We report a time-resolved investigation of the resonant absorption of far-infrared radiation and the subsequent vibrational relaxation processes in a sample of polycrystalline cytosine at 4 K, using terahertz time-domain spectroscopy. The subpicosecond time resolution achieved in our experiments corresponds to a near single-cycle of far-infrared radiation, and this enables us to observe the damped oscillations of the electric field produced by excited molecules as they decay. Furthermore, we

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... w that the progressive absorption and subsequent emission of far-infrared radiation at the frequency of the corresponding vibrational mode can be followed directly as a function of time by means of time-partitioned Fourier transforms of the transmitted signal.