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We carried out theoretical investigation about velocity-selective atomic excitation on long–lived (metastable) levels of an atomic vapour in a thin cell by a monochromatic laser beam, running in the normal direction. The regime of coherent Rabi oscillations is considered on the light-induced transition from a sublevel of the ground quantum term to a metastable atomic level. On the basis of density matrix equations for the two-level system, we analysed the atomic population density of the metastable level, when the sample is irradiated by resonant monochromatic laser beam with an annular cross-section versus atomic velocities and versus the detuning, the amplitude, and the geometry of the laser beam. It is shown that, in the centre of the annular region, it can be obtained a population distribution on the metastable level as a function of the laser detuning, characterized by a sharp narrow resonance profile, whose width is reduced with respect to the thermal Doppler width roughly by the ratio between the diameter of the irradiated region and the inner thickness of the cell. We suggest high-sensitive schemes, in order to detect these sub-Doppler resonances, by probing the population of the metastable state with a second laser beam, resonant with a transition leaving from the metastable level. The case of 1S0 → 3P1 spin-forbidden transition of Ca is discussed in more detail

We investigate the photoexcitation of molecules of a gas, caused by the change of the state of the coherent population trapping of the open Λ-system 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 basis of the Schrodinger equation and in the formalism of the density matrix of molecules in the case of homogeneously broadened spectral lines. The maximum photoexcitation takes place at the two-quantum resonance between lower long-lived states of the Λ-system. It is shown, that the narrow, high-contrast peak of the photoexcitation appears on the comparatively low and wide background, caused by collisions of molecules. Conditions are determined, when the influence of this background is minimum and the spectral width of the photoexcitation peak is much less than the homogeneous widths of the optical lines of resonance transitions in the Λ-system. Such photoexcitation may be used in the technology of isotope (isomer) separation, selective photochemistry and photobiology even at essential overlap of optical spectra of different molecules.

We have detected and analysed interesting features of sub-Doppler resonances, which are induced in absorption of the weak monochromatic probe light beam (with the scanned frequency) by the counterpropagating two-frequency pumping radiation implementing the phenomenon of the coherent population trapping (CPT) on resonant.-systems of atomic levels. Our experimental investigations have been carried out on example of spectral components of the Doppler broadened D-2 line of cesium atoms. Obtained results may be used for optimization of laser frequencies stabilization on atomic transitions.

We have detected and analyzed narrow, high contrast coherent population trapping resonances, which appear in transmission of the probe monochromatic light beam under action of the counterpropagating two-frequency laser radiation, on example of the nonclosed three level {\Lambda}-system formed by spectral components of the Doppler broadened D2 line of cesium atoms (in the cell with the rarefied Cs vapor). These nontrivial resonances are determined directly by the trapped atomic population on the definite lower level of the {\Lambda}-system and may be used in atomic frequency standards, sensitive magnetometers and in ultrahigh resolution laser spectroscopy of atoms and molecules., Comment: 4

We have detected and analyzed narrow, high contrast coherent population trapping (CPT) resonances, which appear in transmission of the probe monochromatic light beam under action of the counter propagating two frequency laser radiation, on example of the nonclosed three level Lambda-system formed by spectral components of the Doppler broadened D-2 line of cesium atoms. Given resonances may be used in atomic frequency standards, sensitive magnetometers and in ultrahigh resolution laser spectroscopy of atoms and molecules.