The influence of the bandwidth of the radiation source on light-induced drift (LID) of Na is studied experimentally. Broadband radiation can be used to eliminate optical hyperfine pumping on the one hand, and to provide more efficient excitation due to an increased velocity coverage, on the other hand. These aspects are highlighted in two separate experiments. An increase of the drift velocity of Na by a factor of 4 compared to monochromatic excitation has been measured. A frequency-modulated (FM) ring dye laser is used as a broadband radiation source, having a bandwidth continuously variable from single mode to multimode with a bandwidth of 10 GHz. Contrary to passive multimode lasers, the spectrum of such a laser is well defined and stable. Various modulation frequencies are used to study the dependence of the drift velocity on the mode spacing of the multimode laser. Only small differences are found. All experimental results are compared with results of a four-level rate-equation model for LID of Na, in which the excited-state hyperfine structure of Na and the detailed shape of the FM spectrum are taken into account. Good agreement between the model and the experimental data is found. The model is also used to show that the FM spectrum yields almost the same values for the drift velocity as a rectangular spectrum, which so far has been considered optimal for LID.
The influence of the bandwidth of the radiation source on light-induced drift (LID) of Na is studied experimentally. Broadband radiation can be used to eliminate optical hyperfine pumping on the one hand, and to provide more efficient excitation due to an increased velocity coverage, on the other hand. These aspects are highlighted in two separate experiments. An increase of the drift velocity of Na by a factor of 4 compared to monochromatic excitation has been measured. A frequency-modulated (FM) ring dye laser is used as a broadband radiation source, having a bandwidth continuously variable from single mode to multimode with a bandwidth of 10 GHz. Contrary to passive multimode lasers, the spectrum of such a laser is well defined and stable. Various modulation frequencies are used to study the dependence of the drift velocity on the mode spacing of the multimode laser. Only small differences are found. All experimental results are compared with results of a four-level rate-equation model for LID of Na, in which the excited-state hyperfine structure of Na and the detailed shape of the FM spectrum are taken into account. Good agreement between the model and the experimental data is found. The model is also used to show that the FM spectrum yields almost the same values for the drift velocity as a rectangular spectrum, which so far has been considered optimal for LID.
We present a model for light-induced drift (LID) in the Na--noble-gas system which should enable direct comparison with experiment. In contrast to previous theories of LID based on a two-level description of the optical absorbers and on a simplified collision treatment, the present model is based on a realistic description of laser-driven Na atoms immersed in a buffer gas. Starting from the generalized Bloch equations, we introduce a rate-equation model for the velocity distributions in the four important Na levels. The velocity-changing and fine-structure-changing collisions are described using composite Keilson-Storer collision kernels in which all adjustable parameters have been eliminated by using available literature data. We apply the model in numerical calculations of LID as a function of all experimentally accessible parameters. It is found that the ground-state hyperfine splitting can have large effects on LID, whereas the excited-state fine-structure splitting has not. The paper establishes criteria for optimum LID effects; when using a single-frequency laser the maximum attainable drift velocity is predicted to be 13.8 m/s. Using a proper set of boundary conditions, we find a pressure dependence of LID qualitatively different from the predictions based on previous work. Finally, the influence of the collision model is investigated. We find that LID is independent of the shape of the collision kernel, indicating that a strong-collision model is always valid.
The use of paraffin-coated cells has led to a breakthrough in the experimental study of light-induced drift (LID) of alkalis in noble gases. In recent experiments isotope separation of the two naturally occurring Rb isotopes through the LID effect has been achieved (enrichment of the rare isotope up to 95%), 1,2 and direct measurements of the drift velocity of Na in noble gases as a function of various experimental parameters have been made.3 In these experiments a single single-mode dye laser, tuned within the Doppler profile of the resonance line, was used to excite a specific velocity group of atoms (isotope selective in the case of Rb). Optical pumping between the ground-state hyperfine levels F = I+½ and F=I−½ plays a dominant (limiting) role in determining the magnitude of the LID effects by strongly depleting the resonantly excited hyperfine substate (of a specific isotope).
Light-induced drift (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
Light-induced drift has many applications in astrophysics, semiconductor physics, and isotope separation. Light-induced drift velocity is a key parameter to characterize the effect of light-induced drift. Laser linewidth exerts a great influence on light-induced drift velocity through influencing the velocity selectivity of atomic excitation, so it is an important factor that cannot be ignored in the study of light-induced drift. However, in existing theoretical studies, the influence of laser linewidth is seldom considered and the exciting light is always treated as monochromatic light. Furthermore, in a few theoretical studies about laser linewidth, the numerical model adopted does not include all the factors of light-induced drift, such as energy level degeneracy, hyperfine structure, and collision model, which will cause the error of calculation. In order to study the influence of laser linewidth on light-induced drift velocity, a four-level rate equation model is established to describe the atomic energy level transition in the process of light-induced drift. In the theoretical model, we introduce strong collision model to describe collisions between atoms and buffer gas. The influences of energy level degeneracy and hyperfine structure are also taken into account. Numerical method is used to calculate the four-level rate equation. According to the calculation results, the influence of laser linewidth on drift velocity of alkali metal atoms is analyzed. The results show that as the linewidth increases, the value of drift velocity first increases and then decreases. There is an optimal linewidth that maximizes the drift velocity. For the best light-induced drift effect, the laser should work under the optimal linewidth condition. When the laser linewidth fluctuates near the optimum linewidth, the laser linewidth should be set to be slightly wider than the optimal linewidth. This can reduce the influence of fluctuation and obtain a better drift effect. In addition, as the laser linewidth increases, the optimum power density corresponding to the maximum drift velocity decreases. When the laser linewidth is narrow, small fluctuations near the optimal laser power density will not have great influence on drift velocity. When the laser linewidth is wide, the power density should be set strictly. The optimum linewidth is related to laser power density, temperature and buffer gas pressure. As the laser power density increases, the value of optimum linewidth first increases rapidly and then decreases slowly. The value of optimal linewidth also increases linearly with the increase of temperature, and it decreases with the increase of buffer gas pressure. In conclusion, the laser linewidth does play a key role in the process of light-induced drift. The results of this study can provide a theoretical basis for future experiments, and be a good reference to the selection of exciting light.
The anomalous light-induced drift (LID) of atoms caused by a velocity dependence of collision frequencies has been investigated in the frame of the theory without adjustable parameters. Our results open the possibility of experimental probing of models of interatomic potentials. The extension of the strong-collision model to the case of velocity-dependent collision rates, is the basis of our approach. The obtained model describes the anomalous LID both in the case of arbitrary mass ratio of absorbing- and buffer-gas particles and in the case of arbitrary ratio of the homogeneous and Doppler widths. In particular, we applied our model to describe the anomalous LID in alkali - noble-gas mixtures (Rb - Kr and Li - Ne systems). Qualitatively another anomaly of the LID velocity was also found in the low-pressure region. As pointed out, the optical pumping effect and the hyperfine structure of the ground electronic state, but not the velocity dependence of the collision rates, are the sources of this new anomaly.
We report an experimental and theoretical study of the optical piston, a consequence of the phenomenon of light-induced drift (LID), observed in binary mixtures of Na vapor and a buffer gas. Good agreement has been found between experiment and existing theory as far as the shape of the Na density profile in the piston is concerned. The piston velocity is anomalously low; this is accounted for by incorporating the effects of adsorption and desorption into the theory. Furthermore, we introduce a rate-equation model for LID which incorporates the fine and hyperfine structure of the level scheme of the Na absorbers; this gives insight into the sublevel velocity distributions and the nonequilibrium part of the total velocity distribution during LID.