Detection of the diatomic dications SiH2þ and AlH2þ

The diatomic dications SiH2þ and AlH2þ were produced by Arþ sputtering of a hydrogenated amorphous silicon (a-Si:H) layer deposited on an Al substrate. These exotic dications were observed in positive SIMS mass spectra taken at low (30Si1H2þ at m/z 15.5) and at high mass resolution. More intense signals of SiH2þ or AlH2þ could be produced by toluene (C7H8) vapor flooding of an Arþ bombarded Si or Al surface. The ion energy distributions of SiH2þ for both a-Si:H/Al and toluene-flooded Si show that it is formed at or near the sputtered surface. Ion formation by collisional processes in the gas phase several micrometers away from the surface is found to be negligible. Exposure of the sputtered a-Si:H surface to oxygen reduced the intensity of SiH2þ. This argues strongly against an association of Siþ and Hþ at or near the surface. It is conjectured that ion formation of SiH2þ may be caused by Auger shake-off decay of Si2p core-excited molecules of SiH* a few nanometers in front of the sputtered surface. # 2004 Elsevier B.V. All rights reserved