Desorption Time Measurements using ac Modulation Techniques

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Abstract
A technique was developed to measure the desorption time for ion emission from a heated surface using phase-sensitive lock-in detection of modulated alkali metal atom beams. This technique is described, together with the derivations of equations which relate the desorption time to experimental parameters. The relationships obtained show the dependence of the amplitude and phase of the ion emission ac signal upon the desorption time (i.e., the mean time that adsorbed particles spend upon a heated surface before being emitted as ions.) The measurable effects upon the signal as the desorption time increases are the attenuation of the amplitude and the phase shift of the modulated wave. A description of ac ion emission characteristics is included along with the results of ac modulation measurements of cesium and rubidium ion desorption times from tungsten at various temperatures. The ac modulation technique can be used to determine other mean times involving particle-surface interactions and in general the effective time constants for the passage of ac signals.