Chemi-ionization in alkali-heteronuclear halogen collisions: Role of excited molecular ion states

Total cross sections have been measured for the production of negative halogen ions in collisions between the alkali atoms K, Na and Li and the heteronuclear halogen molecules lBr and ICl at relative energies from threshold up to about 20 eV. By means of a mass spectrometer the cross sections for negative parent ion and fragment ion production could be measured separately. The total cross sections for the IX- and X- production show features similar to those reported earlier for M + X-2 --> M+ + X-2(-), M+ + X + X-. The most striking feature or this work is the presence of the I- production. This is interpreted to occur via an excited state of IX- ((2)Pi(2/2)). The general behaviour of the I- cross section is characteristically dependent on the alkali collision partner; for K collisions the I- cross section peaks just above threshold and falls rapidly, for Na the cross section is just about the experimental, uncertainty and rat Li negligible. This behaviour will be discussed in terms of a modified Landau-Zener theory. From these measurements we have inferred the adiabatic electron affinity of IX (ICI: 2,41 eV, [Dr. 2.55 eV) and the well depth of IX-((2) Sigma(+)) (ICI-: 0.95 eV, IBr-: 1.05 eV).