Collision-induced Lamb dips in laser Stark spectroscopy

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Abstract
While observing inverse Lamb dips in infrared laser Stark spectra, we have found unexpected extra dips which lie midway between the ’’genuine’’ dips. These center dips are similar to those observed by Uzgiris, Hall, and Barger in that they are caused by a crossover of two different transitions due to the Doppler effect, but are different in that the two transitions do not share a common level. Thus these newly observed center dips are the result of a four−level infrared−infrared double resonance effect and they provide information on rotational transitions as well as on the velocity changes which occur during intermolecular collisions. Since the transitions which cause the center dips involve different M components of the same rotational level, this method is specially suited for the study of reorientation collisions, that is, collisions in which the direction of the total angular momentum changes without changing its magnitude. A theory is developed in which the relative intensities of the center dips with respect to those of ’’genuine’’ dips are related to the relative importance of reorientation collisions. The experiments were done on NH3, H2CO, and CH3F using the CO and the CO2 lasers. The following conclusions are obtained from analysis of the results: (a) The observation of sharp center dips confirms our previous conclusion (Ref. 3) that, as a result of weak collisions, rotational quantum states of molecules can change without appreciable change in velocity. (b) The variation of relative intensities for CH3F, H2CO, and NH3 indicates that parity changing reorientation transitions are preferred to parity conserving transitions. (c) Analysis of very large signals for J = K levels of CH3F indicates that, for those levels, the rate of ΔM = ±1 reorientation collisions relative to all inelastic collisions is of the order of 70%. The consideration of ΔM ≳ 1 transitions would further increase the relative importance of reorientation collisions. (d) Analysis of ΔM ≳ 1 signals in H2CO indicates that rather large reorientations of molecules can occur during a single collision.