Rotational branching ratios in (1+1) resonant-enhanced multiphoton ionization of NO via theAΣ+2state

The rotational branching ratios resulting from (1+1) resonant-enhanced multiphoton ionization of NO via the $A{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ Rydberg state are analyzed. Theoretical results using ab initio molecular parameters agree reasonably well with recent experimental data. More importantly, the analysis underscores the importance of the molecular nature of the problem and its resulting complexities. It is shown that, for photoionization of a $\Sigma$ state that leaves the ion in a $\Sigma$ state, the allowed rotational states of the ion satisfy the selection rule $\Delta N+l=\mathrm{odd}$, where $\Delta N$ is the difference in (electronic + rotational) quantum numbers for the neutral and for the ion, and $l$ is the partial wave of the electron. Based on this selection rule, it follows that the predominantly gerade $3s\sigma$ Rydberg orbital of the $A{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ state couples only to the ungerade channel in the continuum ($l$ odd), thereby suppressing the $\Delta N=\pm 1$ peaks, in agreement with experiment. The molecular nature of the ionic potential leads to strong $l$ mixing in electronic continuum orbitals. In fact, the influence of a nearby shape resonance causes the $f$ wave to be dominant in the $\sigma$ channel.