(a) A schematic sketch of the Knudsen layer formed above the surface following the laser induced thermal desorption. The desorption flux is assumed to be large enough for the post-desorption collisions to take place and to further form the Knudsen layer. The arrows denote the velocity vectors of desorbed atoms at the surface and at the end of the Knudsen layer. (b) Schematic plots of the velocity distribution of the desorbed atoms in the z (surface normal) direction at the surface and at the end of the Knudsen layer. T S, T K, and u denote the surface temperature, the translational temperature of the desorbed atoms at the end of the Knudsen layer, and the stream velocity, respectively.
A schematic drawing of the experimental setup for the time-of-flight measurement of Xe following the laser induced thermal desorption from Au surfaces. C. Amp., PD, and BS denote fast current amplifier, photo diode, and beam splitter, respectively.
Time-of-flight (TOF) spectra of Xe from Au surfaces following pulsed laser irradiations. The Xe coverage Θ are 10, 6.7, 1.7, and 0.3 ML from top to bottom, respectively. The dashed curves and the solid curves are the Maxwell-Boltzmann (M-B) velocity distribution and the shifted M-B velocity distribution fitted to the experimental results, respectively. The vertical dashed line and the arrows indicate the peak position of each spectrum.
The stream velocity u of the desorbed Xe atoms from Au surfaces following the pulsed laser irradiations as a function of the Xe coverage Θ. The data are obtained by analyzing the time-of-flight spectrum with the shifted Maxwell-Boltzmann velocity distribution. The solid line is a guide for eyes. u becomes constant at above 4 ML which is denoted by the arrow.
Mach number M obtained in the present study and previously reported values by simulation and theory on the Knudsen layer.
Mean gas density , mean free path λ, and Knudsen number Kn in the vicinity of the surface at the moment of laser desorption as a function of Θ.
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