Optical–optical double resonance studies of rotational autoionization of NO

Abstract

Optical–optical double resonance spectroscopy is used to probe Rydberg series converging to the first ten rotational levels of NO+ X 1Σ+, v+=0. Above the lowest ionization threshold, rotational autoionization of Rydberg series converging to higher thresholds is observed. Predissociation of these Rydberg states is found to compete with rotational autoionization in much the same manner as predissociation competes with vibrational autoionization in the region of the first few vibrational limits of NO+. The presence of this competing decay process, which has a decay rate similar to that of rotational autoionization, permits the comparison of rotational autoionization rates for different changes in rotational quantum number (ΔN+). Rotational autoionization by ΔN+=2 is found to be faster than by ΔN+=1 or 3. This results from the requirement that ΔN+=even processes require interactions between levels that both have even or both have odd values of orbital angular momentum l, while ΔN+=odd processes require interactions between levels of which one has even l and the other has odd l. In NO, the latter interactions are known to be quite weak. The electric field dependence and pressure dependence of the ionization threshold are also discussed.