Collision-induced Lamb dips in laser Stark spectroscopy

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.

Collisional Attenuation of Focused CH3Cl Molecular Beams in a Hexapole Filter

“…Oka et al , investigated the selection rules that apply during rotationally inelastic collisions in NH 3 , H 2 CO, and CH 3 F using inverse Lamb dips in infrared laser Stark spectroscopy. Collision-induced transitions with Δ M > 1 were identified in the H 2 CO system, although cascading could not be totally eliminated.…”

Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes

“…From this graph, we can see that at buffer gas pressures of less than 20 Torr, the value of a, is much greater than 1, showing that the magnitude of the selfinduced heterodyne signal is more dependent upon the sweep rate than upon collisions with the buffer gas. In collisions at low gas pressure, Shoemaker et al 18 and Johns et al 19 showed that M J changing collisions occurred at large intermolecular separations. The similarity of the effects of He and Ar in damping the gain spike suggests that the main effects of these collisions are occurring at long range.…”

High-resolution saturation spectroscopy of singly-ionized iron with a pulsed UV laser

“…If both peaks are real, one possibility is that they correspond to the 54 Fe isotopic line and to a collisioninduced crossover resonance [32]. It is unlikely that 57 Fe could have been observed in this experiment, because its abundance is only 2.1% and its nuclear spin of 1/2 causes hyperfine splitting.…”

“…The small features to the right of the main saturation signal peak in Figure 5 are probably due at least in part to the 54 Fe isotope (5.8 % abundance [31]), but the signal to noise ratio is not quite sufficient to be certain of their existence, so the isotope shift cannot be determined from this demonstration experiment. If both peaks are real, one possibility is that they correspond to the 54 Fe isotopic line and to a collisioninduced crossover resonance [32]. It is unlikely that 57 Fe could have been observed in this experiment, because its abundance is only 2.1% and its nuclear spin of 1/2 causes hyperfine splitting.…”