 fit 2

2.30 TB (K)

0.77 1.00 0.73 1.00 0.81 1.00 0.92 0.96 0.74 0.99

300 371 300 400 300 359 300 348 300 403

fB

11.2 TB (K)

0.64 0.87 0.53 0.88 0.45 0.71 0.27 0.52

300 401 300 492 300 467 300 552

fB

71.0 TB (K)

0.68 0.78 0.53 0.67 0.34 0.37 b

0.13

300 363 300 377 300 342 b

456

fB

120.5 TB (K)

0.61 0.79 0.62 0.69 0.39 0.50 b

0.18

300 388 300 353 300 391 b

474

b

b

b

b

b

b

b

b

b

b

b

b

a fB is the fraction of the Boltzmann component in the fit to P(Ef), and TB is the temperature of this component. For fit 1, TB is fixed at 300 K, the initial temperature of the H-SAM. For fit 2, TB is allowed to vary. b Means that fitting cannot be performed, that is, fB ≈ 0.

apparent Boltzmann component in P(Ef), that is, fB is approximately zero. It is interesting that, for θi e 45°, the fB values become independent of the collision energy for Ei g 11.2 kcal/ mol. It is particularly remarkable that, for the hyperthermal collision energy of 120.5 kcal/mol (∼5 eV), fB is near to or even larger than 50% for θi up to 45°. The values of fB vs Ei and θi are summarized in Figure 4. B. Trajectory Types. To understand the dependence of the P(Ef) distribution on Ei and θi, it is important to identify the different types of trajectories and the roles they play in the energy transfer. Listed in Table 5 are the fractions of trajectories

Figure 4. Bar graph of the fraction of the Boltzmann component in P(Ef), for different Ei and θi values. The temperature of the Boltzmann component, TB, does not equal 300 K in the fit.

which directly scatter off the top of the surface with only one inner turning point (Nhop ) 0), those which physisorb on the top of the surface with Nhop > 0, and those which penetrate the outer C-atoms of the CH3 groups and get inside the surface. For identifying surface penetration, the surface top is defined as a height of 14.5 Å. In comparison, the average height of the terminal C-atoms at 300 K is 14.7 Å. Thus, a penetrating O-atom is defined as one that descends below the height of the methyl groups. Furthermore, trajectories that penetrate the surface are subdivided into five different types. They are identified by Nhop ) 0 or Nhop > 1 and by whether they penetrate but do not reach the Au substrate (i.e., inside), reach the Au substrate (i.e., bottom), or remain trapped inside the surface when the trajectory is concluded at 60 ps. The fractions for these five types are listed in Table 5 and their sum is unity.


Tasic´ et al.

TABLE 5: Fractions of Different Trajectory Types fractions (all trajectories)a

fractions (penetrating trajectories)b

Ei

θi

〈Ef〉

directc

physisorbd

penetratee

Nhop ) 0

inside Nhop > 0

bottomg Nhop ) 0

bottom Nhop > 0

trapped h

0.12

15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75

1.22 1.23 1.19 1.19 1.18 1.44 1.51 1.41 1.47 1.63 2.02 2.38 2.96 4.24 6.20 3.40 5.51 11.2 24.4 51.5 4.04 6.16 13.8 36.4 85.3

0.22 0.23 0.21 0.24 0.20 0.29 0.32 0.27 0.33 0.37 0.26 0.36 0.43 0.59 0.65 0.15 0.19 0.39 0.70 0.95 0.10 0.13 0.24 0.56 0.95

0.66 0.67 0.70 0.64 0.69 0.45 0.49 0.53 0.57 0.55 0.21 0.17 0.29 0.30 0.33 0.02 0.02 0.02 0.02 0.04 0.01 0.01 0.01 0.01 0.02

0.12 0.11 0.09 0.12 0.11 0.27 0.19 0.19 0.10 0.08 0.53 0.47 0.28 0.11 0.01 0.84 0.79 0.59 0.28 0.01 0.90 0.87 0.75 0.43 0.03

0.12 0.06 0.13 0.12 0.11 0.36 0.36 0.37 0.41 0.23 0.33 0.45 0.46 0.39 0.14 0.17 0.30 0.53 0.74 1.00 0.15 0.25 0.43 0.76 0.91

0.71 0.75 0.74 0.67 0.65 0.51 0.54 0.61 0.49 0.65 0.52 0.48 0.49 0.56 0.71 0.35 0.42 0.29 0.19 0.00 0.31 0.35 0.32 0.17 0.06

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.04 0.02 0.01 0.00 0.00

0.00 0.02 0.04 0.03 0.07 0.04 0.01 0.01 0.00 0.05 0.03 0.02 0.02 0.00 0.00 0.18 0.09 0.08 0.02 0.00 0.21 0.16 0.11 0.03 0.03

0.17 0.17 0.09 0.18 0.18 0.09 0.09 0.01 0.10 0.08 0.12 0.06 0.03 0.06 0.14 0.30 0.18 0.10 0.05 0.00 0.28 0.22 0.13 0.03 0.00

2.30

11.2

71.0

120.5

insidef

a The fractions of all trajectories add up to unity. b The fractions of penetrating trajectories add up to unity. c “Direct” trajectories are those with Nhop ) 0 and hmin g 14.5 Å. d “Physisorption” trajectories are those with Nhop > 0 and hmin g 14.5 Å. e “Penetrating” trajectories are those with hmin < 14.5 Å. f “Inside” means 2.3 Å < hmin < 14.5 Å. g “Bottom” means hmin e 2.3 Å. h “Trapped” trajectories means that O(3P) does not desorb by 60 ps.

Table 5 illustrates the complexity and richness of detail in the O(3P) + H-SAM collision dynamics. The nature of the scattering dynamics depends on Ei. For the low collision energy Ei of 0.12 kcal/mol, the collision dynamics are insensitive to the incident angle θi and the most important trajectory type is physisorption. For Ei of 2.30 kcal/mol, the probability of physisorption is smaller but remains the dominant event. Its fraction is nearly independent of θi, while the fractions for the direct and penetrating trajectories increase and decrease, respectively, with larger θi. The scattering dynamics for Ei of 11.2 kcal/mol is similar to that at 2.30 kcal/mol, but physisorption is no longer the dominant event. Dependence of the direct and penetrating trajectories on θi is more pronounced, with penetration the dominant event for small θi and direct scattering dominating at the larger θi. There is a major transition in the gas-surface collision dynamics between Ei of 11.2 and 71.0 kcal/mol. For Ei ) 11.2 kcal/mol, physisorption is important at all θi, constituting 2030% of the trajectories, while physisorption is negligible at Ei values of 71.0 and 120.5 kcal/mol. At the high Ei values, the interplay between the direct and penetrating trajectories is more pronounced, with negligible penetration for θi ) 75°. For the trajectories that penetrate the surface, the “inside” type, which do not reach the bottom or remain trapped, dominates all Ei and θi. For Ei ) 0.12 kcal/mol, the fractions of the different types of penetrating trajectories are independent of θi. For Ei of 0.12, 2.30, and 11.2 kcal/mol, there are no apparent trends vs θi in the fractions of trajectories which reach the bottom of the surface or remained trapped. Their respective fractions range from 0 to 0.07 and 0.03-0.18. For Ei values of 2.3 and 11.2 kcal/mol, the “inside” types, with Nhop ) 0 and Nhop > 0, have fractions which depend on θi. These fractions decrease and increase, respectively, with increase in θi. Increasing Ei from 11.2 to 71.0 kcal/mol gives rise to a significant change in the penetration dynamics. For Ei values

of 71.0 and 120.5 kcal/mol, the fractions of “inside” type trajectories with Nhop ) 0 and Nhop > 0 increase and decrease, respectively, as θi is increased. These trends are opposite of what is found for the lower Ei. As compared with Ei values of 11.2 kcal/mol and smaller, for the large Ei, more trajectories reach the bottom of the surface or remain trapped, with fractions as large as 0.21 and 0.30, respectively. These fractions decrease with increases in θi, becoming very small for θi ) 75°. A very interesting feature of the O(3P) + H-SAM scattering are the trajectories where the O(3P) atoms move through the H-SAM and reach the Au(s) substrate. It is perhaps surprising that, even for the low Ei values of 0.11 and 2.30 kcal/mol, up to 7% of the trajectories reach the substrate for some of the θi. Averaged over all the θi, ∼3% of the trajectories hit the substrate for these low Ei. At the highest Ei of 120.5 kcal/mol, and for near perpendicular collisions with θi ) 15°, 4% of the trajectories hit the Au(s) substrate and scatter with Nhop ) 0, that is, have only one inner turning point in the O(3P) + Au(s) substrate relative motion. Listed in Table 6 is the average final translational energy 〈Ef〉j for each of the trajectory types. These average energies are related to 〈Ef〉 by

〈Ef〉 )

∑j fj〈Ef〉j

(4)

where fj is the fraction of trajectory type j. For Ei ) 0.12 kcal/ mol and all θi, each trajectory type leads to an 〈Ef〉j equal to the thermal desorption energy of 2RT ) 1.2 kcal/mol. For Ei ) 2.3 kcal/mol and all θi, the trajectories which physisorb or penetrate have near thermal 〈Ef〉j values. At Ei of 11.2 kcal/mol, the trajectories which penetrate acquire thermal energies, while the 〈Ef〉j values for the other two trajectory types depend on θi. At the high Ei values of 71.0 and 120.5 kcal/mol, the 〈Ef〉j values for all the trajectory types depend on θi. For the direct



TABLE 6: Average Final Translational Energies for Different Trajectory Typesa 〈Ef〉j (all trajectories)

〈Ef〉j (penetrating trajectories)

Ei

θi

〈Ef〉

direct

physisorb

penetrate

inside Nhop ) 0

0.12

15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75

1.22 1.23 1.19 1.19 1.18 1.44 1.51 1.41 1.47 1.63 2.02 2.38 2.96 4.24 6.20 3.40 5.51 11.2 24.4 51.5 4.04 6.16 13.8 36.4 85.3

1.3 ( 0.1 1.1 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.8 ( 0.1 1.9 ( 0.1 1.5 ( 0.1 1.9 ( 0.1 2.0 ( 0.1 3.2 ( 0.2 3.5 ( 0.2 4.1 ( 0.2 5.4 ( 0.2 7.0 ( 0.2 10.8 ( 0.7 15.4 ( 1.0 19.8 ( 0.9 29.7 ( 0.6 51.4 ( 0.4 16.6 ( 1.2 23.9 ( 1.3 34.7 ( 1.4 51.5 ( 1.1 86.6 ( 0.7

1.2 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.2 ( 0.0 1.2 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.4 ( 0.1 1.6 ( 0.1 1.8 ( 0.2 2.3 ( 0.2 3.0 ( 0.2 4.9 ( 0.3 3.6 ( 0.8 2.2 ( 0.8 5.8 ( 3.2 22.1 ( 4.6 59.2 ( 1.7 NA 1.9 ( 0.7 2.7 ( 1.4 24.4 ( 13.5 101.5 ( 3.0

1.2 ( 0.1 1.0 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.1 ( 0.1 1.4 ( 0.1 1.5 ( 0.1 1.5 ( 0.1 1.4 ( 0.2 1.4 ( 0.1 1.6 ( 0.1 1.7 ( 0.1 1.8 ( 0.1 1.6 ( 0.2 NA 2.1 ( 0.2 3.2 ( 0.2 5.7 ( 0.4 10.9 ( 0.7 NA 2.7 ( 0.2 3.6 ( 0.2 7.2 ( 0.4 16.9 ( 0.9 39.7 ( 5.1

NAb NA NA NA NA 1.4 ( 0.1 2.1 ( 0.2 1.9 ( 0.2 2.0 ( 0.3 NA 2.1 ( 0.1 2.3 ( 0.1 2.4 ( 0.2 2.5 ( 0.4 NA 5.3 ( 0.7 7.2 ( 0.7 9.6 ( 0.7 14.2 ( 0.8 NA 7.5 ( 0.7 9.6 ( 0.6 14.5 ( 0.8 21.7 ( 1.1 43.3 ( 5.2

2.30

11.2

71.0

120.5

inside Nhop > 0

bottom Nhop ) 0

bottom Nhop > 0

1.2 ( 0.1 1.1 ( 0.1 1.3 ( 0.2 1.1 ( 0.1 1.1 ( 0.1 1.5 ( 0.1 1.0 ( 0.1 1.3 ( 0.1 1.1 ( 0.1 1.3 ( 0.2 1.5 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.0 ( 0.1 NA 1.6 ( 0.1 1.5 ( 0.1 1.6 ( 0.2 1.8 ( 0.2 NA 1.8 ( 0.1 1.7 ( 0.1 1.9 ( 0.1 1.8 ( 0.2 NA

NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 8.3 ( 1.5 6.8 ( 2.4 NA NA NA

NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA 1.2 ( 0.1 1.4 ( 0.2 1.3 ( 0.2 NA NA 1.4 ( 0.1 1.4 ( 0.1 1.5 ( 0.1 1.2 ( 0.2 NA

a The errors are the standard deviations of the means. b “NA” means that there are less than 10 data points (trajectories) available, insufficient for analysis.

TABLE 7: Fractions of Trajectories vs the Number of Hops fraction Ei

θi 〈Nhop〉 Nhop ) 0

0.12 15 30 45 60 75 30 45 60 75 11.2 15 30 45 60 75 71.0 15 30 45 60 75 120.5 15 30 45 60 75

6 7 6 7 7 5 4 4 4 13 6 4 2 1 48 28 13 3 0 47 38 20 3 0

0.24 0.23 0.22 0.25 0.21 0.39 0.35 0.37 0.38 0.43 0.57 0.56 0.64 0.65 0.29 0.43 0.70 0.91 0.96 0.27 0.36 0.57 0.89 0.98

1

2

3

4

5

0.16 0.16 0.17 0.13 0.14 0.16 0.13 0.18 0.16 0.14 0.13 0.11 0.16 0.15 0.07 0.07 0.05 0.02 0.02 0.06 0.05 0.06 0.02 0.02

0.16 0.11 0.11 0.09 0.11 0.08 0.12 0.10 0.11 0.06 0.04 0.06 0.06 0.06 0.03 0.05 0.04 0.01 0.01 0.04 0.04 0.04 0.01 0.00

0.10 0.08 0.08 0.09 0.11 0.08 0.10 0.08 0.07 0.05 0.04 0.07 0.04 0.04 0.02 0.03 0.02 0.01 0.01 0.03 0.03 0.02 0.01 0.00

0.07 0.08 0.08 0.08 0.06 0.03 0.06 0.05 0.06 0.03 0.02 0.04 0.02 0.02 0.02 0.03 0.01 0.01 0.00 0.03 0.02 0.02 0.01 0.00

0.06 0.07 0.07 0.04 0.05 0.05 0.04 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.01 0.00 0.00 0.02 0.03 0.01 0.01 0.00

6-10 11-20 > 20 0.11 0.15 0.14 0.19 0.18 0.13 0.12 0.10 0.11 0.12 0.07 0.06 0.05 0.04 0.07 0.10 0.05 0.01 0.00 0.09 0.08 0.06 0.02 0.00

0.07 0.09 0.09 0.08 0.09 0.06 0.06 0.06 0.05 0.05 0.04 0.04 0.01 0.01 0.07 0.04 0.02 0.01 0.00 0.07 0.07 0.04 0.01 0.00

0.03 0.04 0.03 0.04 0.04 0.03 0.02 0.02 0.02 0.09 0.05 0.02 0.01 0.00 0.39 0.23 0.11 0.02 0.00 0.39 0.31 0.18 0.03 0.00

and physisorption trajectories, the 〈Ef〉j values are above the thermal value, with physisorption providing significantly more relaxation than direct scattering for the smaller θi. An analysis of the penetrating trajectories shows that those with Nhop > 0 have near-thermal 〈Ef〉j values, independent of θi. The penetrating trajectories, which directly scatter from the surface with Nhop ) 0, have strongly nonthermal 〈Ef〉j. However, it should be noted that these latter values are substantially smaller than those for the trajectories which directly scatter without penetration. The trajectories may also be analyzed in terms of their Nhop values and this information is given in Tables 7 and 8. Table 7 shows that, for Ei of 11.2 kcal/mol and lower, the probability of hopping decreases in an exponential-like fashion with

increasing number of hops. For the low Ei values of 0.12 and 2.30 kcal/mol, the average number of hops and the fraction of trajectories for a particular Nhop value are independent of the incident angle. Overall, the Nhop distributions at Ei ) 11.2 kcal/ mol are similar to those at the lower Ei, except that the probability of Nhop ) 0 increases as θi is increased. Increasing Ei from 11.2 to 71.0 kcal/mol results in a significant change in the “hopping” dynamics. Both 〈Nhop〉 and the Nhop probability distribution become strongly dependent on θi. For large θi, nearly all the trajectories have Nhop ) 0. However, for small θi, it is the trajectories with extensive hopping (Nhop > 20) that dominate. As shown in Table 5, this is a result of the importance of surface penetration events at high Ei and small θi. Table 8 lists the average final translational energy 〈Ef〉j as a function of j ) Nhop. These values may be used in eq 4, with fj to obtain 〈Ef〉. For Ei ) 0.12 kcal/mol, 〈Ef〉j is independent of both θi and Nhop and equals 2RT. As Ei is increased, more “hops” are required to thermalize the final translational energy and the extent of relaxation is associated with the value of Nhop. In addition, the 〈Ef〉j values increase with increase in θi. At Ei of 11.2 kcal/mol, near-thermalization occurs at Nhop g 3 for collisions with θi of 15-60°. For the larger θi of 75°, nearthermalization requires Nhop g 6. The results for the larger Ei are similar to those for Ei ) 11.2 kcal/mol, except the dependence of 〈Ef〉j on θi is more pronounced for Nhop values of 0 and 1. C. Scattering Angles. The final polar angle θf and change in azimuthal angle ∆χ, of the scattered O(3P) atoms, provide important information about the collision dynamics. For direct elastic collisions with a smooth surface, the scattering is specular with θf ) θi and ∆χ ) 0. In contrast, for trapping desorption, the atoms leave the surface in random directions with a probability proportional to sin θf cos θf at each ∆χ between 0° and 360°. Representative plots of P(θf) and P(∆χ) are given in Figures 5 and 6 for O(3P) collisions with the H-SAM at Ei values of 0.12, 11.2, and 120.5 kcal/mol and θi of 15° and 75°. For θi ) 15°, the P(θf) and P(∆χ) distributions are random for each


Tasic´ et al.

TABLE 8: Average Final Translational Energies for Trajectories vs the Number of Hopsa 〈Ef〉j Ei

θi

〈Ef〉

Nhop ) 0

1

2

3

4

5

6-10

11-20

> 20

0.12

15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75

1.22 1.23 1.19 1.19 1.18 1.44 1.51 1.41 1.47 1.63 2.02 2.38 2.96 4.24 6.20 3.40 5.51 11.2 24.4 51.5 4.04 6.16 13.8 36.4 85.3

1.3 ( 0.1 1.0 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.7 ( 0.1 1.9 ( 0.1 1.6 ( 0.1 1.9 ( 0.1 2.0 ( 0.1 2.8 ( 0.1 3.1 ( 0.1 3.7 ( 0.1 5.2 ( 0.2 7.0 ( 0.1 8.2 ( 0.6 10.9 ( 0.6 15.2 ( 0.6 26.2 ( 0.5 51.1 ( 0.4 10.8 ( 0.6 14.5 ( 0.7 22.9 ( 0.8 40.5 ( 0.9 85.2 ( 0.7

1.3 ( 0.1 1.3 ( 0.1 1.1 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.4 ( 0.1 1.5 ( 0.1 1.3 ( 0.1 1.4 ( 0.1 1.7 ( 0.1 1.9 ( 0.2 1.7 ( 0.2 2.8 ( 0.3 3.5 ( 0.3 6.3 ( 0.4 2.7 ( 0.3 2.0 ( 0.2 3.7 ( 1.4 16.4 ( 4.3 60.4 ( 1.3 2.4 ( 0.2 2.8 ( 0.3 2.8 ( 0.4 5.6 ( 2.5 95.7 ( 6.5

1.4 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.2 ( 0.1 1.2 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.9 ( 0.3 2.1 ( 0.4 2.4 ( 0.3 3.0 ( 0.4 4.9 ( 0.6 1.7 ( 0.5 1.7 ( 0.2 2.0 ( 0.6 10.4 ( 5.1 NA 2.0 ( 0.2 2.2 ( 0.3 2.1 ( 0.3 1.8 ( 0.4 NA

1.0 ( 0.1 1.1 ( 0.2 1.1 ( 0.1 1.1 ( 0.1 1.2 ( 0.1 1.3 ( 0.2 1.2 ( 0.1 1.2 ( 0.1 1.2 ( 0.1 1.4 ( 0.2 1.5 ( 0.2 1.4 ( 0.3 1.5 ( 0.2 1.7 ( 0.3 4.6 ( 0.8 1.3 ( 0.3 1.3 ( 0.4 NA NA NA 1.5 ( 0.2 2.0 ( 0.2 1.5 ( 0.2 NA NA

1.0 ( 0.1 1.7 ( 0.2 1.1 ( 0.1 1.1 ( 0.1 1.0 ( 0.1 1.3 ( 0.2 1.4 ( 0.3 1.3 ( 0.2 1.4 ( 0.2 1.1 ( 0.1 1.1 ( 0.2 1.5 ( 0.3 2.1 ( 0.5 2.4 ( 0.7 3.4 ( 0.8 1.4 ( 0.2 1.6 ( 0.4 NA NA NA 1.4 ( 0.2 1.2 ( 0.2 1.9 ( 0.3 1.4 ( 0.3 NA

1.2 ( 0.1 0.9 ( 0.1 1.1 ( 0.1 1.0 ( 0.2 1.5 ( 0.2 1.2 ( 0.2 1.3 ( 0.2 1.4 ( 0.2 0.9 ( 0.1 1.6 ( 0.2 1.6 ( 0.3 1.5 ( 0.3 1.9 ( 0.5 NA 2.9 ( 0.8 1.8 ( 0.3 1.7 ( 0.4 NA NA NA 1.9 ( 0.3 1.4 ( 0.2 1.7 ( 0.3 NA NA

1.1 ( 0.1 1.4 ( 0.1 1.1 ( 0.1 1.1 ( 0.1 1.2 ( 0.1 1.2 ( 0.1 1.1 ( 0.1 1.3 ( 0.1 1.0 ( 0.1 1.3 ( 0.1 1.2 ( 0.1 1.1 ( 0.1 1.3 ( 0.2 1.4 ( 0.3 1.5 ( 0.3 1.3 ( 0.2 1.4 ( 0.2 1.2 ( 0.1 1.1 ( 0.2 NA 1.4 ( 0.1 1.4 ( 0.1 1.4 ( 0.1 1.9 ( 0.3 NA

1.1 ( 0.1 1.1 ( 0.1 1.5 ( 0.2 1.1 ( 0.1 1.1 ( 0.1 1.3 ( 0.1 1.0 ( 0.2 1.5 ( 0.2 1.2 ( 0.1 1.2 ( 0.2 1.3 ( 0.2 1.2 ( 0.2 1.0 ( 0.2 NA NA 1.4 ( 0.2 1.3 ( 0.3 1.8 ( 0.4 NA NA 1.6 ( 0.1 1.3 ( 0.1 1.5 ( 0.2 NA NA

1.1 ( 0.2 0.9 ( 0.1 1.2 ( 0.2 1.4 ( 0.2 1.1 ( 0.2 1.0 ( 0.2 1.5 ( 0.2 1.2 ( 0.2 NAb NA 1.0 ( 0.1 1.3 ( 0.2 1.3 ( 0.2 NA NA 1.2 ( 0.1 1.2 ( 0.1 1.1 ( 0.1 1.3 ( 0.2 NA 1.3 ( 0.0 1.2 ( 0.0 1.2 ( 0.1 1.0 ( 0.2 NA

2.30

11.2

71.0

120.5

a The errors are the standard deviations of the means. b “NA” means that there are less than 10 data points (trajectories) available, insufficient for analysis.

Ei. The solid curve in the graph of each of these P(θf) distributions is the distribution sin θf cos θf for totally random scattering,21 which provides an excellent representation of the trajectory results. (For only in-plane scattering events, the distribution is cos θf.) For Ei ) 0.12 kcal/mol, the scattering angles are also random for θi ) 75°, but this is not the case for Ei values of 11.2 and 120.5 kcal/mol. For these Ei, there is a strong preference for specular scattering at θf ) 75°, which is particularly pronounced for the higher Ei of 120.5 kcal/mol. The values for the average scattering angles 〈θf〉 and 〈∆χ〉 are listed in Table 9. For Ei values of 0.12 and 2.30 kcal/mol, the 〈θf〉 values are ∼45° for each θi. However, for the higher Ei, 〈θf〉 increases with θi, and ∆χ approaches zero, with near specular scattering at Ei values of 71.0 and 120.5 kcal/mol for θi ) 75°. D. Correlations Between Trajectory Properties. The above results show that there are strong correlations between Ef and both the trajectory types and Nhop values. Further analyses of the trajectories show there are additional correlations between trajectory properties. For each of the Ei, θi simulations there is a strong, near-linear correlation (not shown) between Nhop and τres, the length of time O(3P) interacts with the surface. Thus, the same type of correlation is found between Ef and τres, as discussed above for Ef and Nhop. Another important dynamical property is hmin, the minimum height O(3P) attains above the Au substrate during its collision with the H-SAM/Au surface. Average values of hmin, 〈hmin〉, are plotted in Figure 7 vs θi for the simulations at the different Ei. At 300 K, the average height of the terminal C-atoms above the Au substrate is 14.5 Å, and for Ei ) 0.12 kcal/mol, there is only a small amount of penetration of this outer layer (see Table 5). For Ei ) 2.30 kcal/ mol and θi ) 15-45°, 20-30% of the trajectories penetrate the outer C-atom and this is evident in their 〈hmin〉 values. A somewhat smaller 〈hmin〉 value of 13.1 Å is found for Ei ) 11.2 kcal/mol and θi ) 15°. Deep penetration of the surface is found for the high collision energies of 71.0 and 120.5, where respective 〈hmin〉 values as small as 7.6 and 6.3 Å are found.

Figure 7 also shows that deep surface penetration results in 〈τres〉 as large as 10 ps. For Ei of 0.12 kcal/mol, 〈τres〉 is independent of θi and equals 4-5 ps. The 〈τres〉 values become shorter with increase in θi for Ei values of 11.2, 71.0, and 120.5 kcal/mol. For Ei ) 2.30 kcal/mol and θi ) 15°, for which surface trapping is important, 〈τres〉 is similar to that for Ei of 0.12 kcal/mol. However, for the larger θi, 〈τres〉 is smaller for Ei ) 2.30 kcal/ mol, as a result of less trapping. For Ei values of 71.0 and 120.5 kcal/mol, 〈τres〉 is strongly dependent on θi, as a result of the dramatic decrease in surface penetration with increase in θi. Comparing the plots in Figure 7 with the 〈Ef〉 values in Table 5 shows that the decrease in 〈hmin〉, as θi is decreased for a fixed Ei, is associated with more efficient energy transfer. This is illustrated in Figure 8, where 〈Ef〉 is plotted vs 〈hmin〉 at each θi for the simulations at different Ei. Table 7 shows that the decrease in 〈hmin〉, with decrease in θi, is associated with an increase in 〈Nhop〉. This suggests there is also a correlation between 〈Ef〉 and 〈Nhop〉, and Figure 8 shows this is indeed the case. More details of these calculations are illustrated by the scatter plots in Figure 9 of Ef vs hmin and Nhop vs hmin, for a representative set of simulations with Ei ) 120.5 kcal/mol and θi ) 30°. The Ef values change dramatically at hmin ∼ 13 Å, where for smaller hmin the largest Ef is 10-15 kcal/mol. Thus, for hmin < 13 Å, substantial energy transfer occurs. However, an hmin value less than 13 Å does not necessarily mean a large value for Nhop. For trajectories in which the O(3P) atoms reach the Au substrate (i.e., hmin ∼ 2 Å), Nhop ranges from zero (trajectories that directly depart) to more than 80. Correlations between the final translational energy of the scattered O(3P) atom and its final polar and azimuthal angles depend on the collision dynamics. Examples are illustrated in Figure 10, where histograms are given of 〈Ef〉 vs θf and ∆χ. For the collisions at Ei ) 0.12 kcal/mol and θi ) 15°, the 〈Ef〉 values are independent of θf and ∆χ. Though 22% of trajectories are direct, as shown in Table 5, thermal accommodation is sufficiently efficient to give a thermal 〈Ef〉 at each θf and ∆χ. Dependence of 〈Ef〉 on θf and ∆χ, for collisions with Ei ) 120.5



Figure 5. Representative histograms of the probability of θf for different Ei and θi values. All values of ∆χ are included in the P(θf) distributions. The curve in the plots is the distribution sin θf cos θf for thermal desorption.

kcal/mol, is strongly affected by the incident angle. For θi ) 15°, where 90% of trajectories penetrate the surface, the distributions retain some of the characteristics found for Ei ) 0.12 kcal/mol. The 〈Ef〉 value is near thermal for θf ) 0° and then increases with increase in θf. However, for large θf, there is still substantial accommodation of the collision energy. The 〈Ef〉 value is somewhat larger for in-plane and near in-plane scattering, but there is not a dramatic change in its value for out-of-plane scattering. If θi is increased to 75°, for Ei ) 120 kcal/mol, then there is a major change in dependence of the energy transfer on θf and ∆χ. 〈Ef〉 varies from ∼15 kcal/mol for small θf to ∼110 kcal/mol and essentially no energy transfer for large θf. The value of 〈Ef〉 decreases rapidly as the scattering becomes out-of-plane. IV. Discussion A. Comparison with Ne + H-SAM Simulations and Experiments. In previous research,20-25 collisions of rare gas atoms with H-SAM surfaces have been simulated. As discussed in Section IIB and shown in Table 3, the Ne and O(3P) atoms have similar intermolecular potentials with the H-SAM. Thus, they are expected to have similar energy transfer dynamics. This is indeed the case, as illustrated by comparing previous simulations23 for Ne + n-hexylthiolate SAM scattering for Ei ) 10.0 kcal/mol and θi ) 45°, with the current O(3P) +

n-decanethiolate SAM scattering for Ei ) 11.2 kcal/mol and θi ) 45°. For the Ne + H-SAM simulations: 〈Ef〉 ) 3.25 kcal/ mol, fB the fraction of the Boltzmann component in the P(Ef) distribution is 0.72, TB the temperature of the Boltzmann component is 605 K, and 〈Nhop〉 ) 6. The ratio 〈Ef〉/Ei for this simulation is 0.325. For the O(3P) + H-SAM simulation, these properties are 〈Ef〉 ) 2.96 kcal/mol, fB ) 0.71, TB ) 467 K, and 〈Nhop〉 ) 4, with 〈Ef〉/Ei ) 0.264. The somewhat more efficient energy transfer to the H-SAM by O(3P) collisions, as compared with Ne collisions, is consistent with O(3P)’s slightly more attractive interaction with the H-SAM. The well depth is ∼0.1 kcal/mol deeper for O(3P)‚‚‚CH4 than for Ne‚‚‚CH4. However, the above comparison indicates that O(3P) and Ne collisions with H-SAM surfaces have similar energy transfer dynamics. In contrast to the O(3P) simulations reported here, where penetration of the H-SAM is found for Ei values as small as 0.12 kcal/mol, no significant penetration was found in previous simulations and analyses of experiments for Ne + H-SAM scattering with Ei of 0.4-12.9 kcal/mol.25 However, a careful comparison of the two studies shows their differences concerning the importance of surface penetration are overall rather small and may arise in part from differences in the models and conditions for the two simulations. In the current study, an explicit-atom model is used for the H-SAM, and its temperature


Tasic´ et al.

Figure 6. Representative histograms of the probability of ∆χ for different Ei and θi values. All values of θf are included in the P(∆χ) distributions. For thermal desorption, P(∆χ) is uniform.

Figure 7. Plots of the average minimum height 〈hmin〉 and average residence time 〈tres〉 vs the incident polar angle θi, for collision energies Ei of 0.12, 2.30, 11.2, 71.0, and 120.5 kcal/mol.

is 300 K. A united-atom model is used for the H-SAM, at a temperature of 135 K, in the Ne + H-SAM study.25 Previous work has shown that the united-atom and explicit-atom models for the H-SAM give similar scattering results.23 However, the colder 135 K surface is more rigid, with smaller thermal fluctuations providing “openings” for penetration. In the current O(3P) + H-SAM study, penetration is defined as the O(3P) atom attaining a minimum height, hmin, less than 14.5 Å, as compared with the 14.7 Å average distance between the terminal C-atoms and the Au substrate at 300 K. The mechanism for O(3P) penetration at Ei ) 0.12 kcal/mol is first

physisorption on the top of the H-SAM, followed by migration into the surface. Though Table 5 shows the penetration probability is ∼10%, the probability of penetration beyond the interfacial region is very low. To obtain a penetration probability comparable with the Ne + H-SAM study,25 only O(3P) + H-SAM events in the forward direction with |∆χ| < 30° were selected, since only in-plane forward scattering was measured for Ne + H-SAM. For Ei ) 0.12 kcal/mol, these forward scattered O(3P) atoms have a 9% and 5% probability of penetrating deeper than 12.0 Å at θi of 15° and 75°, respectively. Table 5 shows that, for Ei > 0.12 kcal/mol, the penetration



Figure 8. Semilog plots of the final translational energy of outgoing O(3P) as functions of the average minimum height 〈hmin〉 and average number of hops 〈Nhop〉, for different collision energies Ei. The 〈hmin〉 and 〈Nhop〉 are the values for the different θi.

Figure 9. Scatter plots of Ef vs hmin and Nhop vs hmin, for the ensemble of trajectories with Ei ) 120.5 kcal/mol and θi ) 30°.

TABLE 9: Average Scattering Anglesa,b Ei

θi

〈θf〉

〈abs(∆χ)〉

Pforwardc

0.12

15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75 15 30 45 60 75

46 ( 20 44 ( 20 45 ( 19 46 ( 20 45 ( 19 43 ( 20 45 ( 20 44 ( 19 47 ( 19 47 ( 20 45 ( 20 45 ( 19 48 ( 20 53 ( 19 60 ( 18 41 ( 20 42 ( 19 44 ( 19 52 ( 18 68 ( 13 38 ( 20 39 ( 19 42 ( 19 51 ( 19 69 ( 12

87 ( 52 90 ( 51 89 ( 53 89 ( 51 89 ( 51 87 ( 52 89 ( 53 86 ( 51 76 ( 50 74 ( 52 83 ( 50 83 ( 52 74 ( 51 55 ( 45 31 ( 33 89 ( 50 85 ( 52 78 ( 54 51 ( 46 14 ( 13 87 ( 51 85 ( 53 79 ( 52 54 ( 47 14 ( 15

0.53 0.49 0.51 0.49 0.52 0.55 0.50 0.54 0.62 0.65 0.56 0.55 0.66 0.80 0.94 0.50 0.54 0.60 0.82 1.00 0.52 0.54 0.60 0.78 1.00

2.30

11.20

71.00

120.50

a Energy is in kcal/mol and angles are in degrees. b Uncertainties are standard deviations. The standard deviations of the mean are less than 1° for 〈φf〉 and less than 3° for 〈abs(∆χ)〉. c Probability of forward scattering trajectories, those with 〈abs(∆χ)〉 < 90°.

probability depends on θi where a direct penetration pathway, without initial physisorption, becomes important for the smaller θi. The probability of O(3P) penetration at Ei ) 11.2 kcal/mol, for scattering in the forward direction with |∆χ| < 30°, is 11% for hmin < 12.0 Å at θi ) 15°, 2% for hmin < 13.0 Å at θi ) 45°, and 0.3% for hmin < 14.5 Å at θi ) 75°. For Ei ) 2.30 kcal/mol, this probability is 6% for hmin < 12.0 Å at θi ) 15° and is smaller for the larger θi. The only initial conditions, for

which there is significant O(3P) penetration of the H-SAM beyond the interfacial region and which are representative of the Ne + H-SAM study,25 are those with θi ) 15° and Ei < 11.2 kcal/mol. However, there is not a direct comparison between these initial conditions and those of the Ne + H-SAM study. In future work, it would be of interest to investigate in detail the effect of the H-SAM temperature on the penetration dynamics. As for a gas-phase reaction, there is a reaction path and potential energy barrier for an atom to penetrate the surface. If the surface is in its classical potential energy minimum, then this barrier can only be overcome by the atom’s collisional energy. However, in a manner similar to the way reactant vibrational energy may promote a gas-phase reaction, increasing the temperature of the surface may promote penetration. For a gas-phase reaction, the vibrational enhancement effect arises from a coordinate displacement. For the H-SAM, the effect of increasing the temperature is to promote “openings” in the surface, which lower the penetration barrier and, thus, enhance penetration. B. Comparison with Previous O(3P) + H-SAM Simulations. In a previous study,19 Troya and Schatz performed QM/ MM direct dynamics simulations to study the reactive and inelastic scattering dynamics for hyperthermal (5 eV) O(3P) atoms colliding with an octyl-thiolate H-SAM adsorbed on Au{111}. The QM component of the simulation model was represented by the MSINDO semiempirical model,53 which provides reasonable potential energy surface properties for the O(3P) + H-SAM reactive channels. MSINDO was not reparametrized to fit ab initio potential energy surface properties. The 5 eV ) 115.3 kcal/mol collision energy considered by Troya and Schatz is similar to the 120.5 kcal/mol collision energy of the current study, and it is of interest to compare the inelastic scattering dynamics found from the two studies. However, first, several differences between the studies are described. As described above, the current study uses a potential


Tasic´ et al.

Figure 10. 〈Ef〉 vs θf and ∆χ for the trajectories at Ei values of 0.12 and 120.5 kcal/mol and at θi values of 15° and 75°. Ef values for all ∆χ are included for each θf bar, and similarly, Ef values for all θf are included for each ∆χ bar.

energy function that only includes O(3P) + H-SAM inelastic channels. Also, in comparison with the current study where impact sites are randomly selected on the H-SAM surface and the azimuthal angle is selected randomly, random initial conditions were not chosen in the QM/MM study of Troya and Schatz. The initial azimuthal angle χi was set to either 0° or 180°, with the former and latter resulting in near-parallel and near-perpendicular collisions with the alkyl chains, respectively (see Figure 3 of ref 19). In addition, the surface target region for trajectories was a 4.2 × 2.6 Å2 region at the top of the CH3 group of the central alkyl chain of the H-SAM. This sampling emphasizes the reactive region of the H-SAM and neglects other regions such as channels between the alkyl chains. Values of 〈Ef〉/Ei and θf obtained from the QM/MM simulations and the current work are compared in Table 10. The 〈Ef〉/

Ei values for the two studies are in qualitative agreement, with the QM/MM results for χi ) 0° in better agreement with the current study. The agreement between the 〈θf〉 values of the two studies is less satisfactory. For this comparison, the QM/ MM results for χi ) 180° are in the overall better agreement with the current study. The disagreement in comparing the 〈θf〉 is not particularly surprising, since the initial azimuthal angle and surface impact site should have an important effect on 〈θf〉. Neither is chosen randomly in the QM/MM simulations. There are similarities in the atomic-level dynamics of the previous QM/MM and the current study. For both studies, trajectories which are near-parallel to the alkyl chains have a propensity for surface penetration and trapping. In addition, penetration followed by trapping leads to substantial energy transfer but does not thermalize the O(3P) atoms (see Table 5).



Figure 11. Comparison of experimental and current simulation values of 〈Ef〉 vs θf. The experimental results are for in-plane scattering off liquid squalane. The simulations results identified by the points do not restrict ∆χ. The solid, bold lines restrict ∆χ to (10° (see text).

Two simulations have investigated the dynamics of the O(3P) reaction with the H-SAM. Li et al.18 studied incident energies of 19.1 kcal/mol and less, and Troya and Schatz19 investigated the hyperthermal energy of 115.3 kcal/mol (5 ev). The zeropoint energy corrected barriers for abstraction of primary and secondary H-atoms from an alkyl chain are 5-8 kcal/mol,14,16-18 while the barriers for C-C and C-H bond rupture are 45-50 kcal/mol.13,14,16,17 As discussed above, the simulations have restricted the O(3P) collisions to a surface target area on top of a CH3 group and have only considered the azimuthal angles of 0° and 180°. No attempt was made to randomly sample the gassurface collisions. In the Li et al. simulation,18 H-atom abstraction from the terminal CH3 group was observed and, for a small number of these events, the OH product abstracted an H-atom to form H2O. Given the limited target area, no penetration of the surface was observed and, thus, there was no H-atom abstraction from -CH2- groups within the surface. The simulations reported here indicate, that with proper random sampling, penetration of the H-SAM occurs for low collision energies and, thus, presumably H-atom abstraction will occur in the surface interior. At a 5 eV collision energy, Troya and Schatz19 find H-atom abstraction from the terminal C-atom (C1) and the interior carbons C2-C4. The C-H bond rupture channel occurs primarily at C1, with a small probability at C3. C-C bond rupture occurs between C1 and C2. These simulations show that the relative importance of the abstraction and bond rupture channels depends on the azimuthal and incident angles, since direct access to their TS’s depends on the values for these angles. The model used for these simulations allows no reaction below

C4 and an appreciable fraction of the trajectories results in O(3P) and OH trapped in the surface interior. With a complete model, a sizable fraction of these radicals will abstract H-atoms. More surface penetration is expected if the sampling is extended beyond the limited target area. It will be of interest to see if this increased penetration primarily enhances abstraction, at the expense of the bond rupture channels. C. Comparison with Experiments. Minton and co-workers have performed experiments7 to study the dynamics of O(3P) collisions with liquid squalane at Ei values of 71.0 and 120.5 kcal/mol and θi values of 30°, 45°, and 60°. They measured the in-plane, that is, ∆χ ) 0°, energy distribution P(Ef) for the scattered O(3P) atoms as a function of θf, and 〈Ef〉 vs θf is plotted in Figure 11 for the different Ei, θi experiments. For each set of experiments, there is a near-linear increase in 〈Ef〉 with θf. For a fixed θf, 〈Ef〉 increases with an increase in θi. It is of interest to compare these experimental results with the current simulations at the same Ei and θi. The simulations are for a different surface, that is, the n-decanethiolate H-SAM instead of squalane, and use a model PES which does not include the O(3P) + surface reactive channels. These differences in the experiments and simulations may affect the comparison. Two sets of simulations results are included in Figure 11. The points are values of 〈Ef〉 vs θf without any constraint on ∆χ, in contrast to the in-plane experimental measurements. These simulation results follow the trends of the experimental curves; that is, there is an increase in 〈Ef〉 with either θi or θf. However, the 〈Ef〉 values from the simulations are considerably smaller. To obtain a more direct comparison with the experiments, 〈Ef〉 values were determined from the simulation results


Tasic´ et al.

TABLE 10: Comparison of Simulations of Inelastic Scattering: 〈Ef〉 and 〈θf〉a QM/MM, MSINDOb property

χi ) 0°

〈Ef〉/Ei 〈θf〉

0.068 22.3


0.11 28.0


0.23 33.8

χi ) 180° θi ) 30° 0.11 27.3 θi ) 45° 0.23 38.8 θi ) 60° 0.43 52.5

this work χi random

TABLE 11: 〈θf〉 for Penetrating O3(P) Atomsa 〈θf〉b Ei

θi

Nhop ) 0

Nhop > 0

11.20

15 30 45 60 75 15 30 45 60 75 15 30 45 60 75

35 ( 8 28 ( 17

39 ( 18 36 ( 15 43 ( 23 59 ( 13

35 ( 20 33 ( 16 38 ( 18 33 ( 11

38 ( 19 37 ( 21 39 ( 20 37 ( 18

30 ( 16 31 ( 14 33 ( 18 36 ( 15

37 ( 20 37 ( 20 36 ( 19 43 ( 19

0.051 39.2 0.11 42.2

71.00

0.30 50.9

a Simulations from Table 4 of ref 19. The initial azimuthal angle χ i is set either to 0° or 180°; see Figure 3 in ref 19. b Ei for the QM/MM, MSINDO, and current calculations is 115 and 120.5 kcal/mol, respectively. 〈θf〉 is in degrees.

for ∆χ in the range of (10°. This restriction on ∆χ reduces the number of available trajectories in the sample and, thus, introduces considerable scatter. To provide some “smoothing” to this scatter, the resulting 〈Ef〉 vs θf plots were each fit with a quadratic function 〈Ef〉 ) a + bθf + cθf 2, and these fits are presented in Figure 11 by the solid, bold lines. Restricting ∆χ is seen to give better agreement with experiment for the large θf. This is particularly the case for θi values of 45° and 60°. There are two possible origins of the differences between the experimental and simulation plots of 〈Ef〉 vs θf. The higher 〈Ef〉 values for the experiment may be a result of less efficient energy transfer to liquid squalane as compared with the H-SAM. Differences in the energy transfer dynamics are not unexpected for such two quite distinctive surfaces, despite some of their similarities. The H-SAM consists of ordered, parallel chains that are free to move laterally (sideways), whereas squalane has a disordered liquid structure. The ability of O(3P) to penetrate deep between the chains of the H-SAM may enhance energy transfer. The O(3P) + H-SAM simulations show that the efficiency of energy transfer depends on whether the trajectories directly scatter, physisorb on the surface, or penetrate the surface. The relative probability of these trajectory types is expected to be different for O(3P) + squalane scattering, giving rise to a different overall energy transfer efficiency. Additionally, for O(3P) + squalane scattering, the energy transfer may be less dependent on θi, as compared with O(3P) + H-SAM, because of the former’s irregular surface structure. The other possibility is that O(3P) + hydrocarbon surface reactions preferentially occur for collisions which transfer a substantial energy to the surface. Oxygen atoms that give up less energy to the surface, that is, that retain more of their initial kinetic energy in translation, may be less likely to undergo a reaction. Thus, these less-relaxed oxygen atoms preferentially scatter and contribute to the experimental P(Ef) distribution for inelastic O(3P) scattering, whereas the more-relaxed oxygen atoms preferentially react. Since reactions are not allowed in the current simulations, all oxygen atoms contribute equally to P(Ef) regardless of how much energy they transfer to the surface. This may artificially lower the 〈Ef〉 values in the simulations as compared with the experiments. O(3P) atoms which penetrate the surface and are trapped for a period of time are very likely to react. In the current simulation, these events do not lead to reaction and instead scatter with appreciable transfer of energy. This may shift the P(Ef) distribution to lower Ef. D. Dynamics of O(3P) + H-SAM Penetrating Trajectories. Trajectories show that a hyperthermal, that is, Ei values of 71.0 and 120.5 kcal/mol, O(3P) atom may move through multiple “channels” of the alkyl chains as it descends at a high speed.

120.50

a Penetrating trajectories are here defined as those with hmin < 12.2 Å, the average height of the 3rd C-atom from the top. b The uncertainties are the standard deviations of the θf distribution. The standard deviation of the mean is less than 1°.

The likelihood that the atom passes through multiple channels is highest when its direction is perpendicular to the alignment of the alkyl chains and lowest when its direction is aligned with the chains. Once trapped near the Au substrate, the oxygen atom tends to remain inside a particular channel for a significant period of time. However, it is possible for the oxygen atom to move between channels. The O(3P) atom spends most time bouncing between three nearest-neighbor chains, but as the chains undergo their regular thermal motion, an opening occasionally forms between two of the chains, providing an opportunity for the oxygen atom to pass through and enter another channel. The trapped oxygen atom may change channels several times within 50 ps. The O(3P) atoms that penetrate the H-SAM leave the surface with an average scattering angle 〈θf〉 nearly the same as the tilt angle of 34° for the alkyl chains. These results are given in Table 11. Thus, these atoms depart from “channels” of the surface with their velocity vectors nearly parallel to the direction of the backbone of the alkyl chains. This type of scattering is found for the penetrating atoms that scatter with either Nhop ) 0 or Nhop > 0. For the latter Nhop, the scattering becomes more random, with θf ≈ 45°, for the larger θi. The scattering observed here, with 〈θf〉 approximately equal to the alkyl chain tilt for O-atoms that penetrate the surface, is similar to the finding of Gibson et al.52 for Xe-atoms scattering off the 1-decanethiol SAM. They found deep penetration of Xeatoms between the aligned chains of the monolayer, with the angular intensity distribution peaked close to the chain’s tilt angle. However, in contrast to the current O(3P) + H-SAM study, for which physisorbed and penetrating O-atoms have overall similar final energies (see Table 6), the penetrating Xeatoms have much higher final energies than do those that physisorb. This difference between the O + H-SAM and Xe + H-SAM may arise from the larger Xe-atoms, which should give rise to a greater atom + H-SAM repulsive interaction when the Xe-atom penetrates the surface. This penetration/expulsion mechanism is called “directed ejection” for Xe + H-SAM scattering. For the O(3P) + H-SAM scattering, trajectories with only one inner turning point have much higher final energies than do those experiencing multiple hops. Under hyperthermal conditions, the factor determining the O-atom’s final energy appears to be the number of hops, rather than the depth of penetration, and there is no evidence that penetrating atoms are “expelled” from the surface with large kinetic energies.

Collisions of O3(P) Atoms with 1-Decanethiol V. Summary The simulations reported here and comparisons with previous simulations lead to the following conclusions concerning the dynamics of energy transfer in collisions of O(3P) with alkylthiolate self-assembled monolayer (H-SAM) surfaces. (1) Ne + H-SAM energy transfer dynamics have been studied in previous simulations for collision energies Ei of 12.9 kcal/ mol and lower.20-25 For these Ei, O(3P) + H-SAM scattering gives similar energy transfer dynamics. This is expected, since the Ne‚‚‚H-SAM and O(3P)‚‚‚H-SAM intermolecular potentials are similar. (2) There are three principal types of O(3P) + H-SAM trajectories: direct scattering from the top of the H-SAM, physisorption on the top of the H-SAM followed by desorption, and penetration into the H-SAM. Physisorption is the dominant event at the low Ei values of 0.12 and 2.30 kcal/mol. For the larger Ei values of 11.2, 71.0, and 120.5 kcal/mol, the dominant trajectory type depends on the incident angle θi. Surface penetration and direct scattering dominate at small and large θi, respectively. (3) The translational energy distribution of the scattered O(3P) atoms, P(Ef), may be deconvoluted into Boltzmann and nonBoltzmann components. As found in previous work,25 efficient energy transfer to the surface only requires several hops on/in the H-SAM after the initial collision. For large Ei values, the energy transfer to the surface is strongly dependent on θi. (4) The average final translational energy 〈Ef〉 of the scattered O(3P) atoms vs final polar angle is in qualitative, but not quantitative, agreement with O(3P) scattering off liquid squalane. This may result from different energy transfer dynamics for H-SAM and liquid squalane surfaces. Differences in the energy transfer dynamics might be expected, since the H-SAM surface is ordered, while that for squalane is rough and disordered. (5) The efficiency of O(3P) + H-SAM collisional energy transfer may be differentiated by three trajectory types, that is, direct scattering, physisorption on the surface, and penetration of the surface, and by further distinction between the different types of penetrating trajectories. The relative importance of these trajectory types is expected to depend on the surface structure (and possibly also temperature) and understanding their relative importance may provide a valuable approach for comparing collisional energy transfer for different surfaces (e.g., liquid hydrocarbons such as squalane vs self-assembled monolayers). Acknowledgment. This research is supported by the Air Force Office of Scientific Research and the Robert A. Welch Foundation. We are grateful to Tim Minton and Jianming Zhang for providing the experimental results in Figure 11. The authors wish to thank Oleg Mazyar and Asif Rahaman for identifying the potential parameters used in the simulations. References and Notes (1) Leger, L. J.; Visentine, J. T. J. Spacecr. Rockets 1986, 23, 505. (2) Minton, T. K.; Garton, D. J. In Chemical Dynamics in Extreme EnVironments; Dressler, R. A., Ed.; World Scientific: Singapore, 2001; p 420. (3) Rasoul, F. A.; Hill, D. T.; George, G. A.; O’Donnell, J. H. Polym. AdV. Technol. 1998, 9, 24. (4) Murad, E. J. Spacecr. Rockets 1996, 33, 131. (5) Garton, D. J.; Minton, T. K.; Alagia, M.; Balucani, N.; Casavecchia, P.; Volpi, G. G. Faraday Discuss. 1998, 108, 387. (6) Garton, D. J.; Minton, T. K.; Alagia, M.; Balucani, N.; Casavecchia, P.; Volpi, G. G. J. Chem. Phys. 2000, 112, 5975. (7) Zhang, J.; Garton, D. J.; Minton, T. K. J. Chem. Phys. 2002, 117, 6239. (8) Wagner, A. J.; Wolfe, G. M.; Fairbrother, D. H. J. Chem. Phys. 2004, 120, 3799.

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Dynamics of Energy Transfer in Collisions of O (3P) Atoms with a 1

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