Microwave detected, microwave–optical double resonance spectra of NO2: A test of Hardwick’s ergodicity conjecture

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Abstract
In a recent paper, Hardwick predicted that the optical spectrum of NO2 should display ‘‘quantum ergodicity’’, meaning that almost all eigenstates allowed by total symmetry should be seen in absorption. We have performed a test of this prediction by using the technique of microwave detected, microwave–optical double resonance. We have determined the spectrum over the range 16 810–17 100 cm− 1 for the two ground vibrational states 91,9(F 1) and 100,10(F 1) of NO2. We observed 324 transitions from the 100,10 state and 364 transitions from the 91,9 state. The number of observed transitions is a factor of 8 greater than that expected if only the allowed rotational transitions to any B 2 symmetry vibronic level were observed. The number of observed lines is a factor of 3 less than the number predicted if all selection rules were broken as predicted by Hardwick. Thus we see that Hardwick’s ‘‘ergodicity’’ conjecture is a useful starting point for understanding the spectrum of NO2, but the spectrum is intermediate between the regular and the ergodic limits.