We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O-2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1-19.4 eV, yielding accurate ionization energies for the transitions O-2(+)(b (4) Sigma(g)(-), upsilon(+) = 0-9, N+ = 1) <-- O-2(X (3) Sigma(g)(-), upsilon = 0, N = 1).