Measurement of fast desorption kinetics of D2 from tungsten by laser induced thermal desorption

A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D2 from a polycrystalline tungsten sample. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D2 from W at rates of 5x107 monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 105 times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbate-desorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3x10-8 sec and surface temperature rise of 300 to 3000 K. (Auth.)