Intensity and Energy Distributions of Neutral Chromium Clusters Sputtered by Low-Energy Argon Ions.

The technique of sputtered neutral mass spectrometry (SNMS) was used to study energy distributions of neutral chromium dimers (Cr(,2)) and yield distributions of neutral chromium dimers and trimers (Cr(,3)) sputtered from a polycrystalline chromium target by normally incident singly charged argon ions (Ar('+)) with energies from 200-900 electron volts. The argon ions were generated in a thermionically sustained, magnetically confined Ar plasma operating at a pressure of approximately 2 x 10('-3) torr. The plasma also served as a means of efficiently positionizing the sputtered neutral cluster. The slopes of high energy tails of the sputtered chromium dimer energy distributions were calculated and were found to give results consistent with a statistical recombination model of neutral cluster formation. The yield distribution of the neutral Cr(,2) molecules was also found to be in agreement with the predicted dependence of dimer yield on total sputtering yield for the recombination model, i.e., that the dimer yield should be proportional to the square of the total sputtering yield for chromium. The neutral Cr(,3) molecules observed were estimated to be less than 10('-5) of the total sputtering yield. The distributions of the sputtered trimers versus the total sputtering yield were not found to be in good agreement with the predicted dependence by the recombination model as the cube of the total sputtering yield. The dependence was, however, more consistent with the statistical recombination model than with the direct emission model of neutral cluster formation, which predicts a dependence on the first power of the total yield. The results of this study of energy and intensity distributions of sputtered neutral chromium dimers and of intensity distributions of sputtered neutral chromium trimers supports the statistical recombination model of neutral metal cluster formation, as opposed to the direct emission model, as the more correct view of this physical phenomenon.