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.
|Number of pages||1|
|Journal||Journal of Electron Spectroscopy and Related Phenomena|
|Publication status||Published - 1983|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Atomic and Molecular Physics, and Optics
- Surfaces and Interfaces