E. B. Stechel

Multidimensional dynamics in the electron stimulated desorption of ammonia from Pt(111)

We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more ‘‘spinning’’ rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the ν2 symmetric deformation or ‘‘umbrella’’ mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a1 electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two‐dimensional potential energy surfaces (intramolecular bond angle and surface bond length)...

Ab initio calculations of the energetics of the neutral Si vacancy defect

Ab initio plane-wave pseudopotential calculations for the neutral silicon vacancy indicate a formation energy of 3.6 eV, with the surrounding lattice undergoing a tetragonal distortion with the nearby atoms forming two dimers having bond lengths 2.91 A. Close in energy is a tetrahedrally distorted structure in which the nearby atoms relax towards the vacancy by 12.6% of the bulk bond length. Additional distortions with trigonal symmetry were also investigated, but no stable structures were found. The symmetry, energetics, and geometry are found to be a sensitive function of the computational basis-set and supercell used in the plane-wave calculations.

Chemisorbed-molecule potential energy surfaces and DIET processes

Abstract We report the use of the local-density approximation, with and without gradient corrections, for the calculation of ground-state potential energy surfaces (PESs) for chemisorbed molecules. We focus on chemisorbed NO and ammonia on Pd(1 1 1) and compare our results with the latest experimental information. We then turn to two aspects of excited-state PESs. First, we compare first-principles calculations of the forces on an ammonia ion as a function of distance from the surface. We find that the image-charge model fails significantly at distances which are the most relevant for dynamics, closer than ∼3 A, and discuss reasons for the failure. We then summarize a purely electronic adiabatic model of the moleuule-surface bond and use empirical parameters to estimate hot carrier-produced excited states of chemisorbed NO. We find multiple PESs and a novel interpretation of the π ∗ resonance, seen in inverse photoemission.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

Abstract We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO 2 dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ o ) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO 2 on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the ∼ 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO 2 is N-bonded, ∼ 6° off-normal beams are observed. When the substrate is precovered with θ o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (∼9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.

DIET in the bulk: Evidence for hot electron cleavage of Si-H bonds in SiO2 films

Abstract The observed increase in leakage current through SiO2 films after hot electron exposure is ascribed to dissociation induced by electronic transitions (“DIET”) of bulk SiH bonds, producing mobile hydrogen. We use ab initio supercell bandstructure calculations at the local density functional level to locate features produced by hydrogen-containing defects in α-SiO2. The edge of the SiH σ∗ resonance is found to be about 2.7 eV above the conduction band rise, in good agreement with the observed threshold for hot electron induced damage in amorphous SiO2 films grown on Si substrates. The OH σ∗ resonance is almost 4 eV higher. Removing H from OH in the supercell does not affect the gap region (O− forms); however, removing H from SiH produces a mid-gap state, suggesting leakage current by hopping conductivity between Si dangling bonds. A Morse potential model is used to explore the dynamics of bond scission by short-lived ( σ∗ capture. Supercell calculations on interstitial atomic hydrogen indicate the energy cost to break an embedded SiH bond is about 0.6 eV less than in the gas phase. The DIET yield is substantially increased by reducing both ground and electron-attached state binding by this amount. While uncertainty over the displaced equilibrium in the electron-attached excited state remains, the computed DIET cross-section for reasonable parameters is ≈10−18 cm2, and is in agreement with the semi-empirically derived value for trap creation. Comparisons are made to surface DIET processes involving SiH bonds.

Electron-stimulated dissociation of ammonia on Pt(111): observation of gas-phase atomic hydrogen

We characterize the electron-stimulated dissociation of chemisorbed NH 3 and ND 3 on Pt(111) by time-of-flight (TOF) laser detection of the neutral gas-phase H and D products, respectively. We detect ground-state atomic hydrogen via 2 + 1 resonance-enhanced multiphoton ionization (REMPI) through the 2 2 S 1/2 level ; we do not observe any nascent metastable (2 2 S 1/2 ) hydrogen products. We assign the 14 eV threshold for ground-state hydrogen detection to excitation of a le adsorbate electron. Considering that the N-H bond axis is ∼68° off-normal for upright, N-down adsorbed ammonia, we find that the REMPI signal is surprisingly strong for hydrogen trajectories <45° off-normal. The presence of tilted adsorbates is expected to contribute to this signal ; however, we argue that it will also arise from an appreciable contribution to the product momentum from zero-point bending motion. Thus some of the product momenta from untilted adsorbates will be closer to the surface normal than suggested by the bond directions. To this end, we develop a general theoretical analysis of relevant trajectories (momenta) in laser-detected TOF distributions. We find that theory is consistent with the distinctly non-Maxwellian experimental observations. In addition, we find that the observed H/D yield ratio can be attributed to two effects : (1) The difference in the time scales for H and D motion while building momentum in the repulsive excited state. (2) The difference in zero point bending momentum for the two isotopic molecules.

An adiabatic model of chemisorbed molecules: electron spectroscopy and excited-state potential-energy surfaces

Abstract We review models that have been used to understand excited states of chemisorbed species, focussing on CO and NO, and encounter problems in attempting to fit all observables. We then introduce a new model. We show that a purely-electronic adiabatic approximation leads to an accurate solution for the system wavefunction in the limit that the molecule-substrate interaction is weak. This produces a configuration-interaction theory that has a Hubbard-like form. We derive semi-empirical parameters for the NO:Pt(111) system and find that the transfer integral for the 2π-substrate interaction is small for all choices of the screened electron-electron interaction, U. This suggests, for this system, that the substrate indeed adiabatically follows the fluctuations in adsorbate charge which are inherent in a covalent bond. We propose that our model is robust and applies to many adsorbed molecules. We then investigate low-lying excited-states of the metal-molecule bond.

DIET and the Electronic Structure of Chemisorbed Molecules and Physisorbed Rare Gases

Most observables which characterize stimulated desorption, such as the yield (or cross-section) and the trans I ationa1 and internal energy distributions, result from dynamical processes which are now being quantitatively modeled [1,2]. However, such calculations require realistic ground and excited-state potential energy surfaces and excitation lifetimes, which are in turn determined by the electronic structure of the surface-adsorbate system. We also wish to understand the electronic dynamics of excitation creation and decay and the strength of adsorbate-adsorbate interactions which influence the localization (or self-trapping) probability of an excitation [2,3]. Fortunately, some general principles have emerged recently which simplify the interpretation of electronic structure in terms relevant to DIET. We summarize and illustrate these concepts, drawing upon our studies of chemisorbed CO and NO:Pt(111) and physisorbed rare gases on metals.

Aspects of electronically stimulated processes of chemisorbed molecules

We discuss recent insights into the dynamical nature of electron- and photon-stimulated surface processes for chemisorbed molecules. Much of what we have learned stems from the correlation of quantum-resolved data on the products with the nature of the excited state(s), the excitation lifetimes, and the multidimensional potential energy surfaces. The latter are particularly important when two or more degrees of freedom in the adsorbates determine the dynamics and yields. Rapid advances in local-density functional approximation calculations now allow us to characterize the multidimensional aspects of ground state potential energy surfaces for chemisorbed molecules.

Intramolecular motion in DIET: desorption and dissociation of chemisorbed ammonia☆

Abstract We show that quantum-specific detection of DIET processes of polyatomic adsorbates reveals the multidimensional dynamics of intramolecular motion. Specifically, we present an analysis of the 6–350 eV electron-induced desorption and dissociation of chemisorbed NH3 and ND3 on Pt(1 1 1). State-selective detection of the neutral DIET products is accomplished by 2 + 1 resonance-enhanced multiphoton ionization (REMPI). Desorption and dissociation occur as a result of distinct electronic excitations that result in different, uncoupled, modes of intramolecular motion. We find that desorption results from 3a1−1-induced inversion motion. Trajectories on a two-dimensional potential energy surface reveal that the excited molecule fully inverts; upon deexcitation, the inverted molecule is sufficiently high on the hard wall of the substrate interaction to have enough energy to desorb. Given the short excitation lifetime, the time scale in which the (H) D atoms reach the inversion geometry directly affects the desorption yield and results in an appreciable enhancement of NH3 desorption over that of ND3. In general, multidimensional molecule-surface potential energy surfaces should be considered in DIET processes involving molecular adsorbates.