High-resolution pulses field ionization photoelectron study of O2:: Predissociation lifetimes and high-n Rydberg lifetimes converging to O2+(c 4Σu-, v+=0,1)

We have measured the pulsed field ionization photoelectron (PFI-PE) spectrum of O-2 in the energy range of 24.53-25.0 eV at a PFI-PE resolution of 11 cm(-1) (full Width at half maximum, FWHM), The PFI-PE bands for O-2(+)(c (4)Sigma(u)(-), nu(+) = 0 and 1) obtained at O-2 rotational temperatures of 35 and 298 K have been simulated using the Buckingham-Orr-Sichel. model. Only the Delta N = -3, -1, +1, and +3 (or N, P, R, and T) rotational branches are observed, indicating that the outgoing electron continuum channels with angular momenta l=0, 2, and 4 dominate in-the threshold ionization transitions O-2(+)(c (4)Sigma(u)(-), nu(+) = 0 to 1, N+)<-- O-2(X (3)Sigma(g)(-), nu "=0, N "). The simulation yields natural rotational linewidths of 19.6+/-2.0 and 77+/-8 cm(-1) (FWHM) for the respective nu(+) = 0 and 1 PFI-PE bands of the O-2(+)(c (4)Sigma(u)(-)) state. These linewidths make possible the determination of the predissociation lifetimes for the nu(+) = 0 and 1 levels of O-2(+)(c (4)Sigma(u)(-)) to be (2.7+/-0.3) X 10(-13) and (6.9+/-0.7) X 10(-14) s, respectively. This experiment also provides accurate ionization energies of 24.5622(7)+/-0.0005 and 24.7544(5)+/-0.0005 eV for transitions to O-2(+)(c (4)Sigma(u)(-), nu(+)=0, N+=0) and O-2(+)(c (4)Sigma(u)(-), nu(+)=1, N+=0); from O-2(X (3)Sigma(g)(-), nu ", N "=1), respectively. The rotational constants of 1.58+/-0.02 and 1.54+/-0.04 cm(-1) obtained here for the O-2(+)(c (4)Sigma(u)(-) nu(+)=0) and O-2(+)(c (4)Sigma(u)(-), nu(+)=1) states allow the calculation of their corresponding equilibrium bond distances to be 1.155+/-0.011 and 1.170+/-0.015 Angstrom. The (nominal) effective lifetimes for,high-n Rydberg states converging to the O-2(+)(c (4)Sigma(u)(-), nu(+)=0 and 1) states are measured to be approximate to 0.33 mu S, which are significantly shorter than the values of approximate to 1.9 mu s measured for the O-2(+)(b (4)Sigma(g)(-), nu(+)=0-5) states. The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O-2(+)(c (4)Sigma(g)(-), nu(+)=0 and 1) observed are attributed to the higher kinetic energy releases (or velocities) of O+ fragments resulting from predissociation;of the O-2(+) ion cores. (C) 1998 American Institute of Physics.