State Selected Ion/Surface Scattering

The detailed mechanisms of many surface reactions can be revealed through dynamical studies on well-defined systems. In particular, the initial electronic, vibrational, and rotational energies, as well as the molecule’s internuclear axis alignment and point of impact at the surface, are important in determining whether a molecule will react upon collision with a surface. These effects are delineated through state-resolved experiments involving a molecular beam, laser excitation, an ion beam, and angle-resolved detection techniques.


Dissociative Scattering: NO+ + GaAs(110) –> O-, NO-

The effects that collision energy, initial vibrational state, and surface impact parameter have on dissociation were measured for NO+ on GaAs(110). The translational energy threshold for O- emergence is approximately 25 eV despite the low 4.6 eV endoergicity of the above reaction. Classical trajectory calculations demonstrate that the relatively high threshold is consistent with an impulsive dissociation mechanism. Here, a portion of the incident translational energy is transferred upon impact into rovibrational energy within the diatom. If the internal energy is sufficient to break the bond, the diatom will dissociate within half a vibrational period (~10 fs) after impact. The measured kinetic energy of the O- fragment indicates that approximately 60% of the incident NO+ energy is absorbed by the surface. Energetic arguments suggest that electron transfer to NO+ and subsequently to NO produces only the ground electronic states of NO and NO-, respectively.

The emergence of O- in the above reaction is greatly enhanced by the addition of 6 quanta of vibration in the incident NO+ ion. Only 1.7 eV of vibrational energy results in a 140% enhancement in the O- yield; yet, if the same amount of initial energy is instead deposited as translational energy, an enhancement of merely 14% is observed. This suggests that under these conditions, vibrational energy is an order of magnitude more effective than translational energy in producing scattered O- ions.

Classical trajectory calculations fail to predict a vibrational enhancement in dissociation; however, they do not adequately treat the electron transfer dynamics. Alternatively, a time-dependent quantum mechanical approach quantitatively predicts the observed vibrational enhancement. The latter simulations provide the following picture: neutralization on the imcoming trajectory forms a vibrational coherence within the impinging neutral NO molecule. The time required for the molecule to translate ~2 Angstroms from the region of neutralization to the classical turning point is approximately one half of a vibrational period. Consequently, the NO+(v=6) molecule becomes more compressed upon impact than does the NO+(v=0) species. Furthermore, NO+(v=6) recoils faster than does NO+(v=0), and an electron is more easily attached as the dissociating diatom departs from the surface.

Only the v=0 vibrational level of the ground electronic state of NO- is detected in this study. The scattered NO- yield shows no dependence on the number of quanta in the incident NO+ ion across a wide range (v = 0–6). Time-dependent quantum mechanical simulations confirm this observation and suggest that although more NO- is formed as the number of vibrational quanta in incident NO+ is increased, the result is a hotter distribution of NO- vibrational populations. The yield of the ground vibrational level of NO- remains unaffected by the incident NO+(v) vibrational level.

The surface science community frequently assigns particular binding sites on a catalytic surface to be favorable "active sites." We performed a set of scattering experiments to directly probe the site-dependence to O- formation in the reaction of NO+ with a corrugated GaAs(110) surface. Our experiments showed an intriguing asymmetric scattering distribution for the O- products. Classical trajectory simulations were combined with an empirical surface opacity function for electron attachment, i.e., a function which relates the probability for electron attachment to the last point of impact the oxygen atom makes with the surface before scattering. Together, experiment and simulation implicate the dangling bond states of GaAs(110) as being solely responsible for electron attachment to O in the formation of O-.

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Dissociative Scattering: NO+ + Ag(111) –> O- , NO-

Molecular alignment refers to an anisotropic distribution of a molecule's internuclear-axis direction. In these experiments, we scatter an aligned sample of molecules on a flat surface in order to determine the relationship between molecular approach geometry and reaction probability. Resonance enhanced multiphoton ionization (REMPI) of NO forms NO+ in virtually a single rovibrational quantum state. Moreover, performing REMPI with linearly polarized laser light will photoselect only those molecules having a preferential direction in their rotational angular momentum vectors. Although the resulting NO+ ions will rotate millions of times in the vacuum before striking the surface, the rotational angular momentum vector will be preserved, because NO+ is formed in a closed-shell singlet sigma state. Classically, a diatom rotates in a plane perpendicular to the rotational angular momentum vector. Quantum mechanically, the internuclear-axis probability distribution of NO+ will be anisotropic and is controlled in part by the laser polarization direction (E) and the particular resonant transition excited in the REMPI process. The above figure represents four distinct internuclear-axis probability distributions which can be presented to the surface. By measuring the product yields corresponding to each one of these incident NO+ alignment distributions, we learn how the reaction probability depends on the molecular approach geometry.

The 20 eV threshold energy for O- emergence implicates an impulsive dissociation mechanism. To further explore the reaction dynamics in this system, we measured how the reaction probability depends on the internuclear-axis direction of incident NO+. This dependence is most easily quantified by an alignment preference parameter, termed Beta. A positive value of Beta indicates that NO+ is more reactive when it collides with it's internuclear axis perpendicular ("end-on" approach) rather than parallel ("side-on" approach) to the surface. A negative value of Beta represents the opposite preference, and Beta=0 suggests no alignment preference for the reaction. The data indicates that the reaction proceeds more efficiently when the incident molecule strikes the surface with an end-on type geometry rather than a side-on geometry. This alignment preference diminishes at higher collision energies. Although, we can not definitively measure the reaction probability as a function of molecular orientation, a quantitative determination of Beta restricts the set of possible reagent orientation preference functions. For example, at a collision energy of 35 eV, any of the following functional forms are consistent with the measured Beta=0.3.

In each of these curves, near end-on collisions have a higher reaction probability than side-on approaches; in fact, the reaction probability for the optimal orientation is more than double that for the least reactive orientation. Classical trajectory calculations suggest that the optimal orientation for impulsive dissociation is approximately 25 degrees off normal, because this orientation induces the greatest degree of rovibrational excitation in the diatom.

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Dissociative Scattering: OCS+ + Ag(111) –> S-, O-, SO-

State-selectively prepared OCS+(v1, v2, v3) was directed at a clean Ag(111) surface. Three major scattered anionic products were observed: O-(2P), S-(2P), OS-. The sharp threshold behavior for O- emergence suggests that collision induced dissociation of ground state neutralized OCS produces O(1D) which attaches an electron as it scatters from the surface. This was confirmed through a measurement of the scattered O- kinetic energy distribution.

In contrast to O- emergence, the low threshold and final kinetic energy distribution of scattered S-(2P) suggests that neutralization of OCS+ also populates one or more electronically excited dissociative states of neutral OCS. Electron attachment to the resulting S(1D) fragment would produce S-(2P) with a low appearance threshold.

The OS- product channel is particularly interesting, because it arises from removal of the central carbon atom. The appearance threshold for OS- is at 30 eV, suggestive of a sharply bent transition state, accessed only at high collision energies when impact severely distorts the incident molecule.

For this system, state-selective ionization indicated that small initial excitations of the C–O stretch, the C–S stretch, and the O–C–S bend vibrations had no perceivable effect on the ion's reaction probability with the surface. However, only excitations of 1 or 2 quanta (<0.25 eV) could be achieved by the REMPI state-preparation technique.

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Atom-Abstraction: NO+ + O/Al(111)

Atom abstraction is an elementary process by which an atom is transferred to or from an incident molecule as the molecule impacts a surface. Gas surface reactions are often assigned to one of two limiting mechanisms: Langmuir-Hinshelwood or Eley-Rideal. In the Eley-Rideal mechanism, an incident gas particle reacts directly with a surface adsorbate, whereas in the Langmuir-Hinshelwood mechanism both reagents thermally equilibrate with the surface prior to reaction. Distinguishing between these two mechanisms is difficult unless one carefully probes the reaction dynamics.

In the figure to the right, the NO2– yield is plotted as a function of NO+ collision energy for an Al(111) surface dosed with 750 L O2. The NO2– yield exhibits a threshold energy, 9 ± 1 eV, above which the yield increases linearly with NO+ translational energy. The observed threshold is consistent with thermodynamic estimates of the reaction barrier. The NO2– yield also scales with the total coverage of oxygen on the surface. At the high coverages discussed here, most of the abstracted atoms leading to NO2– formation originate from islands of chemisorbed oxygen/oxide species on the Al surface.

In a Langmuir-Hinshelwood mechanism, the incident NO would thermally accommodate on the surface, diffuse to a chemisorbed oxygen site, react, and desorb as NO2. However, the velocity distribution of scattered NO2– products(points) is nonthermal and cannot be described by a Maxwell-Boltzmann distribution at the surface temperature (red curve) or by the NO+ incident velocity distribution (blue curve). Moreover, the mean translational energy of scattered NO2– increases with NO+ collision energy. This correlation implies that the reaction occurs via a direct collision between an incident NO molecule and an adsorbed oxygen atom. The data indicate that neither the incident NO molecule nor the scattered NO2– product resides on the surface long enough to become thermally accommodated. It is proposed that NO2– is formed by a three-step mechanism: incident NO+ is neutralized close to the surface; nascent NO impacts an adsorbed oxygen atom; and O– is abstracted by NO to form NO2– via an Eley-Rideal mechanism. A statistical model is currently being tested to see if it can accurately simulate the experimental data.

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Charge Transfer - An Anomolous Resonance: Br+ + Pt(111) –> Br-

In contrast to conventional charge-transfer theory, the scattering of state-selected Br+(3P2) on Pt(111) shows a dramatic enhancement in the yield of Br-(1S0) at an impact energy of 26 eV. Coincident with this resonance, the Br-(1S0) product scatters with additional translational energy. The observed scattering behavior is consistent with a collision-induced deformation of the lattice that rebounds coherently with the departing projectile. The experimental data demonstrate the strong coupling between the motion of the platinum lattice and the surface electronic states responsible for charge transfer.

Classical trajectories are performed to identify the amount of energy transfer associated with Br striking different impact sites on the surface. The simulations indicate that the resonance feature is consistent with scattering from the three-fold hollow site. As seen in the movies below, the collision deforms the surface to create a large vacancy site while the atom is still proximate to the surface. The lattice distortion perturbs the local electronic structure, facilitating efficient charge transfer.

Click on frames to see quick time movies of a trajectory for 25 eV Br striking a 3-fold hollow site on Pt111). The movies differ only in the camera angle.


Dissociative Scattering: Br2+ + Pt(111) –> Br-, Br2-

Both translational energy and vibrational energy play an important role in the emergence of Br- and Br2- when Br2+ collides with Pt(111). A similar resonance is observed as that seen for Br+(3P2) on Pt(111); however, the resonance is shifted to higher energy. Stay tuned for more details.

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