Formation of metastableN2−andCO−anions in sputtering

Sputtering the surface of a TiAlN(C,O) specimen with a $14.5-{\mathrm{keV}\mathrm{}\mathrm{Cs}}^{+}$ ion beam produces negative molecular ions of ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}}.$ They are detected in a double-focusing mass spectrometer operated at a mass resolution of $\sim 13000;$ this provides a separation of interfering species in this mass region (e.g., ${\mathrm{Si}}^{\mathrm{-}},$ ${\mathrm{AlH}}^{\mathrm{-}}$) and a mass assignment at a precision of $\sim {10}^{-4}\mathrm{amu}.$ The flux of ${\mathrm{N}}_2^{\mathrm{-}}$ relative to the stable ${\mathrm{N}}_3^{\mathrm{-}}$ amounts to about 0.3. On average, one ${\mathrm{N}}_2^{\mathrm{-}}$ is formed for ${10}^7$ N atoms sputtered and a similar rate is observed for ${\mathrm{CO}}^{\mathrm{-}}.$ The flight time through the mass spectrometer ($\sim 10\mu \mathrm{s}$) 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 ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}}:$ 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 ${\mathrm{N}}^{\mathrm{-}}$ is found to be less than one ${\mathrm{N}}^{\mathrm{-}}$ per ${10}^{10}$ nitrogen atoms ejected, in agreement with the very short lifetimes of the excited ${\mathrm{N}}^{\mathrm{-}}$ anions.