Measurement of the Xenon σ3/2:σ1/2 Branching Ratio within the Xe 5s5p66p(1P1) Resonance

Photoionization of the 5p6 shell in xenon leaves the ion in either the 5p(P3/2) or np(P1/2) state. Measurements of the P3/2 : P1/2 branching ratio are reported within the Xe 5s5p6p(P1) resonance and fitted to a new theoretical expression. The maximum value of the ratio as determined by a two-parameter fit of theory to experiment is 8.8 ± 0.5 within the resonance, almost six times the nearly constant value of 1.55 found outside any resonance. Photoionization of the outer np(S0) subshell of the rare gases leaves the ion in either the np(P3/2) or np(P1/2) state. The ratio of the cross sections for these two alternative reactions has been found experimentally to be remarkably constant and non-statistical over a photon energy range of approximately 25 eV above the np(S0) ionization threshold (Samson and Cairns 1968, Samson et al. 1975). These earlier measurements, the analysis of which required some estimated values for the asymmetry parameter β, indicated that a large change in the ratio may occur within autoionizing resonances, presumably due to strong electron correlations, but since only discrete emission lines were available at that time, it was impossible to study the ratio systematically across a resonance. This letter reports the first measurement of this ratio as a function of wavelength within a resonance; the experimental data presented is unaffected by any variations in the photoelectron angular distribution. The particular resonance chosen is the xenon 5s5p6p(P1) autoionizing level at 20.95 eV. Whereas the off-resonance ratio σ3/2 : σ1/2 has been found to be nearly constant at 1.55 ± 0.08 (Samson and Cairns 1968, Samson et al. 1975), well below the statistical ratio of 2.0, we find that within the resonance this ratio increases sharply from its off-resonance value to values greater than three. The experimental data have been fitted to a two-parameter theoretical formula of Starace (1977), and related to the Fano profile parameters determined by Ederer (1971, 1976) for this resonance. The present experimental data demonstrate the strength of electron correlation within small energy regions, as found previously for the photoelectron asymmetry parameter within a resonance (Samson and Gardner 1973, Dill 1973). Furthermore the present branching-ratio data, together with the line-profile data, provide experimental information on the theoretical matrix elements needed to characterize the autoionizing resonance Xe 5s5p6p(P1) completely.