Detection of the diatomic dications SiH ²⁺ and AlH ²⁺

The diatomic dications SiH ²⁺ and AlH ²⁺ were produced by Ar ⁺ 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 ( ³⁰ Si ¹ H ²⁺ at m/z 15.5) and at high mass resolution. More intense signals of SiH ²⁺ or AlH ²⁺ could be produced by toluene (C ₇ H ₈ ) vapor flooding of an Ar ⁺ bombarded Si or Al surface. The ion energy distributions of SiH ²⁺ 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 SiH ²⁺ . This argues strongly against an association of Si ⁺ and H ⁺ at or near the surface. It is conjectured that ion formation of SiH ²⁺ may be caused by Auger shake-off decay of Si2p core-excited molecules of SiH* a few nanometers in front of the sputtered surface.