On the selective photoexcitation of molecules within the homogeneous widthof optical lines

We investigate the photoexcitation of molecules of a gas, caused by-the cha nge of the state of the coherent population trapping of the open Lambda-sys tem of quantum levels at the sharp change of phase and amplitude parameters of the two-frequency laser pumping. Analysis is carried out both on the ba sis of the Schrodinger equation and in the formalism of the density matrix of molecules in the case of homogeneously broadened spectral lines. The max imum photoexcitation takes place at the two-quantum resonance between lower long-lived states of the,l-system. It is shown, that the narrow, high-cont rast peak of the photoexcitation appears on the comparatively low and wide background, caused by collisions of molecules. Conditions are determined, w hen the influence of this background is minimum and the spectral width of t he photoexcitation peak is much less than the homogeneous widths of the opt ical Lines of resonance transitions in the Lambda-system. Such photoexcitat ion may be used in the technology of isotope (isomer) separation, selective photochemistry and photobiology even at essential overlap of optical spect ra of different molecules. (C) 2000 Elsevier Science B.V. All rights reserv ed.