Line-Shape Transition of Collision Broadened Lines

Using the newly developed technique of THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensemble of collision broadened ground state rotational lines of methylchloride vapor out to more than 200 linewidths from resonance, corresponding to frequency offsets as much as 5× 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 at higher frequencies a transition occurs from the regime of the van Vleck--Weisskopf theory to the regime of the Lorentz theory.