Rotationally resolved pulsed field ionization photoelectron study of O-2(+)(B-2 Sigma(-)(g), (2)Sigma(-)(u); v(+) = 0-7) at 20.2-21.3 eV

We have obtained rotationally resolved pulsed field ionization photoelectro n (PFI-PE) spectra of O<INF>2</INF> in the energy range of 20.2-21.3 eV, co vering the ionization transitions of O<INF>2</INF><SUP>+</SUP>(B <SUP>2</SU P>Sigma<INF>g</INF><SUP>-</SUP>, upsilon+ = 0-7, N<SUP>+</SUP>) <-- O<INF>2 </INF>(X<SUP></SUP> <SUP>3</SUP>Sigma<INF>g</INF><SUP>-</SUP>, upsilon " = 0, N "). Only the Delta N = -2, 0, and +2 (or O, Q, and S) rotational branc hes are observed in the PFI-PE bands for O<INF>2</INF><SUP>+</SUP>(B <SUP>2 </SUP>Sigma<INF>g</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-7), indicating that the outgoing electron continuum channels with angular momenta l = 1 a nd 3 dominate in the ionization transitions. This experiment allows the det ermination of accurate spectroscopic constants, such as ionization energy ( 20.2982<INF>5</INF> +/- 0.0004 eV) for the formation of O<INF>2</INF><SUP></SUP>[B <SUP>2</SUP>Sigma<INF>g</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0 , N<SUP>+</SUP> = 1 (F<INF>2</INF>)] from O<INF>2</INF>(X <SUP>3</SUP>Sigma <INF>g</INF><SUP>-</SUP>, upsilon " = 0, N " = 1), vibrational constants (o mega<INF>e</INF><SUP>+</SUP> = 1152.91 cm<SUP>-1</SUP>, omega<INF>e</INF><S UP>+</SUP>chi<INF>e</INF><SUP>+</SUP> = 20.97 cm<SUP>-1</SUP>_, and rotatio nal constants (B<INF>e</INF><SUP>+</SUP> = 1.255 +/- 0.001<INF>5</INF> cm<S UP>-1</SUP>, alpha<INF>e</INF><SUP>+</SUP> = 0.0241 +/- 0.0003<INF>7</INF> cm<SUP>-1</SUP>_ for O<INF>2</INF><SUP>+</SUP>(B <SUP>2</SUP>Sigma<INF>g</I NF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-6) are measured to be approximate to 0.2-0.6 mu s, which are significantly shorter than those of approximate to 1.9 mu s observed for O<INF>2</INF><SUP>+</SUP>(b <SUP>4</SUP>Sigma<INF> g</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-5). The shorter (nominal) effe ctive lifetimes for high-n Rydberg states converging to O<INF>2</INF><SUP></SUP>(B <SUP>2</SUP>Sigma<INF>g</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0 -6) are attributed to the higher kinetic energy releases (or velocities) of O<SUP>+</SUP>+O fragments resulting from predissociation of the O<INF>2</I NF><SUP>+</SUP>(B <SUP>2</SUP>Sigma<INF>g</INF><SUP>-</SUP>, upsilon<SUP>+< /SUP> = 0-6) ion cores. Rotationally resolved PFI-PE measurements also make possible the identification of the weak vibrational progression with the o rigin at 20.35 eV as associated with transitions to O<INF>2</INF><SUP>+</SU P>(<SUP>2</SUP>Sigma<INF>u</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-7). T he analysis of the rotationally resolved PFI-PE bands for O<INF>2</INF><SUP >+</SUP>(<SUP>2</SUP>Sigma<INF>u</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0 and 1) has yielded accurate rotational constants and IE values for these s tates. The rotational structures resolved in the O<INF>2</INF><SUP>+</SUP>( <SUP>2</SUP>Sigma<INF>u</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0 and 1) P FI-PE bands are contributed overwhelmingly by the Delta N = -3, -1, +1, and +3 (or N, P, R, and T) rotational branches, showing that the angular momen ta for the outgoing photoelectron are restricted to l = 0, 2, and 4. Based on simulation of the observed rotational structures, we also obtain the pre dissociative lifetimes for O<INF>2</INF><SUP>+</SUP>(B <SUP>2</SUP>Sigma<IN F>g</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-7) and O<INF>2</INF><SUP>+</ SUP>(<SUP>2</SUP>Sigma<INF>u</INF><SUP>-</SUP>, upsilon<SUP>+</SUP> = 0-7) and O<INF>2</INF><SUP>+</SUP>(<SUP>2</SUP>Sigma<INF>u</INF><SUP>-</SUP>, up silon<SUP>+</SUP> = 0-1) to be in the range of 0.45-2 ps. (C) 1999 American Institute of Physics. [S0021-9606(99)00801-6].