Terahertz studies of collision-broadened rotational lines

Via terahertz (THz) coherent transients and THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensembles of collision-broadened ground state rotational lines of methyl bromide, methyl chloride, and methyl fluoride vapors out to more than 200 line widths from resonance, corresponding to frequency offsets as much as 5 times the resonant frequency. On these far wings the measured absorption is approximately an order of magnitude less than that predicted by the van Vleck-Weisskopf theory. Our observations show that as the offset frequency is increased, a transition occurs from the regime of the van Vleck-Weisskopf theory to the regime of the Lorentz theory. These measurements are fit to a new molecular response theory which explicitly includes the molecular orientation time during a collision. Due to the broad bandwidth of the THz pulses, we demonstrate the validity of this molecular response theory for the far-wing absorption of methyl fluoride, chloride, and bromide. The excellent theoretical fit to our measurements encompassing the frequency range over which this transition occurs indicates a molecular response time on the order of 200 fs. These measurements also permit determination of the line-width dependence on the rotational quantum number J.