Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0-38) in the energy range of 12.05-18.15 eV

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 (2)Pi(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 (2)Pi(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 (2)Pi(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 approximate to 20 and 220 K allows the unambiguous identification of O-2(+)(X (2)Pi(1/2,3/2g), v(+)greater than or equal to 21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O-2(+)(A (2)Pi(u), v(+)=0-12) and O-2(+)(a (4)Pi(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 (2)Pi(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 (2)Pi(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 (2)Pi(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. (C) 1999 American Institute of Physics. [S0021-9606(99)00929-0].