Resonances in photodetachment of sodium halide negative ions

The cross sections for photodetachment of the alkali halide negative ions NaCl−, NaBr−, and NaI− have been measured in the photon energy range 2.0–2.3 eV. In this energy range, well above the photodetachment threshold at approximately 0.6 eV, the only processes energetically accessible are photodetachment to produce the ground electronic state neutral alkali halide, and photodissociation. For each of the sodium halides, we observe a single isolated resonance in the cross section, occurring approximately 160 cm−1 to the high energy side of the neutral sodium atom 3s–3p energy difference. This resonance is interpreted as autodetachment of a 2Π state of the negative ion imbedded in the photodetachment continuum. The X 2Σ–A 2Π energy difference in NaX− is almost completely halide independent, providing strong support for the qualitative picture of the negative ion as being composed of a neutral sodium atom adjacent to a closed shell halide ion. Qualitative fits of the experimental data utilizing the Fano formalism indicate that the lifetime of the autodetaching state is on the order of 10 −12 sec and that the shape of the potential well for the 2Π state is virtually identical to that of the negative ion ground state.