. ge n - ph ] 2 4 M ay 2 00 1 On the physical nature of the fluorescence depletion effect

The physical nature of the fluorescence depletion (FD) effect and the possible way of its experimental study are discussed. The FD effect is used for many years as an efficient time-domain method of rotational spectroscopy for large polyatomic molecules [1-3]. In this method, the picosecond laser pulse is split into the pump and the probe pulses, which are focused at the molecular jet. Time and frequency-integrated fluorescence intensity from the sample is studied as a function of the delay between the pump and the probe laser pulses. The observed rotational recurrences allow to obtain the corresponding rotational constants of the ground and excited states of the molecule [4-6]. However, the physical nature of the FD effect is practically unknown for present day. There exists rather formal phenomenological description, which is based on the concepts of coherency, quantum interference and quantum beats [1]. It is worth to note, that this situation is rather typical in nonlinear optics, where phenomenological description often takes the place of the physical explanation. As a result, some problems appear , if one wants to understand the physical sense of such a descriptions [7]. A real physical explanation of the FD effect must be based on the supposition that the laser pulse " interacts preferentially with those species whose transition dipoles are aligned along its polarization vector at the instant that it passes through the sample " [1]. But this correct idea should be extended further: the absorption cross-section of laser radiation by molecules depends on the orientation of the molecule in space, on the angle between the axes of the molecule and that of the laser beam. How strong is this dependence? How sharp is it? These are obviously the most interesting questions now. The FD effect provides us an excellent possibility to obtain such information. It is surprising that no one has studied the nature of FD effect till now.