  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 [Wurz et al., 2010] for PSD, and the Weibull distribution. We use all
available measurements reported by [Yakshinskiy and Madey, 2000, 2004] 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 distribution 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 kappa=1.7, the speed offset v0=575 m/s, and the
surface temperature to model PSD distributions at planetary bodies.
