RESONANT EXCHANGE OF MOLECULAR ROTATIONAL COHERENCE

By means of a time-resolved microwave spectroscopy experiment using the Stark switching technique, the authors have pointed out an exchange of rotational coherence from a level pair (a-a') of an A molecule to a level pair (b-b') of a B molecule in a gas at low pressure. The experiment exploits the linear Stark effect of the (a-a') line, the frequency of which is successively switched on resonance with the applied CW field to bring a coherence on the (a-a') level pair or with the (b-b') line frequency which is Stark insensitive and always off resonance with the applied CW field. The coherence exchange efficiency is maximum when the two line frequencies (a-a') and (b-b') are switched in exact resonance during a time around the transverse relaxation time T2. The radiation trapping effects are calculated and explain fairly well the experimental results obtained in a long cell. The collisional effects are shown to give a contribution which differs from the previous one only by its phase and its cell-length dependence. An experiment in a short cell is exploited to give a tentative upper limit for the collisional coherence transfers.