  The kinetic energy distributions of photo-electrons emitted from gold
surfaces under illumination by UV-light close to the threshold are measured and
analyzed. Samples are prepared as chemically clean through Ar-Ion sputtering
and then exposed to atmosphere for variable durations before Quantum Yield
measurements are performed after evacuation. During measurements the bias
voltage applied to the sample is varied and the resulting emission current
measured. Taking the derivative of the current-voltage curve yields the energy
distribution which is found to closely resemble the distribution of total
energies derived by DuBridge for emission from a free electron gas. We
investigate the dependence of distribution shape and width on electrode
geometry and contaminant substances adsorbed from the atmosphere, in particular
to water and hydro-carbons. Emission efficiency increases initially during air
exposure before diminishing to zero on a timescale of several hours, whilst
subsequent annealing of the sample restores emissivity. A model fit function,
in good quantitative agreement with the measured data, is introduced which
accounts for the experiment-specific electrode geometry and an energy dependent
transmission coefficient. The impact of large patch potential fields from
contact potential drops between sample and sample holder is investigated. The
total quantum yield is split into bulk and surface contributions which are
tested for their sensitivity to light incidence angle and polarization. Our
results are directly applicable to model parameters for the contact-free
discharge system onboard the LISA Pathfinder spacecraft.
