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

We characterize the electron-stimulated dissociation of chemisorbed NH₃ and ND₃ 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 2²S_1/2 level; we do not observe any nascent metastable (2²S_1/2) hydrogen products. We assign the 14 eV threshold for ground-state hydrogen detection to excitation of a 1e 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.