In this work, two kinds of THz absorption are identified from the polar molecule-formed plasma, namely, the broad band resonant absorption induced by the separation of electrons and ions within plasma, and the rotational transitions between different rotational states. Interestingly, we observe the enhanced THz absorption of the rotational transition lines within the resonant absorption frequency range. Gas pressure is varied to show that the enhanced THz absorption lines are related to the plasma frequencies. This work is not only a new observation on the THz absorption within plasma, but also a glimpse of the coherent nature of laser-prepared plasma.
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