Formation of metastable N-2(-) and CO- anions in sputtering

Sputtering the surface of a TiAlN(C,O) specimen with a 14.5-keV Csi ion beam produces negative molecular ions of N-2(-) and CO-. They are detected in a double-focusing mass spectrometer operated at a mass resolution of similar to 13 000; this provides a separation of interfering species in this mass region (e.g., Si-, AlH-) and a mass assignment at st precision of similar to 10(-4) amu. The flux of N-2(-) relative to the stable N-3(-) amounts to about 0.3. On average, one N-2(-) is formed for 10(7) nT atoms sputtered and a similar rate is observed for CO-. The flight time through the mass spectrometer (similar to 10 mu s) sets a lower Limit for the stability of these molecular anions. The abundant production of electronically excited molecules in sputtering is proposed to constitute the prerequisite for the formation of N-2(-) and CO-: these excited molecules may capture a surface electron forming thus a temporary negative-ion state (Feshbach resonance). By contrast, the upper limit for the emission of N- is found to be less than one N- per 10(10) nitrogen atoms ejected, in agreement with the very short lifetimes of the excited N- anions.