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

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_+−N_i) and smaller, but measurable, ΔN=±2 peaks. The authors [K. S. Viswanathan et al., J. Phys. Chem. 90, 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’’ R_(21) and R_(22), and the ‘‘mixed’’ R_(12)+Q_(22) and R_(11)+Q_(21) branches. We find the mixed R_(12)+Q_(22)(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.