Abstract

Resonantly enhanced multiphoton ionization of NO has been investigated using two tunable dye laser beams oppositely directed and having a common focus. The focus lies between two electrodes in a cell containing 4-15 torr of NO. The first laser beam pumps a rovibronic level of either the A or C state by a two or three photon process. The second laser is scanned over a spectral region leading finally to ionization by one or two photons sometimes assisted by field or collisional ionization. We have obtained high resolution data on high Rydberg and non-Rydberg states below and above the ionization limit with special attention to the Rydberg-non-Rydberg perturbations and dynamic processes such as autoionization, predissociation, collisional and field ionization and rotational relaxation. The spectral simplification resulting from the rotational selectivity of the double resonance technique has yielded easily analyzed spectra in cases for which the ordinary one-photon spectrum is hopelessly congested or overlayed by strong, broadened lines. One of many examples is the strongly perturbed v=8 level of the L valence state. Decay processes can yield either peaks or dips often with measurable widths. Thus the N=1, v=1 level of the 5pσ state, which is obscured in the one-photon spectrum is found to have a predissociation width of about 6cm^-1. In contrast, the v=3 levels of the 4f, 4d, 6s and 7s Rydberg states, all of which lie above the ionization limit for v=0 or v=1 and hence are subject to vibrational autoionization, are found to have widths < .5cm^-1 in agreement with our calculations. We have studied linear and quadratic, static and dynamic Stark effects as well as pressure shifts and broadening of Rydberg states with values of n up to ~35. Pulse delay studies have shown adiabatic following of dressed states under conditions of strong power broadening and A.C. Stark shifts. Resonant structures which appear as either peaks or dips as a function of laser power are explained in terms of Fano theory.

© 1982 Optical Society of America