Electron-stimulated dissociation of ammonia on Pt(111)

observation of gas-phase atomic hydrogen

Ellen Stechel, A. R. Burns, D. R. Jennison

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

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 22S1/2 level; we do not observe any nascent metastable (22S1/2) 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.

Original languageEnglish
Pages (from-to)71-87
Number of pages17
JournalSurface Science
Volume340
Issue number1-2
DOIs
Publication statusPublished - Oct 10 1995

Fingerprint

Ammonia
ammonia
Hydrogen
Momentum
Gases
dissociation
vapor phases
momentum
Adsorbates
Electrons
hydrogen
products
electrons
Ground state
Ionization
Trajectories
trajectories
ionization
ground state
Lasers

Keywords

  • Ammonia
  • Electron stimulated desorption (ESD)
  • Molecular dynamics
  • Platinum
  • Resonance enhanced multiphoton ionization mass spectroscopy ( REMPI MS)

ASJC Scopus subject areas

  • Materials Chemistry
  • Surfaces, Coatings and Films
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Physical and Theoretical Chemistry

Cite this

Electron-stimulated dissociation of ammonia on Pt(111) : observation of gas-phase atomic hydrogen. / Stechel, Ellen; Burns, A. R.; Jennison, D. R.

In: Surface Science, Vol. 340, No. 1-2, 10.10.1995, p. 71-87.

Research output: Contribution to journalArticle

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AB - 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 22S1/2 level; we do not observe any nascent metastable (22S1/2) 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.

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