High-resolution photoelectron spectroscopy using multibunch synchrotron radiation:: rotational-resolved photoelectron bands of O2+(b4Σg-,υ+)

We report rotational-resolved single-photon threshold photoelectron and pulsed field ionization zero kinetic energy (PFI-ZEKE) photoelectron (PE) spectra of O-2 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 O-2(+)(b (4)Sigma(g)(-) v(+) = 0-9) have been simulated using the Buckingham-Orr-Sichel model derived for rotationally resolved single-photon ionization cross-sections. Only the Delta N = - , 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 O-2(+)(b (4)Sigma(g)(-), v(+) = 0-9, N+) <-- O-2(X (3)Sigma(g)(-), v" = O, N"). The relative rotational branch intensities for O-2(+)(b (4)Sigma(g)(-), v(+) = 4 and 5) are found to be drastically different from those for O-2(+)(b (4)Sigma(g)(-), v(+) = 0-3, 6 and 7). Considering that the energies of O-2(+)(b (4)Sigma(g)(-), v(+) = 4 and 5) are close to the dissociation limit of O+(S-4) + O(P-3) and that the crossing location of the O-2(+)(b (4)Sigma(g)(-)) and d (4)Sigma(g)(+) potential curves is shown to be in the vicinity of O-2(+)(b (4)Sigma(g)(-), v(+) = 4 and 5), we suggest that the latter observation is thee result of predissociative perturbations by the d (4)Sigma(g)(+) state. Within the uncertainties of this experiment, the ionization energies for the formation of O-2(+)(b (4)Sigma(g)(-), v(+) = 0-9, N+ - 1) can be satisfactorily characterized using a Morse potential. The effective lifetimes for high-n Rydberg states converging to O-2(+)(b (4)Sigma(g)(-), v(+) = 0, 2-5) prepared in the present experiment are nearly constant with values in the range of 1.8-2.0 mu s The observation that the effective lifetimes for high-n Rydberg states converging to O-2(+)(b (4)Sigma(g)(-), 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 O-2 ion core and the high-n Rydberg electron involved are negligibly small. (C) 1998 Elsevier Science B,V. All rights reserved.