On the origin of broad band optical emission from Mo, Nb, and W bombarded by heavy ions

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Abstract
Continuum emission has been observed when Mo, W, and Nb targets were bombarded with 10–30 keV, Ne+ and Ar+. The emission occurs only when the environment of the target chamber contains an oxygen partial pressure of 2×10−9 torr or above; for lower pressures the emission is absent. Identical continua are also observed when the metal oxides are bombarded with Ne+ and Ar+; similar spectral features, though with changed relative intensities, are seen when the targets are bombarded with O+ and O2+ ions in the absence of O2 gas in the vacuum environment. A significant feature of these continua is that the emitting species extend for some mm beyond the bombarded surface. We suggest that the source of the emission is sputtered oxides of the target. The variation of emission intensity with oxygen partial pressure and bombarding ion beam current is consistent with a mechanism where the oxide is formed by the simultaneous sputtering of an adsorbed oxygen atom and a metal atom from the bulk material.