Light-induced Drift Effect Research Articles

Enhancement of the sensitivity of the light-induced drift effect to interatomic interaction potentials using a mixture of two buffer gases

Based on five different nonempirical (calculated ab initio) interaction potentials for pairs of colliding Li – Ne particles and three different interaction potentials for pairs of colliding Li – Ar particles, we have theoretically investigated the spectral features of the light-induced drift (LID) rate of Li atoms in the buffer Ne gas in a mixture of buffer Ne + Ar gases. The calculations of LID of Li atoms in the buffer Ne gas for two interaction potentials predict anomalous LID and, as a result, strong sensitivity of the spectral shape of the LID lines to the differences in these potentials. For three other potentials (out of the five in question), the shape of the LID line of Li atoms in Ne is insensitive to the shape of the potential, since calculations with these potentials predict a normal LID effect. In this case, as it turned out, by adding a small fraction (approximately 10 %) of Ar to Ne, one can go from normal LID to anomalous LID and thereby radically increase the sensitivity of the LID line shape of Li atoms to the difference in these interaction potentials. The results obtained enable high-precision testing of the interatomic interaction potentials in experiments on anomalous LID.

On the Possibility of the Precise Test of Interatomic Interaction Potentials Using the Effect of Anomalous Light-Induced Drift

The spectral features of the velocity of light-induced drift (LID) of lithium atoms in a neon buffer gas have been theoretically studied using four known ab initio calculated interatomic interaction potentials for the Li–Ne system of colliding particles. The calculations with each of these four potentials predict a strong temperature dependence of the spectral shape of the LID signal of Li atoms in the Ne atmosphere in various temperature intervals. The results allow the high-precision test of the interatomic interaction potentials in experiments on anomalous LID.

Segregation of Calcium Isotopes in the Atmospheres of CP Stars as a Consequence of Light-Induced Drift

A mechanism for the segregation of calcium isotopes in the atmospheres of chemically peculiar (CP) stars due to light-induced drift (LID) of singly charged 48Ca+ ions is discussed. One peculiarity of Ca+ is that an adequate description of the effect of LID requires taking into account several energy levels of Ca+, and thus several pairs of relative differences (ν i − ν k )/ν i for the transport frequencies for collisions of levels i and k with neutral atoms (hydrogen, helium). The known real (calculated ab initio) interaction potentials are used to numerically calculate the factors (ν i − ν k )/ν i for several states of Ca+ for collisions with H and He atoms. These computations show that, at the temperatures characteristic of the atmospheres of CP stars, T = 6600−12 000 K, fairly high values are obtained for Ca+ ions, (ν i − ν k )/ν i ≈ 0.4−0.6. Simple, transparent computations demonstrate that the LID rates of Ca+ ions in the atmospheres of cool CP stars (Teff = 6600 K) exceed the drift rate due to light pressure by two orders of magnitude. The LID is directed upward in the stellar atmosphere, and the heavy isotope 48Ca is pushed into upper layers of the atmosphere. This can explain the observed predominance of the heavy isotope 48Ca in the upper atmospheric layers of CP stars; according to the radiative-diffusion theory, the action of light pressure alone (in the absence of LID) would lead to sinking of the isotope 48Ca deeper into stellar atmosphere, following the lighter main isotope 40Ca. The 48Ca+ LIDrate decreases and its drift rate due to light pressure increases with growth of the effective temperatures in the atmospheres of CP stars. The manifestations of LID and light pressure are roughly comparable in the atmospheres of CP stars with effective temperatures near Teff = 9500 K.

Influence of laser linewidth on light-induced drift velocity of atom

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.

Accurate measurement of the sticking time and sticking probability of Rb atoms on a polydimethylsiloxane coating

We present the results of a systematic study of Knudsen’s flow of Rb atoms in cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound. The purpose of the investigation is to determine the characterization of the coating in terms of the sticking probability and sticking time of Rb on the two types of coating of high and medium viscosities. We report the measurement of the sticking probability of a Rb atom to the coating equal to 4.3 × 10−5, which corresponds to the number of bounces 2.3 × 104 at room temperature. These parameters are the same for the two kinds of PDMS used. We find that at room temperature, the respective sticking times for high-viscosity and medium-viscosity PDMS are 22 ± 3 μs and 49 ± 6 μs. These sticking times are about million times larger than the sticking time derived from the surface Rb atom adsorption energy and temperature of the coating. A tentative explanation of this surprising result is proposed based on the bulk diffusion of the atoms that collide with the surface and penetrate inside the coating. The results can be important in many resonance cell experiments, such as the efficient magnetooptical trapping of rare elements or radioactive isotopes and in experiments on the light-induced drift effect.

Numerical modeling of light-induced drift in a cloud of interstellar gas

We study velocity distribution of the gas in case of resonance interaction between directed radiation line and the gas in medium of non-resonating buffer gas. We have constructed a 6-parameter model numerically describing the effect of uniform quasi-stationary light-induced drift in a rarefied interstellar gas. Based on numerical calculations, we qualitatively determined the dependence of the light-induced drift velocity on the model parameters in the cases of optically thin and optically thick clouds. We checked that light-induced drift has resonance behavior and requires low collision rate of the gas. Also we showed and explained narrowing and shifting of emission line during propagation in cloud.

On the accuracy of a one-dimensional approach to the solution of kinetic equations with velocity-dependent collision frequencies

The solution of many problems in light-induced gas kinetics can be simplified significantly using quantum kinetic equations in the context of the so-called one-dimensional approximation, in which the initial equations are averaged over transverse (relative to the direction of radiation) velocities. The errors introduced in such an approach are usually assumed to be small; however, this has been confirmed quantitatively only on the basis of the simplest (two- and three-level) particle models. We analyze the accuracy of the one-dimensional approximation for multilevel particles quantitatively for the light-induced drift (LID) effect in cesium atoms in the atmosphere of inert buffer gases. It is shown that in the case of the so-called “normal” LID, one-dimensional kinetic equations can always be used instead of three-dimensional equations without a risk of losing some important fine details in the dependence of the drift velocity on the radiation frequency. In the case of anomalous LID, the error of the one-dimensional approximation is also insignificant, but it can be disregarded only in the case of light buffer particles. For comparable masses of resonant and buffer particles, the one-dimensional approximation may give a noticeable error in determination of drift velocity amplitudes; however, the positions of drift velocity zeros and extrema depending on radiation-frequency detuning can be described successfully. Results show that the error introduced by using the one-dimensional approximation for multilevel particles turns out to be more significant than for the simplest particle models.

Spectral anomalies of the effect of light-induced drift of caesium atoms caused by the velocity dependence of transport collision frequencies

The spectral features of the light-induced drift (LID) velocity of caesium atoms in inert buffer gases are studied theoretically. A strong temperature dependence of the spectral LID line shape of Cs atoms in Ar or Kr atmosphere in the vicinity of is predicted. It is shown that the anomalous LID of Cs atoms in binary buffer mixtures of two different inert gases can be observed at virtually any (including ambient) temperature, depending on the content of the components in these mixtures. The results obtained make it possible to precisely test the interatomic interaction potentials in the experiments on the anomalous LID.

Spectral anomalies of the light-induced drift effect caused by the velocity dependence of the collision broadening and shift of the absorption line

We have theoretically investigated the spectral features of the light-induced drift (LID) effect, arising due to the dependence of the collision broadening γ and shift Δ of the absorption line on the velocity of resonance particles, ν. It is shown that under certain conditions, account of this dependence can radically change the spectral shape of the LID signal, up to the appearance of additional zeros in the dependence of the drift velocity on the radiation frequency.

Manifestation of the light-induced drift effect in chemically peculiar stellar atmospheres

We have calculated the factor (νg − νe)/νg in the temperature range T = 300–20 000 K for the ions Be+, Mg+, Ca+, C+ in atomic hydrogen and for the ions Mg+ in atomic argon using the known interaction potentials. Here νe and νg are the transport collision frequencies for excited- and ground-state particles respectively. Calculations have shown that at T = 10 000–20 000 K, typical temperatures of the atmospheres of chemically peculiar (CP) stars, the values |νg − νe|/νg ≈ 0.1–0.2 can be reached for ions. This causes the light-induced drift (LID) velocity of ions up to ∼0.1 cm/s in the atmospheres of CP stars with temperatures T T > 10 000 K) it is possible to expect approximately identical magnitude of the LID effect and that of radiation pressure. In the very hot stars (T >20 000 K) the LID effect is manifested very weakly.

Observation of Light-Induced Drift Effect of Rubidium by Using Two Diode Lasers for Pumping and Re-Pumping

The light-induced drift (LID) of rubidium was demonstrated by using two diode lasers for the pumping and re-pumping of rubidium atoms. The isotope composition ratio for 85 Rb and 87 Rb was estimated by measuring the absorption spectrum of the rubidium vapor.

ABSORPTION AND REFLECTION EXPERIMENTS ON HIGH-MOBILITY 2DEGs IN THE REGIME OF MICROWAVE-INDUCED RESISTANCE OSCILLATIONS

We have performed microwave absorption and near-field reflection experiments on a high mobility GaAs / AlGaAs heterostructure for the same conditions for which Microwave-Induced Resistance Oscillations (MIROs) are observed. It is shown that the electrodynamic aspect of the problem is important in these experiments. In the absorption experiments a broad CR line was observed due to a large reflection from the highly conductive electron gas. There were no additional features observed related to absorption at harmonics of the cyclotron resonance. In near-field reflection experiments a very different oscillation pattern was revealed when compared to MIROs. The oscillation pattern observed in the reflection experiments is probably due to plasma effects occurring in a finite-size sample. The whole microscopic picture of MIROs is more complicated than simply a resonant absorption at harmonics of the cyclotron resonance. Nevertheless, the experimental observations are in good agreement with the model by Durst et al. involving the photo-assisted scattering in the presence of a crossed magnetic field and dc bias. The observed damping factor of MIROs may be attributed to a change in the electron mobility as a function of temperature. MIROs may be considered as a light-induced drift effect, a broad class of phenomena associated with a light-induced asymmetry in the velocity distribution function.

Ion drift in a magnetic field under the combined action of LID and light pressure

The effect of magnetic field on the ion drift in a weakly ionised gas under the combined action of light-induced drift (LID) and light pressure is theoretically investigated. It is shown that the imposition of an external magnetic field may give rise to a velocity component of light-induced ion drift orthogonal to the direction of radiation propagation. The effect of light pressure in sufficiently strong magnetic fields is found to prevail over the LID effect, while the reverse is true for weak magnetic fields. The dependence of the ion drift velocity on the frequency detuning drastically changes in the magnetic field when ions experience the Lorenz force. It is predicted that the projection of the ion drift velocity on the direction of radiation propagation should change its sign with increasing magnetic field, and an anomalous LID can be observed.

Evidence of the Light-Induced Drift Effect in Lithium Vapor

Dependence of light-induced drift (LID) on the laser wavelength has been studied for lithium vapor. Fluorescence intensity variation for 7 Li atoms clearly indicated unique nonlinear dependence on the laser power at the different wavelengths of the laser beam, indicating the LID effect of 7 Li. We also estimated the drift velocity values from the fluorescence intensity variation presently measured and fair agreement with drift velocity was found.

Frequency stabilisation of a broad-band dye laser by light-induced drift

A new method for frequency locking of a broad-band laser based on light-induced drift is presented. The main advantages of the technique are: good stabilisation provided by sensitivity and good signal-to-noise ratio of the error signal; unnecessary dithering of the laser frequency; use of an atomic transition with low frequency drift. The method is well suited to broad-band applications because the light-induced drift effect can naturally integrate over a broad-band of frequencies (of the order of the Doppler width) to find the average. Tests carried out on Na immersed in a buffer gas of Ar, have allowed long-term frequency stabilisation of the central frequency of a 2.5 GHz-bandwidth dye laser.

Trap for polarized atoms based on light-induced drift.

A polarization of sodium atoms in a simple trap based on the light-induced drift effect is reported. A sapphire capillary with one side closed and heated up to high temperature was used as the trap. The capacity of the trap is about 5\ifmmode\times\else\texttimes\fi{}${10}^{11}$ atoms. The achieved polarization of trapped atoms in the gas phase is about 90%. The full polarization of atoms in the trap is estimated at 25% at the temperature of 1400 \ifmmode^\circ\else\textdegree\fi{}C. The adsorption energy of sodium atoms on a sapphire surface is determined to be 2.5 eV. \textcopyright{} 1996 The American Physical Society.

Enhanced Isotope Separation of Rubidium by Light-Induced Drift Using Backpumping

Isotope separation of rubidium by light-induced drift (LID) have been demonstrated with the use of a backpumping. A Ti:Al2O3 laser and a diode laser were used to induce the LID and for the backpumping, respectively. Enhancement of the LID effect was observed by the backpumping. The 85Rb concentration was estimated to be 94% or more from the absorption spectrum measurement.

Semiclassical collision model for rotating molecules. Application to light-induced drift

A new model is proposed to account for the non-spherical part of the intermolecular interaction in gas kinetics. The model is mathematically simple and is based on a transformation of collision kernels for spherical particles to kernels for nonspherical particles. An application to the Light-Induced Drift effect is given.

Isotope separation of 22Na and 24Na by using the light induced drift effect

The LIDIS (Light Induced Drift Isotope Separation) for two light absorbing isotopes is discussed. It is shown, that the atomization degree of the separated isotopes limits the real separation coefficient. The experimental results of 22Na and 24Na isotopes separation are given. The maximum separation coefficient value 25 has been achieved.

Observation of white light-induced drift separation of Rb isotopes

The observation of the separation of rubidium isotopes by white light-induced drift is reported. A broadband modeless laser with a smooth spectrum is used as a light source. Spectral structure formed by the transport of light through the vapor causes a light-induced drift, pushing the 87Rb and pulling the 85Rb. The separation is measured by spectroscopic isotope analysis. Hyperfine effects are considered and rejected as a possible source of a false separation signal. Also investigated is whether statistical properties of the light beyond the time-averaged intensity could produce a net drift. Although it is found that a net drift is possible even for a flat time-averaged spectrum, this is not expected from the structure of the broadband laser used because the associated time scales are too short. The evidence indicates that the observed separation is due to the predicted white light-induced drift effect.