Velocity distribution of photoelectrons over sunlit moon

Photoelectrons are an integral part of the sunlit lunar plasma environment. This paper formulates the characteristic velocity distribution of the emitted photoelectrons over the sunlit Moon. As an advancement, Fowler's photoemission theory has been coupled with observed solar spectra, photoelectric quantum yield, and latitude-dependent surface temperature in deriving the velocity distribution function (VDF) of the photoelectrons. It is noticed that the dominant contribution in the photoelectron VDF comes from extreme ultraviolet photons of the solar spectra and it significantly deviates from the Maxwellian nature. Moreover, within a photoelectron sheath which refers to finite surface potential, the velocity distributions significantly differ from that of the uncharged surface — VDF has been evaluated in terms of free and trapped photoelectron population densities. The derived VDF is found in close agreement with the measured photoelectron velocity distribution.