Adsorbate-induced transition between different mechanisms of laser-stimulated desorption
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were measured as a function of laser fluence, number of laser pulses, and oxygen exposure. If the laser fluence exceeds 10 mJ/cm2 desorption from clean particles occurs as a thermal reaction. Oxygen exposure as low as 1 L causes a strong decrease in the number of desorbed atoms and dimers. For larger oxygen coverages desorption of Na2O molecules is observed and, surprisingly, the atom signal recovers. At this stage, the underlying mechanism is substantially different from that for clean particles. The results can be explained by a model that takes into account the formation of a Na2O layer around a Na core and diffusion of Na atoms through the oxide layer prior to desorption.
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