Role of spins in molecular photoionization: Spectroscopy and dynamics of autoionizing Rydberg states of ortho-H2

Abstract

A combined experimental and theoretical investigation of the role of electronic and nuclear spins in molecular photoionization is reported. Photoionization spectra of autoionizing $p$ Rydberg states belonging to series converging on the $X$ $^{2}\ensuremath{\Sigma}_{g}^{+}({v}^{+}=0,{N}^{+}=3)$ level of ortho-${\mathrm{H}}_{2}^{+}$ have been measured in the range of principal quantum number $n=50--200$ from the long-lived $H$ $^{1}\ensuremath{\Sigma}_{g}^{+}(v=0,J=3)$ level. The use of a pulsed near-Fourier-transform-limited laser with a bandwidth of less than $10\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ resulted in a Doppler-limited linewidth of $25\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ which sufficed to partially resolve the hyperfine structure of the Rydberg states. Below $n\ensuremath{\approx}70$, the exchange interaction between the ion core and Rydberg electrons is larger than the hyperfine interactions in the ion core and the observed levels can be understood in terms of Hund's angular momentum coupling case (d). With increasing value of $n$, the hyperfine interactions in the core lead to a mixing of singlet and triplet characters of the Rydberg states and eventually to a complete decoupling of the Rydberg electron spin from the core spins that results in distinct series converging on the hyperfine components of the ion. Several intermediate coupling cases have been identified and two of them completely characterized. Most interestingly, the total spin angular momentum has been found to be a good quantum number up to $n\ensuremath{\approx}150$ at least, i.e., far beyond the region where the ionic hyperfine structure starts dominating the coupling hierarchy. Multichannel quantum defect theory including nuclear and electron spins has been extended to treat autoionization and predict spectral intensities. The comparison with the experimental spectra has revealed a satisfactory agreement between calculated and measured line positions, linewidths, and intensities and has enabled us to extract, by extrapolation, a more accurate term value for the $H$ $^{1}\ensuremath{\Sigma}_{g}^{+}(v=0,J=3)$ level. The calculations have been used to characterize the role of hyperfine, spin-rotational, and pf interactions in rotational autoionization and have revealed a very strong dependence of the autoionization lifetimes of high Rydberg states on the value of the total angular momentum quantum number $F$.