Detailed experiments confirm the narrowing of ac-Stark-broadened 2+2 multiphoton-ionization spectra of nitric oxide (NO) at thermal equilibrium in a cell at pressures on the order of 1 Torr. Experiments with a supersonic beam of NO at comparable densities exhibit much less narrowing. The possibility that the apparent narrowing of lines from the thermal cell is an artifact associated with space charge is examined and found to be inconsistent with our results. However, these experiments do show that the narrowing of the lines in the room-temperature cell is due to collisions, rather than being a density effect. Additional confirmation of the ac Stark mechanism for the asymmetric broadening of the lines is provided by comparison of line characteristics under excitation by linearly and circularly polarized light.

• Received 14 October 1987

DOI:https://doi.org/10.1103/PhysRevA.37.3801