Terahertz coherent transients from methyl chloride vapor

Abstract

A newly developed optoelectronic source for generating well-collimated beams of subpicosecond pulses of terahertz radiation was used to study coherent propagation effects in a spectrally thick medium of methyl chloride vapor. After the passage of the excitation pulse, we observe the subsequent emission of a coherent terahertz pulse train in the free-induction decay of the molecular vapor. The individual pulses of the train are separated by 38 ps, corresponding to the frequency separation between adjacent rotational lines of the excited manifold of more than 70 transitions. An analysis of the data determines the rotational constants and anharmonicity factors of the two naturally abundant methyl chloride isotopes. From the decay and reshaping of the pulse train the coherence relaxation time T2 is obtained as a function of vapor pressure. Our observations indicate a strong nonmonotonic collision broadening and a van Vleck–Weisskopf line shape of the individual rotational lines.