First principles rates for surface chemistry employing exact transition state theory: application to recombinative desorption of hydrogen from Cu(111).

We present first principles calculations of the reactive flux for thermal recombinative desorption of hydrogen from Cu(111). We follow a theoretical paradigm used successfully for gas phase reactions, where electronic structure theory (DFT-GGA) is combined with transition state theory (TST). Classical ab initio molecular dynamics trajectories initiated from a thermal distribution near the transition state provide dynamical corrections to the desorption rate. We use this to calculate and study the recrossing error of TST and to directly simulate thermal desorption experiments based on a high temperature permeation method. Transition state recrossing is strongly temperature dependent and is even important in a frozen Cu-atom model. It is not influenced by inclusion of electron-hole pair excitation at the level of the local density electronic friction approximation. We also present the kinetic energy resolved flux of desorbing H2 at elevated temperature. This provides a more direct way to compare first principles theory to experiment, with no need to invoke detailed balance.

Associative desorption of hydrogen isotopologues from copper surfaces: Characterization of two reaction mechanisms.

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H2, HD, and D2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a "fast" channel and a "slow" channel on both facets. The "fast channel" is promoted by both hydrogen incidence translational and vibrational energy, while the "slow channel" is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the "fast channel" on Cu(111) are in agreement with theory within "chemical accuracy" (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle - quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the "slow channel," which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments.

Evidence for Electron-Hole Pair Excitation in the Associative Desorption of H2 and D2 from Au(111).

The dissociative adsorption reaction of hydrogen on noble metals is believed to be well-described within the Born-Oppenheimer approximation. In this work, we have experimentally derived translational energy distributions for selected quantum states of H2 and D2 formed in associative desorption reactions at a Au(111) surface. Using the principle of detailed balance, we compare our results to theory carried out at the same level of sophistication as was done for the reaction on copper. The theory predicts translational excitation that is much higher than is seen in experiment and fails to reproduce the experimentally observed isotope effect. The large deviations between experiment and theory are surprising because, for the same reactions occurring on Cu(111), a similar theoretical strategy agreed with experiment, yielding "chemical accuracy". We argue that electron-hole pair excitation is more important for the reaction on gold, an effect that may be related to the reaction's later transition state.

Generation of ultra-short hydrogen atom pulses by bunch-compression photolysis.

Ultra-short light pulses enable many time-resolved studies in chemistry, especially when used in pump-probe experiments. However, most chemical events are not initiated by light, but rather by collisions. Time-resolved collisional experiments require ultra-short pulses of atoms and molecules--sadly, methods for producing such pulses are so far unknown. Here we introduce bunch-compression photolysis, an approach to forming ultra-short and highly intense pulses of neutral atoms. We demonstrate H-atom pulses of 1.2±0.3 ns duration, far shorter than any previously reported. Owing to its extraordinarily simple physical principles, we can accurately model the method--the model shows H-atom pulses as short as 110-ps are achievable. Importantly, due to the bunch-compression, large (mm(3)) photolysis volumes are possible, a key advantage for pulse intensity. This technique overcomes the most challenging barrier to a new class of experiments on time-resolved collisions involving atoms and molecules.