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Chinese Physics Letters

A Method for Effective Population Transfer in U Vapor with Doppler Effect and Collision

Xi‐Jing Ning, Chun-yang Jing, Fucheng Lin

Published Aug 1, 1996• 3 citations
Publisher

Abstract

We proposed a method for effective population transfer in an atomic vapor with Doppler effect and collision. Using this method, we have demonstrated that an efficient population transfer occurs in 238 U vapor produced in a hollow-cathode discharge tube. A numerical calculation with density matrix agrees well with our experimental result and indicates that the method can be used to raise the efficiency of atomic vapor isotope separation.

DOI
10.1088/0256-307x/13/8/009

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We proposed a method for effective population transfer in an atomic vapor with Doppler effect and collision. Using this method, we have demonstrated that an efficient population transfer occurs in 238 U vapor produced in a hollow-cathode discharge tube. A numerical calculation with density matrix agrees well with our experimental result and indicates that the method can be used to raise the efficiency of atomic vapor isotope separation.
Population transfer in three-level atomic systems with high efficiency and selectivity is desirable for practical applications in laser spectroscopy, trace analysis, and isotope separation. The effective excitation method for achieving this population transfer in three-level atomic systems using coherent two-photon excitation with two optimally detuned counterpropagating pulsed lasers, initially investigated analytically assuming monochromatic lasers and Doppler-free systems, is studied numerically by solving the density matrix equations incorporating finite laser linewidth and effective Doppler width. A three-level Yb system is chosen for illustration of numerical results. The effects of laser linewidth and effective Doppler width on the values of optimal detunings and maximum third-level population are discussed.
Populations of excited atoms that all have the same z component of velocity can be produced by pumping a vapor with a narrow-band laser. This velocity-selected population is then thermalized by velocity-changing collisions. However, in a pure vapor, even at low densities where velocity-changing collisions can be ignored, the mechanisms of radiation trapping and resonance exchange collisions can still lead to substantial thermalization of excited-atom velocity distributions. In this paper, we present data demonstrating these effects. We compare our data with results from a simple model of these processes. The model is in qualitative agreement with the experimental results.
We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor), quasi-three-level systems (Na vapor), and on more complex multilevel systems (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.
We propose a method of laser isotope separation based on the phenomenon of adiabatic inversion and illustrate the method in a multilevel system consisting of a single ground state and multiplet excited states. In this model, population may be transferred from the ground state to one of the excited manifolds with nearly 100% efficiency by using laser pulses, the envelope of which rises slowly and then decays rapidly on an internal time scale that can be precisely calculated. Adiabatic inversion in this model is strongly dependent on the laser frequency, thus permitting separation of isotopes by faking advantage of small shifts in molecular or atomic energy level spacings. The criterion that determines whether a laser pulse is adiabatic also depends strongly on the laser frequency and thereby enhances the separation. We demonstrate the effectiveness of the method with a solution of the time-dependent Schroedinger equation for a specific model of two isotopes.
Techniques for population transfer are needed to gain more complete insight into the collision dynamics of thermally unpopulated levels, e.g. highly vibrationally excited states of molecules. Very promising new techniques are now becoming available. They invoke coherence properties of the radiation field and rely on the phenomena of adiabatic following. In this presentation, I will discuss one of them, the STIRAP method, which induces efficient and highly selective flow of population from an initially populated level 1 to a final level 3. These levels are coupled by a pump and a Stokes laser, with both laser frequencies tuned to resonance with an intermediate level 2. Despite the resonance, spontaneous emission, which may populate levels other than level 3, is eliminated. The method can be implemented with cw as well as with pulsed lasers. First results from a study of the effect of initial vibrational excitation of a molecule on a chemical reaction will be presented. It will also be shown how this technique allows the orientation of electronic or molecular angular moments. The prospects of using this technique for efficient transfer of photon momenta to an atom may be mentioned.