Laser-induced thermal desorption of CO from clean polycrystalline copper: Time-of-flight and surface diiffusion measurements

R. Viswanathan, D. R. Burgess, Peter C Stair, E. Weitz

Research output: Contribution to journalArticle

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Abstract

Pulsed laser-induced thermal desorption was utilized as a technique to study the dynamics of desorption and surface diffusion of CO on clean polycrystalline copper at 85K in UHV. Single pulses of a focused KrF excimer laser (248nm, 15ns FWHM) were used to desorb the CO molecules, which were subsequently detected by a quadrupole mass spectrometer (effective time constant =4.4μs). The time-of-flight (TOF) plus distribution was recorded using a fast transient recorder (Biomation 610B/8100). The surface diffusion measurements were made using a versatile technique (ref.1) recently developed in our laboratory, which can be used on any system where the adsorbate can be desorbed thermally. In this technique, a small area on the surface is initially depleted of adsorbate with a laser pulse. Measurements of the time dependence of the growth of the integrated desorption flux from subsequent pulses yield a diffusion coefficient. The TOF desorption spectra could be fit to Boltzmann distributions, and effective translational temperatures of the desorbed CO molecules were obtained from these fits. The desorption process was studied as a function of laser power density, and a desorption threshold corresponding to an absorbed power density of 10MW cm-2 was observed. Near threshold, both the desorption flux and velocity distribution varied rapidly with laser power density. At power densities much larger than the threshold (30-75MW cm-2), the peak desorption flux varied approximately linearly with power density. In the range of power densities studied, the most probable velocity of the CO molecules varied between 350 and 500M s-1, corresponding to Boltzmann temperatures between 200 and 400K. The applicability of the classical desorption rate equation for laser induced desorption will be discussed. A preliminary analysis of the diffusion data for CO adsorbed on polycrystalline copper at 110K yielded a value for the diffusion coefficient D = 5 × 10-7cm2 s-1 (±2×10-7cm2 s-1) (ref.1). The temperature dependence of the diffusion coefficient is currently being investigated in detail in order to accurately determine the activation energy for the diffusion process.

Original languageEnglish
Pages (from-to)111
Number of pages1
JournalJournal of Electron Spectroscopy and Related Phenomena
Volume29
Issue number1
DOIs
Publication statusPublished - 1983

Fingerprint

Thermal desorption
Surface measurement
Carbon Monoxide
Copper
Desorption
desorption
copper
Lasers
radiant flux density
lasers
Surface diffusion
Adsorbates
diffusion coefficient
Fluxes
surface diffusion
Molecules
Laser pulses
thresholds
pulses
molecules

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Spectroscopy
  • Atomic and Molecular Physics, and Optics
  • Surfaces and Interfaces

Cite this

Laser-induced thermal desorption of CO from clean polycrystalline copper : Time-of-flight and surface diiffusion measurements. / Viswanathan, R.; Burgess, D. R.; Stair, Peter C; Weitz, E.

In: Journal of Electron Spectroscopy and Related Phenomena, Vol. 29, No. 1, 1983, p. 111.

Research output: Contribution to journalArticle

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