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Optics and Spectroscopy

Sub-Doppler Spectroscopy in Thin Gas Cells upon Excitation of Atoms by Broadband Pulsed Radiation

А. Ч. Измайлов

Published Mar 1, 2020• 0 citations
Publisher

Abstract

Abstract A method of sub-Doppler spectroscopy is proposed, based on the peculiarities of narrowing over time of the velocity distribution of optically excited atoms (molecules) of a rarefied gas medium in a thin cell after exposure to a sufficiently short pulse of broadband radiation. Recording of narrow sub-Doppler resonances at the central frequencies of quantum transitions from the atomic state excited by such a pump pulse is carried out by subsequent probe monochromatic light pulses. The optimal conditions for the implementation of this spectroscopy method occur when the radiation decay time of an optically excited atomic level exceeds the characteristic time of flight of atoms between the walls of the thin cell under consideration, the inner thickness of which is many times smaller than its transverse dimensions.

DOI
10.1134/s0030400x2003008x

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Abstract A method of sub-Doppler spectroscopy is proposed, based on the peculiarities of narrowing over time of the velocity distribution of optically excited atoms (molecules) of a rarefied gas medium in a thin cell after exposure to a sufficiently short pulse of broadband radiation. Recording of narrow sub-Doppler resonances at the central frequencies of quantum transitions from the atomic state excited by such a pump pulse is carried out by subsequent probe monochromatic light pulses. The optimal conditions for the implementation of this spectroscopy method occur when the radiation decay time of an optically excited atomic level exceeds the characteristic time of flight of atoms between the walls of the thin cell under consideration, the inner thickness of which is many times smaller than its transverse dimensions.
The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.
This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.
The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degen- erate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of par- ticles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in compar- ison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub- Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high- selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.
Abstract Excitation of atoms from the ground quantum level to a metastable state by traveling monochromatic radiation in a thin gas cell, the internal thickness of which is much smaller than its radius, is studied theoretically. Sub-Doppler resonances are detected and analyzed that arise in the population of a metastable atomic level due to light-induced Rabi oscillations on a given optical transition and specific features of transit-time relaxation of atoms in a considered cell with a rarified gas upon scanning the radiation frequency. Such nontrivial resonances can be recorded in fluorescence of atoms with the help of an additional probe light beam. A method is proposed for a direct measurement of the Rabi frequency from spectral intervals between detected sub-Doppler resonances. The results obtained can be applied in atomic spectroscopy for the precision determination of components of dipole moment matrix elements, as well as for the Einstein coefficients and oscillator strengths related to these components, for quantum transitions between the ground and metastable levels of atoms.
The excitation of atoms (molecules) of a rarefied gas by stationary broadband optical radiation in a thin cell, the inner thickness of which is many times smaller than its transverse size, is studied theoretically. The features of optical pumping and transit-time relaxation of atomic particles in such cells open new possibilities for ultra-high resolution spectroscopy. The conditions are analyzed under which narrow high-contrast sub-Doppler fluorescence resonances arise at the central frequencies of spontaneous transitions from optically excited atomic (molecular) quantum levels.