High-resolution photoelectron spectroscopy using multibunch synchrotron radiation:: rotational-resolved photoelectron bands of O2+(b 4Σg−, v+) Original Research Article

We report rotational-resolved single-photon threshold photoelectron and pulsed field ionization zero kinetic energy (PFI–ZEKE) photoelectron (PE) spectra of O2 in the energy range of 18.1–20.2 eV measured using high-resolution monochromatized multibunch undulator synchrotron radiation. The PFI–ZEKE PE bands for O2+(b 4Σg−, v+=0–9) have been simulated using the Buckingham–Orr–Sichel model derived for rotationally resolved single-photon ionization cross-sections. Only the ΔN=−2, 0 and +2 (or O, Q and S) rotational branches are observed for these PFI–ZEKE PE bands, indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the threshold ionization transitions O2+(b 4Σg−, v+=0–9, N+)←O2(X 3Σg−, v″=0, N″). The relative rotational branch intensities for O2+(b 4Σg−, v+=4 and 5) are found to be drastically different from those for O2+(b 4Σg−, v+=0–3, 6 and 7). Considering that the energies of O2+(b 4Σg−, v+=4 and 5) are close to the dissociation limit of O+(4S)+O(3P) and that the crossing location of the O2+(b 4Σg−) and d 4Σg+ potential curves is shown to be in the vicinity of O2+(b 4Σg−, v+=4 and 5), we suggest that the latter observation is the result of predissociative perturbations by the d 4Σg+ state. Within the uncertainties of this experiment, the ionization energies for the formation of O2+(b 4Σg−, v+=0–9, N+=1) can be satisfactorily characterized using a Morse potential. The effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=0, 2–5) prepared in the present experiment are nearly constant with values in the range of 1.8–2.0 μs. The observation that the effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=4 and 5) are significantly longer than the known dissociative lifetimes of the corresponding ionic states is in accordance with the expectation that the couplings between the O2+ ion core and the high-n Rydberg electron involved are negligibly small.