Quasiclassical trajectory study of energy transfer and collision-induced dissociation in hyperthermal Ar + CH4 and Ar + CF4 collisions

We present a study of energy transfer in collisions of Ar with methane and perfluoromethane at hyperthermal energies (E(coll) = 4-10 eV). Quasiclassical trajectory calculations of Ar + CX(4) (X = H, F) collisions indicate that energy transfer from reagents' translation to internal modes of the alkane molecule is greatly enhanced by fluorination. The reasons for the enhancement of energy transfer upon fluorination are shown to emerge from a decrease in the hydrocarbon vibrational frequencies of the CX(4) molecule with increasing the mass of the X atom, and to an increase of the steepness of the Ar-CX(4) intermolecular potential. At high collision energies, we find that the cross section of Ar + CF(4) collisions in which the amount of energy transfer is larger than needed to break a C-F bond is at least 1 order of magnitude larger than the cross sections of Ar + CH(4) collisions producing CH(4) with energy above the dissociation limit. In addition, collision-induced dissociation is detected in short time scales in the case of the fluorinated species at E(coll) = 10 eV. These results suggest that the cross section for degradation of fluorinated hydrocarbon polymers under the action of nonreactive hyperthermal gas-phase species might be significantly larger than that of hydrogenated hydrocarbon polymers. We also illustrate a practical way to derive intramolecular potential energy surfaces for bond-breaking collisions by improving semiempirical Hamiltonians based on grids of high-quality ab initio calculations.     

Theoretical study of the effect of surface density on the dynamics of Ar + alkanethiolate self-assembled monolayer collisions

We present a classical-trajectory study of energy transfer in collisions of Ar atoms with alkanethiolate self-assembled monolayers (SAMs) of different densities. The density of the SAMs is varied by changing the distance between the alkanethiolate chains in the organic monolayers. Our calculations indicate that SAMs with smaller packing densities absorb more energy from the impinging Ar atoms, in agreement with recent molecular-beam scattering experiments. We find that energy transfer is enhanced by a decrease in the SAM density because (1) less dense SAMs increase the probability of multiple encounters between Ar and the SAM, (2) the vibrational frequencies of large-amplitude motions of the SAM chains decrease for less dense SAMs, which makes energy transfer more efficient in single-encounter collisions, and (3) increases in the distance between chains promote surface penetration of the Ar atom. Analysis of angular distributions reveals that the polar-angle distributions do not have a cosine shape in trapping-desorption processes involving penetration of the Ar atom into the alkanethiolate self-assembled monolayers. Instead, there is a preference for Ar atoms that penetrate the surface to desorb along the chain-tilt direction.     

Dynamics of energy transfer in peptide-surface collisions

Classical trajectory simulations are performed to study energy transfer in collisions of protonated triglycine (Gly)(3) and pentaglycine (Gly)(5) ions with n-hexyl thiolate self-assembled monolayer (SAM) and diamond [111] surfaces, for a collision energy E(i) in the range of 10-110 eV and a collision angle of 45 degrees. Energy transfer to the peptide ions' internal degrees of freedom is more efficient for collision with the diamond surface; i.e., 20% transfer to peptide vibration/rotation at E(i) = 30 eV. For collision with diamond, the majority of E(i) remains in peptide translation, while the majority of the energy transfer is to surface vibrations for collision with the softer SAM surface. The energy-transfer efficiencies are very similar for (Gly)(3) and (Gly)(5). Constraining various modes of (Gly)(3) shows that the peptide torsional modes absorb approximately 80% of the energy transfer to the peptide's internal modes. The energy-transfer efficiencies depend on E(i). These simulations are compared with recent experiments of peptide SID and simulations of energy transfer in Cr(CO)(6)(+) collisions with the SAM and diamond surfaces.     

Theoretical studies of hyperthermal O(3P) collisions with hydrocarbon self-assembled monolayers

We present a dynamics study of inelastic and reactive scattering processes in collisions of hyperthermal (5 eV) O(3P) atoms with a hydrocarbon self-assembled monolayer (SAM). Molecular-dynamics simulations are carried out using a quantum mechanics/molecular mechanics (QM/MM) interaction potential that uses a high quality semiempirical Hamiltonian for the QM part and the MM3 force field for the MM part. A variety of products coming from reaction are identified, including H abstraction to generate OH, O atom addition to the SAM with subsequent elimination of H atoms, and direct C-C breakage. The C-C breakage mechanism provides a pathway for significant surface mass loss in single reactive events whereas the O addition-H elimination channel leads to surface oxidation. Reaction probabilities, product energy, and angular distributions are examined to gain insight on polymer erosion in low Earth orbit conditions and on fundamentals of inelastic and reactive hyperthermal gas-surface interactions.     

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.     

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.     

Quantifying electron transfer during hyperthermal scattering of C60+ from Au(111) and n-alkylthiol self-assembled monolayers

A tandem time-of-flight mass spectrometer with an intermediate surface was used to quantify electron transfer during glancing incidence scattering of hyperthermal C(60) (+) (E(coll)=250-500 eV, theta(in)=75 degrees ) from (i) self-assembled monolayers of n-alkylthiols on gold (of various chain lengths), (ii) partly fluorinated alkylthiols on gold, as well as (iii) clean gold surfaces. Self-assembled monolayers (SAMs) behave as insulating layers with their thicknesses determining the electron tunneling probability during collision. Correspondingly, a roughly exponential dependence of the neutralization probability on the chain length n was found. A pronounced dependence of the neutral yield on the primary beam kinetic energy indicates that dynamic SAM deformation and associated projectile penetration depth also play a role in determining electron transfer efficiency. Results are consistent with the molecular deformability of SAMs as determined with other experimental methods.     

Translational to vibrational energy conversion during surface-induced dissociation of n-butylbenzene molecular ions colliding at self-assembled monolayer surfaces

Translational to vibrational (T-->V) energy conversion in the course of inelastic collisions of n-butylbenzene molecular ions with thiolate self-assembled monolayer (SAM) gold surfaces is studied to better understand internal energy uptake by the hyperthermal projectile ions. The projectile ion is selected by a mass spectrometer of BE configuration and product ions are analyzed using a quadrupole mass analyzer after kinetic energy selection with an electric sector. The branching ratio for formation of the fragment ions m/z 91 and m/z 92, measured over a range of collision energies, is used to estimate the average internal energy with the aid of calculations based on unimolecular dissociation kinetics [Rice-Ramsperger-Kassel-Marcus (RRKM) theory]. The measured T-->V conversion efficiencies (the fraction of the laboratory kinetic energy converted into internal energy) are 11 approximately 12% for dodecanethiolate SAM (H-SAM) and 19 approximately 20% for 2-perfluorooctylethanethiolate SAM (F-SAM), respectively, over ranges of a few 10s of eV. The values are similar to those reported earlier for other thermometer molecules undergoing surface collisions. Chemical sputtering leading to ionization of the surface is a prominent feature of the surface-induced dissociation (SID) spectra of n-butylbenzene acquired using the H-SAM surface but not the F-SAM surface because of the lower ionization energy of the former.     

Hyperthermal Ar atom scattering from a C(0001) surface

Experiments and simulations on the scattering of hyperthermal Ar from a C(0001) surface have been conducted. Measurements of the energy and angular distributions of the scattered Ar flux were made over a range of incident angles, incident energies (2.8-14.1 eV), and surface temperatures (150-700 K). In all cases, the scattering is concentrated in a narrow superspecular peak, with significant energy exchange with the surface. The simulations closely reproduce the experimental observations. Unlike recent experiments on hyperthermal Xe scattering from graphite [Watanabe et al., Eur. Phys. J. D 38, 103 (2006)], the angular dependence of the energy loss is not approximated by the hard cubes model. The simulations are used to investigate why parallel momentum conservation describes Xe scattering, but not Ar scattering, from the surface of graphite. These studies extend our knowledge of gas-surface collisional energy transfer in the hyperthermal regime, and also demonstrate the importance of performing realistic numerical simulations for modeling such encounters.     

Dissociative scattering of hyperthermal energy CF3+ ions from modified surfaces

Dissociative scattering of CF3+ ions in collision with a self-assembled monolayer surface of fluorinated alkyl thiol on a gold 111 crystal has been studied at low ion kinetic energies (from 29 to 159 eV) using a custom built tandem mass spectrometer with a rotatable second stage energy analyzer and mass spectrometer detectors. Energy and intensity distributions of the scattered fragment ions were measured as a function of the fragment ion mass and scattering angle. Inelastically scattered CF3+ ions were not observed even at the lowest energy studied here. All fragment ions, CF2+, CF+, F+, and C+, were observed at all energies studied with the relative intensity of the highest energy pathway, C+, increasing and that of the lowest energy pathway, CF2+, decreasing with collision energy. Also, the dissociation efficiency of CF3+ decreased significantly as the collision energy was increased to 159 eV. Energy distributions of all fragment ions from the alkyl thiol surface showed two distinct components, one corresponding to the loss of nearly all of the kinetic energy and scattered over a broad angular range while the other corresponding to smaller kinetic energy losses and scattered closer to the surface parallel. The latter process is due to delayed dissociation of collisionally excited ions after they have passed the collision region as excited parent ions. A similar study performed at 74 eV using a LiF coated surface on a titanium substrate resulted only in one process for all fragment ions; corresponding to the delayed dissociation process. The intensity maxima for these fragmentation processes were shifted farther away from the surface parallel compared to the thiol surface. A new mechanism is proposed for the delayed dissociation process as proceeding via projectile ions' neutralization to long-lived highly excited Rydberg state(s), reionization by the potential field between the collision region and entrance to the energy analyzer, and subsequent dissociation several microseconds after collisional excitation. A kinematic analysis of experimental data plotted as velocity Newton diagrams demonstrates that the delayed dissociation process results from the collisions of the ion with the bulk surface; i.e., the self-assembled monolayer surface acts as a bulk surface. A similar analysis for the highly inelastic collision processes shows that these are due to stronger collisions with a fraction of the thiol molecular chain, varying in length (mass) with the ion energy.     

Inelastic scattering dynamics of Ar from a perfluorinated self-assembled monolayer surface

Dynamics of Ar atom collisions with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) surface on gold were investigated by classical trajectory simulations and atomic beam scattering techniques. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface; in the UA model, the CF3 and CF2 units are represented as single pseudoatoms. Additionally the nonbonded interactions in both models are different. The simulations show the three limiting mechanisms expected for collisions of rare gas atoms (or small molecules) with SAMs, that is, direct scattering, physisorption, and penetration. Surface penetration results in a translational energy distribution, P(Ef), that can be approximately fit to the Boltzmann for thermal desorption, suggesting that surface accommodation is attained to a large extent. Fluorination of the alkanethiol monolayer leads to less energy transfer in Ar collisions. This results from a denser and stiffer surface structure in comparison with that of the alkanethiol SAM, which introduces constraints for conformational changes which play a significant role in the energy-transfer process. The trajectory simulations predict P(Ef) distributions in quite good agreement with those observed in the experiments. The results obtained with the EA and UA models are in reasonably good agreement, although there are some differences.     

Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

Reactive scattering of polyatomic ions in the hyperthermal collision energy range (<100 eV) is used to distinguish isomeric oxygenated adsorbates and to quantify their relative amounts when co-adsorbed at a surface. The self-assembled monolayers (SAMs) of interest are constructed from HO-terminated, CH₃O-terminated, and CH₃CH₂O-terminated dialkyl disulfides. Projectile ions used for ion/surface scattering experiments include CF₃⁺, SiCl₃⁺, and the molecular ion of pyridine, C₅H₅N√⁺. Each of these ions exhibits a unique scattered ion profile upon collision with the SAM monolayer surfaces, and so provides different information about the surfaces. Hydrogen atom abstraction by the C₅H₅N√⁺ ion is more prominent at the CH₃CH₂O- and CH₃O-terminated surfaces than the HO-terminated surface, while collisions of SiCl₃⁺ yield reactively scattered products which reflect the chemical composition of these surfaces. For instance, SiCl₂OH⁺ and SiCl₂OCH₃⁺ are scattered from the HO-terminated and CH₃O-terminated surfaces, respectively. Ion/surface collisions involving the CF₃⁺ ion produce chemically sputtered ions from the oxygenated adsorbates, which are valuable for quantitation of those groups. Preferential sputtering of the CH₃O-terminated versus the HO-terminated SAM surface is ascribed to favored thermochemistry and the more accessible CH₃O-terminated adsorbate. Fundamental ion/surface scattering processes, such as inelastic collisions leading to surface-induced dissociation (SID), ion/surface reaction, and chemical sputtering are examined over a range of collision energies for each of the ion/surface types mentioned, and their value in surface analysis is demonstrated.     

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.     

A molecular dynamics simulation of rebound,capture and diffusion in collisions at thermal energies between a rare gas atom and an argon cluster (n=125)

Molecular dynamics calculations have been performed to simulate the low energy collision (0.2 eV) of a rare gas atom (He, Ar, Xe) with a cluster of 125 argon atoms. Depending on its relative mass to argon, the projectile is either deflected (He) or captured (Ar, Xe) by the argon cluster. We have determined the deflection function of the He projectile that is scattered, and for Xe we have determined wether it stays near the surface of the cluster or migrates inside. These results have been discussed in the light of very simple models.     

Direct dynamics simulations of O(3P) + HCl at hyperthermal collision energies

The dynamics of the O(3P) + HCl reaction at hyperthermal collision energies were investigated using the quasiclassical trajectory method. Stationary points on the OClH 3A" and 3A' potential energy surfaces (PESs) were also examined. The lowest transition state leading to OCl + H on the 3A" surface is 2.26 eV above the reagents at the CCSD(T)/cc-pVTZ level of theory. This saddle point is bent and product-like. Direct dynamics calculations at the MP2/cc-pVTZ level of theory were used to investigate the excitation functions for OH + Cl, OCl + H, and O + H + Cl formation. OCl is formed mainly from small-impact-parameter collisions, and the OCl + H excitation function peaks around 5 eV, where it is similar in magnitude to the OH + Cl excitation function. The shape of the OCl + H excitation function is discussed, and features are identified that should be general to hyperthermal collision dynamics.     

Surface induced dissociation of chromium hexacarbonyl on fluorinated alkanethiolate surface in ion cyclotron resonance mass spectrometer: studies of energetics of the process using recursive internal energy distribution search method

Energy transfer in ion-surface collisional activation is characterized for 0–30 eV collisions of chromium hexacarbonyl molecular cations with a monolayer of fluorinated alkanethiolate self-assembled onto a solid gold surface. This surface was mounted on the back trapping plate of the Infinity® cell of a Bruker BioApex 7T ion cyclotron resonance mass spectrometer on the B-field axis orthogonal to the ion beam direction. Internal energy deposition was deduced from fragmentation spectra using a recursive internal energy distribution search method. The efficiency of energy transfer into the ion slowly increases with incident ion energy to a maximum value of 20% at about 23 eV collision energy. Approximate kinetic energy distributions of the fragments were measured by deducing the dependence of ion abundance on trapping potential. From the kinetic energy dependence on mass we infer that rapid decomposition of the molecular cation occurs after it recoils from the surface. Knowledge of both internal and kinetic energy distributions of collisionally activated ions enabled us to deduce the energy deposited into the self-assembled monolayer as a function of collision energy.     

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.     

Experimental and simulation study of neon collision dynamics with a 1-decanethiol monolayer

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV.     

Surface-Induced dissociation from a liquid surface

Mass-selected projectile ions in the tens of electronvolt energy range undergo surface-induced dissociation upon collision with a liquid perfluorinated polyether (PFPE) surface. The efficiency of translational-to-vibrational (T-V) energy transfer is similar to that observed for a fluorinated self-assembled monolayer (SAM) surface. The thermometer ion W(CO)^’ was used to detenrrine an average T-V conversion efficiency of 18% in the collision energy range of 30–50 eV. The surface can be bombarded for several hours without displaying any change in the scattered ion products. Ion-surface reactions occur with some projectiles and are analogous to those seen with the fluorinated SAM surface. For example, WF _? ⁺ (m=1–5) and W(CO)_nF _? ⁺ (n=1–2, m=1–2) are generated upon collisions of W(CO) ₆ ⁺ with the PFPE liquid surface. The ion-surface reactions observed suggest that F atoms and/or CF₃ groups are accessible for reaction while the oxygen atoms lie below the outermost surface layer. Chemical sputtering of the liquid surface also occurs and yields common fluorocarbon fragment ions, including CF ₃ ⁺ , C₂F ₅ ⁺ , and C₃F ₇ ⁺ and the oxygenated product CFO⁺. The liquid surface is remarkably free of hydrocarbon impurities. Collisions of the pyrazine and benzene molecular ions, both probes for hydrocarbon impurities, resulted in very little protonated pyrazine or protonated benzene.     

Tuning Charge Transfer in Ion–Surface Collisions at Hyperthermal Energies

Charge exchange in ion–surface collisions may be influenced by surface adsorbates to alter the charge state of the scattered projectiles. We show here that the positive‐ion yield, observed during ion scattering on metal surfaces at low incident energies, is greatly enhanced by adsorbing electronegative species onto the surface. Specifically, when beams of N⁺ and O⁺ ions are scattered off of clean Au surfaces at hyperthermal energies, no positive ions are observed exiting. Partial adsorption of F atoms on the Au surface, however, leads to the appearance of positively charged primary ions scattering off of Au, a direct result of the increase in the Au work function. The inelastic energy losses for positive‐ion exits are slightly larger than the corresponding ionization energies of the respective N and O atoms, which suggest that the detected positive ions are formed by surface reionization during the hard collision event.