Quantum beat frequency, rotational level

Zeeman quantum beat spectroscopy was used by Gouedard and Lehmann (1979, 1981) to measure the effect of various lu perturbing states on the gj-values [Eq. (6.5.21)] of more than 150 rotational levels of the Se2 B 0+ state (see Section 6.5.2 and Fig. 6.16). In that experiment, the excitation polarization was perpendicular to the applied magnetic field so that quantum beats were observed between nominal B-state components differing in M by 2. The frequencies of these beats increase linearly from 0 MHz at 0 G until the AM — 2 splitting falls... 

This suggests that a quantum beat spectrum of an AT-level system will, because it contains redundant information, be more complicated than the corresponding frequency domain spectrum. However, when the level spacings are approximately integer multiples of a common factor, such as 2B for upper-state A2F(J) = B (iJ + 2) rotational combination differences, then each upper state (J + 1, J — 1) pair of rotational levels coherently excited from all thermally populated lower-state J" levels contributes to a grand rephasing at tn = n [-gj ] (n = 1,2,...). This is Rotational Coherence Spectroscopy (RCS) (Felker and Zewail, 1987 and 1995 Felker, 1992). It provides upper state rotar tional constants without the need for a rotational analysis. 

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