Intracavity laser photoelectron spectroscopy of Cr 2-, Cr 2 H-, and Cr 2 D-

Abstract

The photoelectron spectrum of Cr 2 - shows vibrational levels in the 1 (Sigma) 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 Cr 2 . Low-lying vibrational levels are found to fit a Morse potential with (omega) e equals 479 +/- 2 cm -1 and (omega) e (chi) e equals 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 Cr 2 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 Cr 2 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 Cr 2 is measured to be 0.505 +/- 0.005 eV. An excited electronic state of Cr 2 with a vibrational frequency of 580 +/- 20 cm -1 is observed 14,240 +/- 30 cm -1 above the ground state. For Cr 2 - , we obtain (omega) e equals 470 +/- 25 cm -1 , (omega) e (chi) e equals 20 +/- 10 cm -1 , and r e equals 1.71 +/- 0.01 angstrom. Tentative state assignments of 1 (Sigma) u + or 3 (Sigma) u + for the excited Cr 2 state, and 2 (Sigma) u + for the anion, are discussed. Preliminary results for Cr 2 H - and Cr 2 D - 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.