A secondary electron yield from a polycrystalline A1 surface was measured under Ar+ bombardment. The yield of negatively charged particles gamma(theta) was measured as a function of steady state oxygen coverage theta of target surfaces during Ar+ bombardment. Projectile energy was changed from 20 to 80 keV. The absolute value of effective oxygen coverage theta of a target surface was determined ill situ by means of an optical spectroscopic technique in which light intensities emitted by sputtered excited atoms from the target was measured as a function of current densities of the projectile. Absolute yield of secondary electrons at zero oxygen coverage, gamma(0), was obtained by measuring yield variation as a function of theta.