Ionic rotational branching ratios in resonant enhanced multiphoton ionization of NO via the A 2Σ+(3sσ) and D 2Σ+(3pσ) states

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Abstract
We present the results of ab initio calculations of the ionic rotational branching ratios in NO for a (1+1) REMPI (resonant enhanced multiphoton ionization) via the A 2Σ+(3sσ) state and a (2+1) REMPI via the D 2Σ+(3pσ) state. Despite the atomic‐like character of the bound 3sσ and 3pσ orbitals in these resonant states, the photoelectron continuum exhibits strong l mixing. The selection rule ΔN+l=odd (ΔN≡N+−Ni) implies that the peaks in the photoelectron spectrum corresponding to ΔN=odd (±1,±3) are sensitive to even partial waves while those corresponding to even ΔN probe the odd partial waves in the photoelectron continuum. Recent experimental high resolution photoelectron studies have shown a strong ΔN=0 peak for ionization via the A 2Σ+ and the D 2Σ+ states, indicating a dominance of odd‐l partial waves. While this seems natural for ionization out of the 3sσ orbital, it is quite anomalous for 3pσ ionization. Based on extensive bound calculations, Viswanathan et al. [J. Phys. Chem. 9 0, 5078 (1986)] attribute this anomaly to a strong l mixing in the electronic continuum caused by the nonspherical molecular potential. We have performed ab initio calculations of the rotational branching ratios and compared them with the experimental results. The electronic continuum shows a significant p‐wave component which leads to the large ΔN=0 peak in both cases. Calculations are performed for both rotationally ‘‘clean’’ and ‘‘mixed’’ branches. The relative heights of the peaks are very sensitive to the photoelectron kinetic energy for the D 2Σ+ state and less so for the A 2Σ+ state. This is a direct consequence of the l mixing in the continuum.