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

We have obtained rotationally resolved pulsed field ionization photoelectro n (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 rotati onal-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 spectro scopic data for these states. The simulation of spectra obtained at rotatio nal temperatures of approximate to 20 and 220 K allows the unambiguous iden tification of O-2(+)(X (2)Pi(1/2,3/2g), v(+)greater than or equal to 21) PF I-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 wi th spectroscopic data obtained in the previous emission study and the prese nt PFI-PE experiment, we have obtained accurate Dunham-type expansion coeff icients for ionization energies, vibrational constants, rotational constant s, 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-r esonant 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 observ ation of a long PFI-PE vibrational progression with a relatively smooth ban d 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)) st ates in the Franck-Condon gap region. Since this experiment was carried out under relatively high rotational temperatures for O-2, the PFI-PE data rev eal higher rotational transitions and numerous local intensity enhancements , which were not observed in previous vacuum ultraviolet laser studies usin g a cold O-2 molecular beam. The rotational branches found here indicate th at photoelectrons are formed predominantly in continuum states with orbital angular momenta l=1,3, and 5. (C) 1999 American Institute of Physics. [S00 21-9606(99)00929-0].