(1+1) resonant enhanced multiphoton ionization via the A 2Σ+ state of NO: Ionic rotational branching ratios and their intensity dependence

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Abstract
Recent high resolution photoelectron spectroscopic studies of the (1+1) resonant enhanced multiphoton ionization (REMPI) of NO via the 0–0 transition of the A–X band (γ band) have shown a pronounced ΔN=0 signal (ΔN≡N+−Ni) and smaller, but measurable, ΔN=±2 peaks. The authors [K. S. Viswanathan et al., J. Phys. Chem. 9 0, 5078 (1986)] assign the excitation to be via an R(21.5) line, with no further specification. We have performed ab initio calculations of the rotational branching ratios for the four possible ‘‘R(21.5)’’ transitions, namely, the rotationally ‘‘clean’’ R21 and R22, and the ‘‘mixed’’ R12+Q22 and R11+Q21 branches. We find the mixed R12+Q22(21.5) branch to agree best with the observed photoelectron spectrum collected parallel to the polarization vector of the light. The discrepancy is larger for detection perpendicular to the polarization. To understand this difference, we have assessed the influence of laser intensity and polarization ‘‘contamination’’ on the branching ratios and photoelectron angular distributions.