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We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantum-state selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.

We report kinetic energy distributions of exoelectrons produced by collisions of highly vibrationally excited NO molecules with a low work function Cs dosed Au(111) surface. These measurements show that energy dissipation pathways involving nonadiabatic conversion of vibrational energy to electronic energy can result in electronic excitation of more than 3 eV, consistent with the available vibrational energy. We measured the dependence of the electron energy distributions on the translational and vibrational energy of the incident NO and find a clear positive correlation between final electron kinetic energy and initial vibrational excitation and a weak but observable inverse dependence of electron kinetic energy on initial translational energy. These observations are consistent with a vibrational autodetachment mechanism, where an electron is transferred to NO near its outer vibrational turning point and ejected near its inner vibrational turning point. Within the context of this model, we estimate the NO-to-surface distance for electron transfer.

We describe an advanced and highly sensitive instrument for quantum state-resolved molecule-surface energy transfer studies under ultrahigh vacuum (UHV) conditions. The apparatus includes a beam source chamber, two differential pumping chambers, and a UHV chamber for surface preparation, surface characterization, and molecular beam scattering. Pulsed and collimated supersonic molecular beams are generated by expanding target molecule mixtures through a home-built pulsed nozzle, and excited quantum state-selected molecules were prepared via tunable, narrow-band laser overtone pumping. Detection systems have been designed to measure specific vibrational-rotational state, time-of-flight, angular and velocity distributions of molecular beams coming to and scattered off the surface. Facilities are provided to clean and characterize the surface under UHV conditions. Initial experiments on the scattering of HCl(v = 0) from Au(111) show many advantages of this new instrument for fundamental studies of the energy transfer at the gas-surface interface.

Molecular beam experiments in which gas molecules are scattered from liquids provide detailed, microscopic perspectives on the gas–liquid interface. Extending these methods to volatile liquids while maintaining the ability to measure product energy and angular distributions presents a significant challenge. The incorporation of flat liquid jets into molecular beam scattering experiments in our laboratory has allowed us to demonstrate their utility in uncovering dynamics in this complex chemical environment. Here, we summarize recent work on the evaporation and scattering of Ne, CD4, ND3, and D2O from a dodecane flat liquid jet and present first results on the evaporation and scattering of Ar from a cold salty water jet. In the evaporation experiments, Maxwell–Boltzmann flux distributions with a cosθ angular distribution are observed. Scattering experiments reveal both impulsive scattering (IS) and trapping followed by thermal desorption (TD). Super-specular scattering is observed for all four species scattered from dodecane and is attributed to anisotropic momentum transfer to the liquid surface. In the IS channel, rotational excitation of the polyatomic scatterers is a significant energy sink, and these species accommodate more readily on the dodecane surface compared to Ne. Our preliminary results on cold salty water jets suggest that Ar atoms undergo some vapor-phase collisions when evaporating from the liquid surface. Initial scattering experiments characterize the mechanisms of Ar interacting with an aqueous jet, allowing for comparison to dodecane systems. Key Points: Molecular beam scattering from flat liquid jets is a powerful technique to elucidate mechanistic detail at the gas–liquid interface. Previous dodecane scattering experiments have uncovered angularly-resolved thermal desorption fractions and energy transfer at the interface for several small molecule scatterers. Preliminary results on scattering from cold salty water reveal mechanisms of interaction between argon and an aqueous jet.

Energy transferred in atom‐surface collisions typically depends strongly on projectile mass, an effect that can be experimentally detected by isotopic substitution. In this work, we present measurements of inelastic H and D atom scattering from a semiconducting Ge(111)c(2×8) surface exhibiting two scattering channels. The first channel shows the expected isotope effect and is quantitatively reproduced by electronically adiabatic molecular dynamics simulations. The second channel involves electronic excitations of the solid and, surprisingly, exhibits almost no isotope effect. We attribute these observations to scattering dynamics, wherein the likelihood of electronic excitation varies with the impact site engaged in the interaction. Key Points: Previous work revealed that H atoms with sufficient translational energy can excite electrons over the band gap of a semiconductor in a surface collision.We studied the isotope effect of the energy transfer by H/D substitution and performed band structure calculations to elucidate the underlying excitation mechanism.Our results suggest a site‐specific mechanism that requires the atom to hit a specific surface site to excite an electron‐hole pair. [ABSTRACT FROM AUTHOR]

In this work we seek to examine the nature of collisional energy transfer between HCl and Au(111) for nonreactive scattering events that sample geometries near the transition state for dissociative adsorption by varying both the vibrational and translational energy of the incident HCl molecules in the range near the dissociation barrier. Specifically, we report absolute vibrational excitation probabilities for HCl(v = 0 → 1) and HCl(v = 1 → 2) scattering from clean Au(111) as a function of surface temperature and incidence translational energy. The HCl(v = 2 → 3) channel could not be observed-presumably due to the onset of dissociation. The excitation probabilities can be decomposed into adiabatic and nonadiabatic contributions. We find that both contributions strongly increase with incidence vibrational state by a factor of 24 and 9, respectively. This suggests that V-T as well as V-EHP coupling can be enhanced near the transition state for dissociative adsorption at a metal surface. We also show that previously reported HCl(v = 0 → 1) excitation probabilities [Q. Ran et al., Phys. Rev. Lett. 98, 237601 (2007)]-50 times smaller than those reported here-were influenced by erroneous assignment of spectroscopic lines used in the data analysis. [ABSTRACT FROM AUTHOR]

We have constructed a potential energy surface (PES) for H-atoms interacting with fcc Au(111) based on fitting the analytic form of the energy from Effective Medium Theory (EMT) to ab initio energy values calculated with density functional theory. The fit used input from configurations of the H–Au system with Au atoms at their lattice positions as well as configurations with the Au atoms displaced from their lattice positions. It reproduces the energy, in full dimension, not only for the configurations used as input but also for a large number of additional configurations derived from ab initio molecular dynamics (AIMD) trajectories at finite temperature. Adiabatic molecular dynamics simulations on this PES reproduce the energy loss behavior of AIMD. EMT also provides expressions for the embedding electron density, which enabled us to develop a self-consistent approach to simulate nonadiabatic electron-hole pair excitation and their effect on the motion of the incident H-atoms. For H atoms with an energy of 2.7 eV colliding with Au, electron-hole pair excitation is by far the most important energy loss pathway, giving an average energy loss ≈3 times that of the adiabatic case. This increased energy loss enhances the probability of the H-atom remaining on or in the Au slab by a factor of 2. The most likely outcome for H-atoms that are not scattered also depends prodigiously on the energy transfer mechanism; for the nonadiabatic case, more than 50% of the H-atoms which do not scatter are adsorbed on the surface, while for the adiabatic case more than 50% pass entirely through the 4 layer simulation slab. [ABSTRACT FROM AUTHOR]

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9≤v≤18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10-2 electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces. [ABSTRACT FROM AUTHOR]

We report direct measurements of the vibrational survival probability of NO(ν = 12) suffering collisions with a LiF(001) crystalline surface. Survival probabilities are near unity in all cases. At the lowest reported incidence energies (5.3 kJ/mol) and surface temperatures (290 K), vibrational relaxation is enhanced, angular distributions approach cos(θ), and rotational temperatures of the scattered molecules approach the surface temperature. At higher incidence energies, rotational temperatures are higher than the surface temperature and angular distributions narrow approaching cos[SUP6](θ). These results suggest that trapping desorption is occurring at least at the lowest incidence energies and surface temperatures. Substantial vibrational relaxation appears to occur when residence time are as small as a few ps. This is many orders of magnitude faster than has been reported for diatomic molecules relaxing from ν = 1 on salt crystals. [ABSTRACT FROM AUTHOR]

PUMP–DUMP–PROBE experiments were performed on nitric oxide (NO) in order to study the rotational and spin–orbit energy transfer in a highly vibrationally excited diatomic. Total depopulation as well as state-to-state rotational and spin–orbit energy transfer rate constants were measured for v=8 and 19 of the ground electronic state. By comparing this new set of results with previously measured results for v=2 [Aa. Sudbo and M. M. T. Loy, J. Chem. Phys. 76, 3646 (1982)] the vibrational influence on the rotational and spin–orbit energy transfer dynamics could be evaluated. Total depopulation rate constants for individual rovibrational states were found to be very similar for v=2, 8, and 19. State-to-state rotational energy transfer rate constants could be compared for v=8 and 19, and again only small differences were found. It appears that vibrational energy does not have a significant impact on the rotational and spin–orbit energy transfer dynamics for NO–NO collisions. [ABSTRACT FROM AUTHOR]

Investigates the compensation of rotational motion on the energy defect in near-resonant vibration-vibration energy transfer. Observation of vibrationally elastic energy transfer; Dependence of process efficiency on energy defect; Influence of vibration on rotational energy transfer.

The dependence of vibrational energy transfer on vibrational excitation has been studied using the stimulated emission pumping technique to efficiently prepare a large range of specific vibrational states of the nitric oxide molecule in its ground electronic state. Laser induced fluorescence was used to detect collisionally relaxed NO. The self-relaxation rate constants of NO(v>1) were up to 200 times larger than that of NO(v=1). Multiquantum relaxation was found to be important at high energy and was quantified at 3.8 eV. Self-relaxation rate constants of 15N18O as well as 14N16O were measured and a large isotope effect was observed. Relaxation of NO(v‘=22) with H2 was also investigated. Theoretical explanations of our experimental results were attempted and it is shown that at vibrational energy up to ≊3 eV the qualitative trends observed in these experiments such as the mass effect and the multiquantum relaxation can be explained by Schwartz–Slawsky–Herzfeld theory. A simple explanation of the anomalously high NO self-relaxation rate is given. The large acceleration of the vibrational relaxation rate above 3.0 eV is coincident with the energetic onset of high energy (NO)2 isomer complexes. More theoretical and experimental work is needed to explain the quantitative aspects of these observations. [ABSTRACT FROM AUTHOR]

Electronically nonadiabatic interactions between molecules and metal surfaces are now well known. Evidence is particularly clear from studies of diatomic molecules that molecular vibration can be strongly coupled to electrons of the metal leading to efficient energy transfer between these two kinds of motion. Since molecular vibration is the same motion needed for bond breaking, it is logical to postulate that electronically nonadiabatic influences on surface chemical reaction probabilities would be strong. Still there are few if any examples where such influences have been clearly investigated. This review recounts the evidence for and against the aforementioned postulate emphasizing reacting systems that have yet to receive full attention and where electronically nonadiabatic influence of reaction probabilities might be clearly demonstrated. [ABSTRACT FROM AUTHOR]

We present quantum-state and velocity resolved experiments for molecular beam scattering of acetylene (C2H2) from a single-crystal Au(111) surface, observations that reveal translational, rotational and vibrational inelasticity. The experiments are made possible by a novel (1 + 2) REMPI scheme for acetylene. The scattered molecules' velocity distributions as well as their ro-vibrational quantum-state distributions depend on the translational energy of incidence, EI, providing unambiguous evidence that the scattered molecules were not trapped and equilibrated on the surface. We report the EI-dependence of the collisional excitation of one and two quanta of the trans-bending vibrational mode, ν4 = 0 → 1, 2, which is consistent with a mechanism involving conversion of incidence translational energy to acetylene vibration. Rotationally resolved velocity measurements on scattered acetylene in its ground vibrational state are interpreted in terms of orientation-dependent rotational and vibrational excitation probabilities. [ABSTRACT FROM AUTHOR]

We report on the design and characterization of a new apparatus for performing quantum-state resolved surface scattering experiments. The apparatus combines optical state-specific molecule preparation with a compact hexapole and a Stark decelerator to prepare carrier gas-free pulses of quantum-state pure CO molecules with velocities controllable between 33 and 1000 m/s with extremely narrow velocity distributions. The ultrahigh vacuum surface scattering chamber includes homebuilt ion and electron detectors, a closed-cycle helium cooled single crystal sample mount capable of tuning surface temperature between 19 and 1337 K, a Kelvin probe for non-destructive work function measurements, a precision leak valve manifold for targeted adsorbate deposition, an inexpensive quadrupole mass spectrometer modified to perform high resolution temperature programmed desorption experiments and facilities to clean and characterize the surface. [ABSTRACT FROM AUTHOR]

Abstract: Molecular energy transfer processes at solid surfaces are profoundly important, influencing trapping, desorption, diffusion, and reactivity; in short, all of the elementary steps needed for surface chemistry to take place. In this paper we review recent progress in our understanding of energy transfer at surfaces with a particular emphasis on those phenomena, which are peculiar to solids with delocalized electronic structure, e.g. electronically nonadiabatic energy transfer. This area of study represents an area requiring significant extensions of our theoretical understanding, which is largely based on density functional theory. This review provides an overview of some of the experimental and theoretical tools presently being used in this field and a description of several illustrative examples of work that have helped to shape our understanding. [Copyright &y& Elsevier]

Abstract: The degree of rotational excitation resulting from collisions of NO(v =0) and NO(v =2) with a Au(111) surface is smaller for NO(v =2) that undergoes vibrational relaxation, NO(v =2→ v =1), than it is for NO that scatters in a vibrationally elastic channel, NO(v =2→ v =2) or NO(v =0→ v =0). The rotational excitation is less for the vibrational relaxation channel, despite the fact that vibrational relaxation releases 0.25eV that is potentially available for increased rotational excitation. This difference in amount of rotational excitation for vibrationally inelastic and elastic channels increases with the kinetic energy, E i, of the incident NO molecules. These observations should provide useful comparisons to full dimensional theories of surface interactions. [Copyright &y& Elsevier]

Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science. [ABSTRACT FROM AUTHOR]

Translational motion is believedto be a spectator degree of freedomin electronically nonadiabatic vibrational energy transfer betweenmolecules and metal surfaces, but the experimental evidence availableto support this view is limited. In this work, we have experimentallydetermined the translational inelasticity in collisions of NO moleculeswith a single-crystal Au(111) surfacea system with strongelectronic nonadiabaticity. State-to-state molecular beam surfacescattering was combined with an IR-UV double resonance scheme to obtainhigh-resolution time-of-flight data. The measurements include vibrationallyelastic collisions (v= 3→3, 2→2) aswell as collisions where one or two quanta of molecular vibrationare excited (2→3, 2→4) or de-excited (2→1, 3→2,3→1). In addition, we have carried out comprehensive measurementsof the effects of rotational excitation on the translational energyof the scattered molecules. We find that under all conditions of thiswork, the NO molecules lose a large fraction (∼0.45) of theirincidence translational energy to the surface. Those molecules thatundergo vibrational excitation (relaxation) during the collision recoilslightly slower (faster) than vibrationally elastically scatteredmolecules. The amount of translational energy change depends on thesurface temperature. The translation-to-rotation coupling, which iswell-known for v= 0→0 collisions, is foundto be significantly weaker for vibrationally inelastic than elasticchannels. Our results clearly show that the spectator view of thetranslational motion in electronically nonadiabatic vibrational energytransfer between NO and Au(111) is only approximately correct. [ABSTRACT FROM AUTHOR]

Wemeasured absolute probabilities for vibrational excitation ofNO(v= 0) molecules in collisions with a Au(111)surface at an incidence energy of translation of 0.4 eV and surfacetemperatures between 300 and 1100 K. In addition to previously reportedexcitation to v= 1 and v= 2, weobserved excitation to v= 3. The excitation probabilitiesexhibit an Arrhenius dependence on surface temperature, indicatingthat the dominant excitation mechanism is nonadiabatic coupling toelectron–hole pairs. The experimental data are analyzed interms of a recently introduced kinetic model, which was extended toinclude four vibrational states. We describe a subpopulation decompositionof the kinetic model, which allows us to examine vibrational populationtransfer pathways. The analysis indicates that sequential pathways(v= 0 → 1 → 2 and v= 0 → 1 → 2 → 3) alone cannot adequately describeproduction of v= 2 or 3. In addition, we performedfirst-principles molecular dynamics calculations that incorporateelectronically nonadiabatic dynamics via an independent electron surfacehopping (IESH) algorithm, which requires as input an ab initio potentialenergy hypersurface (PES) and nonadiabatic coupling matrix elements,both obtained from density functional theory (DFT). While the IESH-basedsimulations reproduce the v= 1 data well, they slightlyunderestimate the excitation probabilities for v=2, and they significantly underestimate those for v= 3. Furthermore, this implementation of IESH appears to overestimatethe importance of sequential energy transfer pathways. We make severalsuggestions concerning ways to improve this IESH-based model. [ABSTRACT FROM AUTHOR]

Abstract: Vibrational excitation of HCl molecules (ν =0→1) has been observed when HCl molecular beams at energies of 0.59–1.37eV are scattered from a Au(111) surface at low surface temperature (T s =273K). The incident and scattered HCl is probed state-selectively using 2+1 REMPI. The vibrational excitation probability depends strongly on incidence kinetic energy, E i, exhibiting a threshold near ∼E i =0.57eV. We measured the absolute vibrational excitation probability which varies from 10−6 to 10−5 over this energy range, 1–2 orders of magnitude higher than the thermal equilibrium expectation value. The magnitude of the excitation probability, the near specular angular distributions of the scattered ν =1 molecules and the dramatic narrowing of the angular distribution near threshold are all consistent with a direct translation to vibration (T–V) mechanical energy transfer mechanism. [Copyright &y& Elsevier]