Negative ion photoelectron spectroscopy of Cr2

Sean M. Casey, Doreen G. Leopold

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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 1Σg+ ground state of Cr2 up to v = 9, 3040 cm-1 above the zero point level, fit a Morse potential with ωe = 480.6 ± 0.5 cm-1 and ωeχe = 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 (ΔG1/2) of 574 ± 10 cm-1 and a short 1.65 ± 0.02-Å bond length. It is assigned as a 3Σu+ state with single occupation of the 4sσg orbital and the 4sσu* orbital. For the ground state of the Cr2- anion, we obtain ΔG1/2 = 411 ± 10 cm-1, ωe = 440 ± 20 cm-1, ωeχe = 14 ± 6 cm-1, re = 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 2Σu+ state in which the extra electron occupies the 4sσu* orbital.

Original languageEnglish (US)
Pages (from-to)816-830
Number of pages15
JournalJournal of physical chemistry
Issue number4
StatePublished - 1993


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