Velocity distribution function of Na released by photons from planetary surfaces

Gamborino Uzcanga, Diana; Wurz, Peter (2018). Velocity distribution function of Na released by photons from planetary surfaces. Planetary and space science, 159, pp. 97-104. Elsevier 10.1016/j.pss.2018.04.021

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In most surface-bound exospheres Na has been observed at altitudes above what is possible by thermal release.
Photon stimulated desorption of adsorbed Na on solid surfaces has been commonly used to explain observations at
high altitudes. We investigate three model velocity distribution functions (VDF) that have been previously used in
several studies to describe the desorption of atoms from a solid surface either by electron or by photon
bombardment, namely: the Maxwell-Boltzmann (M-B) distribution, the empirical distribution proposed by [1] for
PSD, and the Weibull distribution. We use all available measurements reported by [2, 3] to test these distributions
and determine which one fits best (statistically) and we discuss their physical validity. Our results show that the
measured VDF of released Na atoms are too narrow compared to Maxwell-Boltzmann fits with supra-temperatures
as suggested by [3]. We found that a good fit with M-B is only achieved with a speed offset of the whole dis-
tribution to higher speeds and a lower temperature, with the offset and the fit temperature not showing any
correlation with the surface temperature. From the three distributions we studied, we find that the Weibull
distribution provides the best fits using the temperature of the surface, though an offset towards higher speeds is
required. This work confirms that Electron-Stimulated Desorption (ESD) and Photon-Stimulated Desorption (PSD)
should produce non-thermal velocity (or energy) distributions of the atoms released via these processes, which is
expected from surface physics. We recommend to use the Weibull distribution with the shape parameter κ ¼ 1.7,
the speed offset v
0
¼ 575 m/s, and the surface temperature to model PSD distributions at planetary bodies

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