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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Possibility of selective photoexcitation of molecules in dense gas media

А. Ч. Измайлов, Mohammad Mahmoudi, H. Tajalli

Published Feb 25, 1999• 1 citation
Publisher

Abstract

The possibility of the high-selective photoexcitation of molecules of a gas medium is shown on the basis of the coherent population trapping of sublevels of the ground term at the optical pumping by the two-frequency laser radiation. Such photoexcitation may be used in the technology of the isotope (isomer) separation, photochemistry and molecular biology, even in cases of comparatively dense media, when the selectivity has few of the usual methods of the laser action.

DOI
10.1117/12.340129

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The possibility of the high-selective photoexcitation of molecules of a gas medium is shown on the basis of the coherent population trapping of sublevels of the ground term at the optical pumping by the two-frequency laser radiation. Such photoexcitation may be used in the technology of the isotope (isomer) separation, photochemistry and molecular biology, even in cases of comparatively dense media, when the selectivity has few of the usual methods of the laser action.
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.
A theoretical investigation is made of the photoionisation of a three-level medium intended for laser isotope separation. The results show that the highest ionisation efficiency can be achieved by dividing the process into two stages: coherent population inversion and photoionisation. A study is made of the possibility of inversion of three-level systems for various detunings of the laser frequencies from the radiative transition frequencies. It is shown that there are detunings for which the population of the third level is maximal for any a priori selected parameters of the medium and field. Such detunings lie near a two-photon resonance and depend on the energy density and duration of the laser pulses, and also on the dipole moments of the transitions. The suitability of this method in the case of inhomogeneously broadened optically dense media is considered. It is shown that the method is most efficient in the case of counterpropagating laser pulses. The efficiency of the method is demonstrated for ytterbium.
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.
It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.
We propose the new method of essential decrease of transit-time broadening of Doppler-free absorption resonances on transitions between long-lived quantum levels of atoms (or molecules) of a rarefied gas medium. This method is based on preliminary optical pumping of the ground term of atoms (molecules) by additional radiation which is spatially separated from the recording light beam. The proposed method may be applied in atomic (molecular) spectroscopy of ultra high resolution and also in frequency and time standards.
Abstract Rather different problems can be lumped together under the general term 'laser control of atoms and molecules'. They include the laser selection of atomic and molecular velocities for the purpose of Doppler-free spectroscopy, laser control of the position and velocity of atoms (i.e. laser trapping and cooling of atoms), and laser control of atomic and molecular processes (ionization, dissociation) with a view of detecting single atoms and molecules and particularly separating isotopes and nuclear isomers. Over the last decades the principal problems posed have been successfully solved, and many of them have evolved remarkably in the subsequent investigations of the international research community. For example, the solution of the problem of laser cooling and trapping of atoms has given birth to the new field of the physics of ultracold matter, i.e. quantum atomic and molecular gases. The laser non-coherent control of uni-molecular processes has found an interesting extension in the field of laser coherent control of molecules. The concept of laser control of position has been successfully demonstrated with microparticles (optical tweezers), concurrently with investigations into atomic control. The laser photo-ionization of molecules on surfaces has led to the development of novel techniques of laser-assisted mass spectrometry of macromolecules, and so on. The aim of this book is to review these topics from a unified or 'coherent' point of view. It will be useful for many readers in various fields of laser science and its applications.