  The production of negative ions in cesium sputter ion sources is generally
considered to be a pure surface process. It has been recently proposed that ion
pair production could explain the higher-than-expected beam currents extracted
from these ion sources, therefore opening the door for laser-assisted
enhancement of the negative ion yield. We have tested this hypothesis by
measuring the effect of various pulsed diode lasers on the O$^-$ beam current
produced from Al$_2$O$_3$ cathode of a cesium sputter ion source. It is
expected that the ion pair production of O$^-$ requires populating the 5d
electronic states of neutral cesium, thus implying that the process should be
provoked only with specific wavelengths. Our experimental results provide
evidence for the existence of a wavelength-dependent photo-assisted effect but
cast doubt on its alleged resonant nature as the prompt enhancement of beam
current can be observed with laser wavelengths exceeding a threshold photon
energy. The beam current transients observed during the laser pulses suggest
that the magnitude and longevity of the beam current enhancement depends on the
cesium balance on the cathode surface. We conclude that the photo-assisted
negative ion production could be of practical importance as it can more than
double the extracted beam current under certain operational settings of the ion
source.
