Negative ion photoelectron spectroscopy of Cr₂

The 476-514-nm photoelectron spectra of Cr₂^-, recorded at 4-meV resolution, show vibrationally resolved transitions to the ground state and to one excited electronic state of neutral Cr₂. The measured electron affinity of Cr₂ is 0.505 ± 0.005 eV. Observed vibrational energies in the ¹Σ_g⁺ ground state of Cr₂ up to v = 9, 3040 cm^-1 above the zero point level, fit a Morse potential with ω_e = 480.6 ± 0.5 cm^-1 and ω_eχ_e = 14.1 ± 0.3 cm^-1. Twenty additional Cr₂ levels that are evenly spaced with a 128-cm^-1 average interval are observed 4880-7320 cm^-1 above the 0-0 transition. Based on the measured isotope shift for ⁵²Cr₂ and ⁵⁰Cr⁵²Cr and other considerations, we assign these as highly excited vibrational levels of the Cr₂ ground state. An RKR potential is reported for this state that fits all of the measured vibrational levels to within experimental error. The observed ∼ 128-cm^-1 intervals are similar to the 110-cm^-1 intervals predicted by Goodgame and Goddard for the 4s-4s bonded outer well of a double-minimum ground-state potential. However, the observed intervals occur at much lower energies above the zero-point level than are predicted, and the fitted potential obtained here differs considerably from the MGVB potential. Intensities of transitions to v ≥ 2 levels in the photoelectron spectrum do not follow a Franck-Condon intensity profile; in addition, they vary dramatically with laser wavelength. Peak spacings from the 0-0 transition, however, are not wavelength dependent. These results indicate that the intensities of these relatively weak spectral features are strongly affected by an indirect photodetachment process involving a resonant autodetaching state of the Cr₂^- anion, a possible assignment for which is proposed. The spectrum also shows a transition to an excited electronic state of neutral Cr₂ 14 240 ± 20 cm^-1 above the ground state. This excited state has a high vibrational frequency (ΔG_1/2) of 574 ± 10 cm^-1 and a short 1.65 ± 0.02-Å bond length. It is assigned as a ³Σ_u⁺ state with single occupation of the 4sσ_g orbital and the 4sσ_u* orbital. For the ground state of the Cr₂^- anion, we obtain ΔG_1/2 = 411 ± 10 cm^-1, ω_e = 440 ± 20 cm^-1, ω_eχ_e = 14 ± 6 cm^-1, r_e = 1.705 ± 0.010 Å, and a bond dissociation energy 0.17 ± 0.01 eV lower than that of Cr₂. The anion ground state is assigned as a ²Σ_u⁺ state in which the extra electron occupies the 4sσ_u* orbital.