Rotational branching ratios at low photoelectron energies in resonant enhanced multiphoton ionization of NO.

We report calculated rotational branching ratios for very low energy (50 meV) photoelectrons resulting from (1+1’) resonant enhanced multiphoton ionization (REMPI) via the Ji =1/2, 3/2, 5/2, and 7/2 levels of the P11 branch of the A 2Σ+ (3sσ) state of NO. Even angular momentum transfer (ΔN≡N+-Ni) peaks are dominant in these rotational distributions, in agreement with the selection rule ΔN+l=odd. Angular momentum coupling in the photoelectron wave function arising from the molecular ion potential leads to smaller but appreciable ΔN=odd peaks. The calculated ΔN=0 to ΔN=+2 peak ratios show the same strong decrease when Ji increases from 1/2 to 3/2 as seen in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra [Sander et al., Phys. Rev. A 36, 4543 (1987)], but do not show the rapid die-off of the ΔN≠0 peaks for higher Ji observed experimentally. The calculated trend in the ΔN=+2 vs ΔN=0 peaks could be understood on the basis of simple angular momentum transfer arguments. These same arguments indicate that this trend in the ΔN=0 and +2 peaks with increasing angular momentum is not generally expected in other branches. Spectra via the R21 ( J) branch are presented to support this assertion. We also present photoelectron angular distributions which show a strong dependence on ΔN reflecting the changing composition of the photoelectron wave function. [ABSTRACT FROM AUTHOR]