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Soviet Journal of Quantum Electronics

Isotopically selective two-photon excitation of a three-level atom

A. G. Bakhtadze, V. M. Vetsko, Andrei N Starostin, T R Khukhunashvili

Published Oct 31, 1983• 2 citations
Publisher

Abstract

A theoretical analysis is made of the process of isotopically selective excitation of atoms in the field of opposite waves when the compensation of the Doppler broadening is incomplete. The density matrix formalism for a homogeneous steady-state case is used to obtain expressions for the total probability of the excitation of resonating and nonresonating isotopes and for the selectivity of the process defined as the ratio of these probabilities. In some cases analytic expressions are obtained and these make it possible to determine the nature of the dependence of the total probability of the excitation and selectivity on the temperature of a vapor, buffer gas concentration, ratio of the Rabi transition frequencies, detuning from a resonance, and isotopic shifts. Numerical estimates are obtained of the atomic parameters for transitions in potassium and rubidium atoms. It is shown that the adopted method is more promising for the separation of the isotopes of alkali metals than is one-photon excitation.

DOI
10.1070/qe1983v013n10abeh004856

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A theoretical analysis is made of the process of isotopically selective excitation of atoms in the field of opposite waves when the compensation of the Doppler broadening is incomplete. The density matrix formalism for a homogeneous steady-state case is used to obtain expressions for the total probability of the excitation of resonating and nonresonating isotopes and for the selectivity of the process defined as the ratio of these probabilities. In some cases analytic expressions are obtained and these make it possible to determine the nature of the dependence of the total probability of the excitation and selectivity on the temperature of a vapor, buffer gas concentration, ratio of the Rabi transition frequencies, detuning from a resonance, and isotopic shifts. Numerical estimates are obtained of the atomic parameters for transitions in potassium and rubidium atoms. It is shown that the adopted method is more promising for the separation of the isotopes of alkali metals than is one-photon excitation.
Results are presented of a theoretical analysis of isotopically selective excitation of atoms in a field of counterpropagating waves with incomplete compensation for Doppler broadening. A density matrix formalism based on a strong collision model in a homogeneous steady-state case is used to obtain expressions for the total excitation probabilities and for the drift velocities of the resonant and nonresonant isotopes. An essentially new possibility for obtaining an asymmetric distribution function as a result of the dynamic two-photon Stark effect is noted whereby a light-induced drift (LID) can also be observed at exact resonance. Simple analytic expressions are derived for many cases and these can be used to analyze the dependence of the excitation and the drift velocity on the main characteristics of the process. Numerical estimates are given of the atomic parameters of alkali metals. The advantages of using this method for isotope separation compared with one-photon LID are discussed.
A system for three-photon Doppler-free excitation of two-level atoms is proposed. It is shown that this is equivalent to the excitation of a two-level system with an effective interaction Hamiltonian. An analysis is made of a system for selecting the detuning of the excitation frequencies that can completely compensate for Doppler broadening. Numerical estimates are made and these show that even for small isotope shifts and high temperatures of the excited gas, excitation having a high probability and a high isotopic selectivity can be achieved.
We consider a gas of two-level atoms of two species (for example, natural Rb vapor consisting of two isotopes) in the field of laser radiation tuned close to resonance with the certain transition of a specific iostope. This radiation may turn to be rather close to a near lying transition of other isotope (within the linewidth of laser radiation), so it is necessary to take into account the possibility of excitation of other isotope in order to evaluate the enrichment degree if it is aimed at the isotope separation by means of selective excitation with subsequent chemcial reaction (in our example the rubidium hydride formation) of the excited isotope which is believed to take place at a rate much higher than that in the ground state of the atom.
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.
A numerical study is made of the interaction of laser radiation with different isotopic forms of potassium, 39K and 40K, for time intervals shorter than the relaxation times. The quantum efficiency and selectivity of the process are determined. The behavior of the atoms is considered using the Schrodinger equations for the probability amplitudes. Account is taken of the effects associated with hyperfine splitting of the levels, their degeneracy, and the Doppler effect. Excitation by a single traveling wave and by two oppositely traveling waves is considered and the excitation characteristics of the two regimes are compared.
The selectivity of three-step pulse photo-ionization of atoms versus the detuning delta of exciting radiation frequencies from intermediate resonance is investigated in powerful laser fields where the power broadening of the resonant transitions is at least of the order of isotope shifts. The Doppler broadening of the resonant transitions is taken into account and it is assumed that the sum frequency of both exciting laser radiations is tuned to the line centre of the two-photon transition of a selected isotope. It is shown that in this situation the photo-ionization selectivity increases so greatly with the absolute detuning mod delta mod that even in such strong fields the process of photo-ionization can be highly selective. The optimization of selective photo-ionization of atoms is discussed in connection with the decrease in the selective isotope yield with increase of the absolute detuning mod delta mod . Analytical consideration of selective three-step pulse photo-ionization in powerful laser fields, based on the density matrix formalism, is presented together with the results of numerical simulation of the 40 Ca- 48 Ca laser isotope separation process as an example.