Multi-resonance effects within a single chirp in broadband rotational spectroscopy: The rapid adiabatic passage regime for benzonitrile

We report here pronounced, stepwise multi-resonance excitations in benzonitrile arising from a single 1 mu s broadband 2-8.3 GHz microwave chirp, observed with our new chirped-pulse broadband rotational spectrometer, COMPACT. Such multi-resonance excitations significantly alter the relative intensity patterns and are a strong indication that, for the given experimental conditions and using benzonitrile as a polar test molecule (mu(A) = 4.5152 D), the rapid adiabatic passage (RAP) regime for strong coupling must be applied. This finding is contrary to previous discussions of chirped-pulse rotational spectroscopy, where the linear fast passage regime of weak coupling has been assumed. (C) 2012 Elsevier Inc. All rights reserved.