Rotationally resolved pulsed field ionization photoelectron bands of O_2~+ (X~2II_(1/2,3/2g),v~+ = 0 - 38) in the energy range of 12.05 - 18.15 eV

摘要

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra for O_2 in the energy range of 12.05 - 18.15 eV, covering ionization transitions O_2~+(X~2II_(1/2,3/2g)v~+ = 0 - 38,J~+) <- O_2(X~3#SIGMA#_g~-, v~+ = 0,N"). While the PFI-PE bands for O_2~+(X~2II_(1/2,3/2g),v~+ = 3 - 5, 9, 11, 12, 22, and 25 - 38) reported here are the first rotational-resolved photoelectron measurements, the PFI-PE bands for O_2~+(X~2II_(1/2,3/2g),v~+ = 25 - 38) represent the first rotationally resolved spectroscopic data for these states. The simulation of spectra obtained at rotational temperatures of approx= 20 and 220 K allows the unambiguous identification of O_2~+ (X~2II_(1/2,3/2g), v~+ >= 21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O_2~+ (A ~2II_u, v~+ = 0 - 12) and O_2~+ (a ~4II_u, v~+ = 0 - 18). Combined with spectroscopic data obtained in the previous emission study and the present PFI-PE experiment, we have obtained accurate Dunham-type expansion coefficients for ionization energies, vibrational constants, rotational constants, and spin-orbit splitting constants covering the O_2~+ (X ~2II_(1/2, 3/2g, v~+ = 0 - 38) states. Significant local intensity enhancements due to near-resonant autoionization were observed in PFI-PE bands for O_2~+ (X ~2II_(1/2,3/2g), v~+ = 0 - 14). The energy region of these states is known to manifest a high density of very strong autoionizing low-n-Rydberg states. The observation of a long PFI-PE vibrational progression with a relatively smooth band intensity profile is also in accord with the direct excitation model for the production of highly vibrationally excited O_2~+(X ~2II_(1/2,3/2g)) states in the Franck-Condon gap region. Since this experiment was carried out under relatively high rotational temperatures for O_2, the PFI-PE data reveal higher rotational transitions and numerous local intensity enhancements, which were not observed in previous vacuum ultraviolet laser studies using a cold O_2 molecular beam. The rotational branches found here indicate that photoelectrons are formed predominantly in continuum states with orbital angular momenta l = 1, 3, and 5.