Intracavity laser photoelectron spectroscopy of Cr2-, Cr2H-, and Cr2D-

The photoelectron spectrum of Cr₂^- shows vibrational levels in the ¹Σ_g⁺ ground state of the neutral molecule up to 7300 cm^-1 above its zero point level. These data, obtained at an instrumental resolution of 5 meV (40 cm^-1), reveal a panoramic view of the controversial ground state potential curve of Cr₂. Low-lying vibrational levels are found to fit a Morse potential with approximately ega_e = 479 ± 2 cm^-1 and approximately ega_eχ_e = 13.5 ± 1.0 cm^-1. This unusually large anharmonicity extrapolates to a dissociation asymptote of only 0.5 eV, considerably lower than the true 1.44 eV value. Between 4875 and 7320 cm^-1 above the zero point level, we observe twenty peaks at 130 ± 15 cm^-1 intervals, which we assign as transitions from the ground electronic and vibrational state of the anion to high vibrational levels of the Cr₂ ground state. Using an RKR inversion procedure, we have obtained a potential curve that fits all of the observed vibrational levels to within our experimental uncertainty. This potential curve is compared with the predictions of Goodgame and Goddard's modified GVB calculation. Transitions to highly excited vibrational levels of the Cr₂ ground state are far more intense than would be expected for a direct photodetachment process, and are also strongly wavelength dependent. These non-Franck-Condon intensities are attributed to a resonance of the laser with one or more metastable states of the negative ion far above the electron detachment threshold. The electron affinity of Cr₂ is measured to be 0.505 ± 0.005 eV. An excited electronic state of Cr₂ with a vibrational frequency of 580 ± 20 cm^-1 is observed 14,240 ± 30 cm^-1 above the ground state. For Cr₂^-, we obtain approximately ega_e = 470 ± 25 cm^-1, approximately ega _eχ_e = 20 ± 10 cm^-1, and r_e = 1.71 ± 0.01 angstrom. Tentative state assignments of ¹Σ_u⁺ or ³Σ _u⁺ for the excited Cr₂ state, and ²Σ_u⁺ for the anion, are discussed. Preliminary results for Cr₂H^- and Cr₂D^- are also presented. The photoelectron spectra of these anions reveal the Cr-Cr and Cr-H stretching frequencies in the neutral molecules, and exhibit partially resolved rotational structure.