Generation of Sub-nanosecond H Atom Pulses for Scattering from Single-Crystal Epitaxial Graphene

Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures and to produce narrow speed distributions. They are particularly useful in combination with pulsed-laser-based detection schemes and have also been used as pump pulses in pump–probe experiments with neutral matter. The mechanical response of pulsed valves and chopper wheels limits the duration of these pulses typically to about 10–100 μs. Bunch compression photolysis has been proposed as a means to produce atomic pulses shorter than 1 ns─an experimental capability that would allow new measurements to be made on chemical systems. This technique employs a spatially chirped femtosecond duration photolysis pulse that produced an ensemble of H atom photoproducts that rebunches into a short pulse downstream. To date, this technique could not produce strong enough beams to allow new experiments to be carried out. In this paper, we report production of pulsed H atom beams consistent with a 700 ps pulse duration and with sufficient intensity to carry out differentially resolved inelastic H scattering experiments from a graphene surface. We observe surprisingly narrow angular distributions for H atoms incident normal to the surface. At low incidence energies quasi-elastic scattering dominates, and at high incidence energy we observe a strongly inelastic scattering channel. These results provide the basis for future experiments where the H atoms synchronously collide with a pulsed-laser-excited surface

Application of an event-based camera for real-time velocity resolved kinetics

We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imaging experiment operated at 1 kHz detection rate to study real-time velocity-resolved kinetics of thermal desorption. Such measurements involve a single gas pulse to initiate a time-dependent desorption process and a high repetition rate laser, where each pulse of the laser is used to produce an ion image. The sequence of ion images allows the time dependence of the desorption flux to be followed in real time. In previous work where a conventional framing camera was used, the large number of megapixel-sized images required data transfer and storage rates of up to 16 GB/s. This necessitated a large onboard memory that was quickly filled and limited continuous measurement to only a few seconds. Read-out of the memory became the bottleneck to the rate of data acquisition. We show here that since most pixels in each ion image contain no data, the data rate can be dramatically reduced by using an event-based/neuromorphic camera. The data stream is thus reduced to the intensity and location information on the pixels that are lit up by each ion event together with a time-stamp indicating the arrival time of an ion at the detector. This dramatically increases the duty cycle of the method and provides insights for the execution of other high rep-rate ion imaging experiments

Velocity‐resolved laser‐induced desorption for kinetics on surface adsorbates

Most experimental methods for studying the kinetics of surface reactions – for example, temperature programmed desorption (TPD), molecular beam relaxation spectrometry (MBRS) and velocity-resolved kinetics (VRK) – employ detection schemes that require thermal desorption. However, many adsorbates – for example reaction intermediates – never leave the surface under reaction conditions. In this paper, we present a new method to measure adsorbate concentrations on catalytic surfaces and demonstrate its utility for studying thermal desorption kinetics. After a short-pulsed molecular beam deposits CO or NH3 on Pt (111), the surface is irradiated with an ultrashort laser pulse that induces desorption. Another tightly focused ultrashort laser pulse ionizes the gas-phase molecules by a non-resonant multiphoton process and the ions are detected. This two-laser signal is then recorded as a function of time after the dosing molecular beam pulse and decays exponentially. First-order thermal desorption rate constants are obtained over a range of temperatures and found to be in good agreement with past reports. Ion detection is done mass selectively with ion-imaging, dispersing the gas phase molecules by their velocities. Since laser-induced desorption (LID) produces hyperthermal gas phase molecules, they can be detected with little or no background. This approach is highly surface-specific and exhibits sensitivity below 10−4 ML coverage. Because the signals are linearly proportional to adsorbate concentration, the method can be employed at lower temperatures than VRK, whose signal is proportional to reaction rate

Dynamical steering in an electron transfer surface reaction: Oriented NO(v = 3, 0.08 < E i < 0.89 eV) relaxation in collisions with a Au(111) surface.

We report measurements of the incidence translational energy dependence of steric effects in collisions of NO(v = 3) molecules with a Au(111) surface using a recently developed technique to orient beams of vibrationally excited NO molecules at incidence energies of translation between 0.08 and 0.89 eV. Incidence orientation dependent vibrational state distributions of scattered molecules are detected by means of resonance enhanced multiphoton ionization spectroscopy. Molecules oriented with the N-end towards the surface exhibit a higher vibrational relaxation probability than those oriented with the O-end towards the surface. This strong orientation dependence arises from the orientation dependence of the underlying electron transfer reaction responsible for the vibrational relaxation. At reduced incidence translational energy, we observe a reduced steric effect. This reflects dynamical steering and re-orientation of the NO molecule upon its approach to the surface

Quantum-state specific scattering of molecules from surfaces.

In my work, I investigated the quantum-state resolved scattering of three different diatomic molecules (NO, CO, N2) from different surfaces, including Au(111) and Pt(111). I focused on measurements of the energy transfer between the various degrees of freedom available using both state-of-the-art and new methods developed in the course of this work. I strove to investigate a few simple model systems with the goal of discovering generally valid rules for the coupling between different degrees of freedom of these simple model systems. As a first system, I investigated vibrationally inelastic scattering of nitric oxide (NO) from a single crystal Au(111) surface, a system that has been extensively studied in the past and is thought to be well understood. I measured absolute vibrational excitation probabilities for v = 0→1, 2, 3 scattering as a function of surface temperature and incidence translational energy and compared the results to first-principles independent electron surface-hopping (IESH) theory as well as to an empirical state-to-state kinetic rate model. The excitation probabilities of NO(v =1, 2, 3) increase with surface temperature (TS) in an Arrhenius-like fashion under all conditions of my work. For each final vibrational state, I find that the Arrhenius activation energy is equal to the vibrational energy required for excitation which shows that the NO vibrational energy is taken from a thermal bath. Narrow angular distributions and early, narrow arrival time profiles indicate a direct scattering mechanism leading to fast recoiling molecules. The experimental observations allow for the conclusion that excitation into all vibrational states occurs upon coupling of the NO vibration to electron-hole pairs (EHPs) of the metal surface and that adiabatic (mechanical) coupling to phonons or translation is negligible. The comparison to predictions of first-principles IESH theory reveals quantitative agreement for v = 0→1 excitation but the theoretical predictions slightly underestimate the probabilities for v = 0→2 excitation and clearly underestimate v = 0→3 excitation. A detailed comparison of the excitation mechanisms reveals that this disagreement for scattering into final vibrational states vf > 1 results from an underestimation of overtone excitations in the scattering process. Further failures of the current implementation of the IESH model appear in a comparison to measurements of incidence energy (EI) dependent NO(v = 3→1, 2, 3) relaxation probabilities. The experiments show that the probabilities for vibrational relaxation increase with incidence energy while the IESH simulations predict the opposite trend. A detailed trajectory analysis reveals that the theoretical model predicts a large fraction of multi-bounce collisions that increases with decreasing EI. A selection of only single-bounce collisions improves the EI dependence but still does not reproduce the experimental observations. The single bounce results predict relaxation probabilities that do not depend on EI. My results indicate that the overestimation of multi-bounce collision in the IESH model is probably related to a corrugated potential energy surface (PES) because multi-bounce artifacts occur also for simple adiabatic calculations on the ground-state PES. The failure might be directly related to a failure of the density-functional theory (DFT) calculations from which the PES was obtained. As a final study on the NO/Au(111) system, I performed state-to-state time-of-flight experiments on scattering of incident NO(v = 2, 3) from Au(111) into different final vibrational and rotational states at various incidence energies. For the first time, my data shows that vibrationally inelastic scattering of NO from a metal surface can influence the final translational energy (Ef) of the scattered molecules. I find that vibrational excitation leads to a decrease of Ef while vibrational relaxation increases Ef. The amount of vibrational energy that couples to the translational motion (T↔V coupling) depends on incidence energy as well as on surface temperature. I speculate that the T↔V coupling results from an EHP mediated energy transfer mechanism in which vibrational energy is first released (taken) into (from) EHPs which then couple to the translation motion. Furthermore, I observe that the dependence of Ef on the final rotational energy (Erot) depends on incidence energy as well as on the final vibrational state. At higher EI, the decrease of Ef with Erot is similar for all vibrational channels. With decreasing incidence energy,Ef gradually becomes independent of Erot. This effect is more pronounced and occurs already at higher EI for vibrationally inelastic scattering. The mechanism for this observation remains unclear but the observations are in agreement with the expectation for dynamical steering effects or multi-bounce collisions that might become important at low EI. Nevertheless, the data can act as an ideal benchmark for future new or improved theoretical models, which have to treat nonadiabatic as well as adiabatic interactions of the NO molecules with the Au(111) surface correctly in order to obtain reasonable agreement. As next model systems, I investigated the scattering of CO molecules from Au(111) and Pt(111). The experiments on CO/Au(111) scattering involve measurements of v = 0→1 excitation probabilities as well as measurements of v = 2→2, 1 branching ratios. In both cases, I find that the probabilities for vibrational (de-)excitation first decrease with increasing EI but then increase again for EI > 0.4 eV. Overall, the absolute excitation probabilities are about a factor of three lower than observed for NO/Au(111). The results on v = 0→1 excitation are partly in agreement with expectations for trapping followed by desorption at low EI if one assumes complete equilibration with the surface. However, the time-of-flight distributions for scattering of incident CO(vi = 2) show that the assumption of complete equilibration with the surface in trapping-desorption is probably not valid in this system. The experimental data shows that incident CO(vi = 2) molecules can be trapped at the surface but are desorbed in vf = 1, 2 prior to complete equilibration. This observation is direct evidence for vibrationally hot molecules, often referred to as hot precursors, at the surface. The experiments raise the question about the vibrational lifetime of CO adsorbed on Au(111) and whether it is similar to observed picosecond lifetimes found for CO/Pt(111) or CO/Cu(100). I further measured CO(v = 0→1) excitation probabilities in scattering from Pt(111). The CO/Pt(111) system exhibits broad angular distributions and TS dependent arrival time distributions. The excitation probabilities agree with the thermal expectation and reflect complete equilibration with the surface. The data supports vibrational excitation occurring due to trapping followed by desorption after equilibration with the surface. This is further supported by measured speed distributions for desorbing/ recoiling CO(vf = 0, 1), which only show significant direct scattering for v = 0→0 scattering. Furthermore, I used a new velocity selected residence time technique to investigate the desorption kinetics of CO from Pt(111) in real-time with microsecond resolution. I measured the time dependent flux of molecules leaving the surface at well-defined final velocity, sf, as a function of surface temperature. I compare the results of previous studies to the experimental data using a simple first-order kinetic rate model. The comparison demonstrates the capability of the method to judge the reliability of previous results; it is very sensitive to the choice of the kinetic parameters. Furthermore, the experimental data shows clear deviations of the experimental data from the first-order desorption kinetics reported previously. By comparison to a kinetic model involving surface diffusion and adsorption at step sites, I am able to assign the two processes to direct desorption from terraces and to step-to-terrace diffusion followed by desorption from the terrace sites. Finally, I derive a binding energy of E0 = 1.43-1.51 eV for CO adsorption at Pt(111) terraces using transition-state theory; the value is in agreement with recent heat of adsorption measurements. As a last system, I measured vibrational excitation probabilities for N2 scattering from Pt(111) at various incidence energies ranging from 0.1-1.1 eV. I find again an Arrhenius-like dependence of the v = 0→1 excitation probability on the temperature of the surface with an activation energy equal to the vibrational energy uptake. The Arrhenius prefactors increase with increasing incidence energy with zero threshold and are about one order of magnitude lower than for NO/Au(111). Narrow and TS independent angular and time-of-flight distributions clearly indicate a direct scattering mechanism. Consequently, the experimental results exhibit all possible fingerprints of nonadiabatic V-EHP coupling for a molecule-surface system in which the very low electron affinity of the gas phase molecules seems to make electron transfer processes very unlikely

The dynamics of molecular interactions and chemical reactions at metal surfaces: Testing the foundations of theory.

We review studies of molecular interactions and chemical reactions at metal surfaces, emphasizing progress toward a predictive theory of surface chemistry and catalysis. For chemistry at metal surfaces, a small number of central approximations are typically made: (a) the Born-Oppenheimer approximation of electronic adiabaticity, (b) the use of density functional theory at the generalized gradient approximation level, (c) the classical approximation for nuclear motion, and (d) various reduced-dimensionality approximations. Together, these approximations constitute a provisional model for surface chemical reactivity. We review work on some carefully studied examples of molecules interacting at metal surfaces that probe the validity of various aspects of the provisional model

Electron hole pair mediated vibrational excitation in CO scattering from Au(111): Incidence energy and surface temperature dependence.

We investigated the translational incidence energy (E i ) and surface temperature (T s ) dependence of CO vibrational excitation upon scattering from a clean Au(111) surface. We report absolute v = 0 → 1 excitation probabilities for E i between 0.16 and 0.84 eV and T s between 473 and 973 K. This is now only the second collision system where such comprehensive measurements are available – the first is NO on Au(111). For CO on Au(111), vibrational excitation occurs via direct inelastic scattering through electron hole pair mediated energy transfer – it is enhanced by incidence translation and the electronically non-adiabatic coupling is about 5 times weaker than in NO scattering from Au(111). Vibrational excitation via the trapping desorption channel dominates at E i = 0.16 eV and quickly disappears at higher E

Incidence energy dependent state-to-state time-of-flight measurements of NO(v=3) collisions with Au(111): the fate of incidence vibrational and translational energy.

We report measurements of translational energy distributions when scattering NO(v(i) = 3, J(i) = 1.5) from a Au(111) surface into vibrational states v(f) = 1, 2, 3 and rotational states up to J(f) = 32.5 for various incidence energies ranging from 0.11 eV to 0.98 eV. We observed that the vibration-to-translation as well as the translation-to-rotation coupling depend on translational incidence energy, E-I. The vibration-to-translation coupling, i.e. the additional recoil energy observed for vibrationally inelastic (v = 3 -> 2, 1) scattering, is seen to increase with increasing E-I. The final translational energy decreases approximately linearly with increasing rotational excitation. At incidence energies E-I > 0.5 eV, the slopes of these dependencies are constant and identical for the three vibrational channels. At lower incidence energies, the slopes gradually approach zero for the vibrationally elastic channel while they exhibit more abrupt transitions for the vibrationally inelastic channels. We discuss possible mechanisms for both effects within the context of nonadiabatic electron-hole pair mediated energy transfer and orientation effects