Anomalous Light-induced Drift 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.

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 of rubidium atoms caused by the velocity dependence of transport collision frequencies

The spectral features of the light-induced drift (LID) velocity for rubidium atoms (85Rb and 87Rb) in an argon buffer medium and in binary buffer mixtures of noble gases (Ne + Ar, Ne + Kr, Ne + Xe, He + Ar, He + Kr, and He + Xe) have been investigated theoretically. A strong temperature dependence of the spectral shape of the LID signal for Rb atoms in an Ar atmosphere is predicted in the temperature range 450 K < T < 800 K. It is shown that the anomalous LID of Rb atoms in binary buffer mixtures of noble gases can be observed at almost any temperature (including the room one) depending on the fractions of neon or helium in these mixtures. The results obtained enable a highly accurate testing of the interatomic interaction potentials used to calculate the drift velocity for anomalous LID in LID experiments.

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.

Light-induced drift of ions in a magnetic field

The effect of a magnetic field on the ion drift in a weakly ionized gas under the combined action of the light-induced drift and light pressure is studied theoretically. It is shown that, under the action of light, a component of ion drift velocity transverse to the direction of propagation of radiation may appear in a weakly ionized gas upon the application of an external magnetic field. It is shown that the Lorentz force acting on ions in the magnetic field radically changes the dependence of the ion drift velocity on the radiation frequency detuning. It is predicted that the ion drift velocity component along the direction of radiation must reverse its sign upon an increase in the magnetic field and an anomalous light-induced drift may be observed.

On the collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field

The collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field is investigated theoretically. It is shown numerically that the transfer effect is weak. This makes it possible to use simpler approximate one-dimensional quantum kinetic equations instead of three-dimensional equations to solve spectroscopy and light-induced gas kinetics problems, where it is important to take account of the velocity dependence of the collision frequency. It is shown for anomalous light-induced drift, calculations of which are most sensitive to neglecting the transfer effect, that in a wide range of spectroscopy and light-induced gas kinetics problems the transfer of nonequilibrium can be neglected without risking the loss of important fine details of the phenomena being described.

Anomalous light-induced drift of lithium atoms in a mixture of noble gases

This paper is a theoretical study of the spectral features of the velocity of light-induced drift (LID) of lithium atoms (7Li and 6Li) in a binary mixture of noble gases: Ne + Ar, Ne + Kr, and Ne + Xe. The spectral shape of the LID signal is predicted to depend strongly on the fraction ξ of neon in the buffer mixture in the range ξ≈0.8–0.9 (ξ=NNe/Nb, where NNe is the neon concentration, and Nb is the total concentration of the buffer particles). When the velocity of anomalous LID is treated as a function of the radiation frequency, it is found to have one, three, five, or seven zeros and to differ substantially from the dispersion-curve-like behavior with one zero predicted by the standard LID theory with velocity-independent transport collision rates. The reason for these additional zeros of the drift velocity is the alternating-sign dependence on the lithium-atom velocity of the relative difference of transport rates of collisions between buffer particles and excited and unexcited atoms. What is also established is that the anomalous LID of lithium atoms can be observed at almost all temperatures, depending on the value of ξ. At a fixed temperature, anomalous LID can be observed only in a narrow range of values of the fraction of neon in the buffer mixture, Δξ≈0.02. The results make possible highly precise testing in the LID experiments of the interatomic potentials used in calculations of the velocity spectrum of anomalous LID.

Exact solution of the one- and three-dimensional quantum kinetic equations with velocity-dependent collision rates: Comparative analysis

The interaction of a plane monochromatic traveling wave with two-level particles suffering collisions with buffer-gas particles is considered. Collision rates are assumed to be velocity dependent. The collision integral is obtained on the basis of the strong-collision model, generalized to the case of velocity-dependent collision rates (the so-called ``kangaroo'' model). We obtained the exact analytical solution of the problem for arbitrary intensity of radiation, arbitrary ratio of homogeneous and Doppler widths of the absorption line, and arbitrary mass ratio between absorbing- and buffer-gas particles. The obtained analytical solutions of the quantum kinetic equations allowed us to analyze the spectral shape of the strong-field absorption line as well as the probe-field absorption line (the nonlinear part of the work done by the probe field) and the frequency dependence of the light-induced drift (LID) velocity. A comprehensive comparative analysis for the three- and one-dimensional versions of the model is given. On the basis of this analysis, we reach the conclusion that the one-dimensional quantum kinetic equation has quite a wide range of application. We also reveal the conditions for the strongest manifestation of the velocity dependence of the collision rates, which affects most strongly the anomalous LID.

Anomalous light-induced drift of linear molecules

The sudden approximation in energy is used to derive analytic formulas that describe the anomalous light-induced drift (LID) of linear molecules absorbing radiation in the rovibrational transition nJi−mJf (n and m are the ground and excited vibrational states, and Jα is the rotational quantum number in the vibrational state α=m, n). It is shown that for all linear molecules with moderate values B≲1 cm−1 of the rotational constant, anomalous LID can always by observed under the proper experimental conditions; temperature T, rotational quantum number Ji, and type of transition (P or R). The parameter γ=B[Ji(Ji+1)−Jf(Jf+1)] νn/2kBT(νm−νn) is used to derive a condition for observing anomalous LID: γ∼1 (kB is the Boltzmann constant and να is the transport rate of collisions of molecules in the vibrational state α and buffer particles at moderate molecular velocities Open image in new window , where \(\bar \upsilon _b\) is the most probable velocity of the buffer particles). For νm>νn anomalous LID can be observed only in P-transitions, while for νm<νn it can be observed only in R-transitions. It is shown that anomalous LID is possible for all ratios β=Mb/M of the masses of the buffer particles (Mb) and of the resonant particles (M) and any absorption-line broadening (Doppler or homogeneous). The optimum conditions for observing anomalous LID are realized when the absorption line is Doppler-broadened in an atmosphere of medium-weight (β∼1) and heavy (β≫1) buffer particles. In this case, anomalous LID can be observed in the same transition within a broad temperature interval ΔT∼T. If the buffer particles are light (β≪1) or if the broadening of the absorption line is homogeneous, anomalous LID in the same transition can be observed only within a narrow temperature range ΔT≪T.

Spectral anomalies of light-induced drift of rubidium and lithium under monochromatic excitation

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.

Anomalous light-induced drift of potassium in Ne plus rare-gas mixtures

We report anomalous light-induced drift of potassium vapor in mixtures of Ne with another rare gas. The drift velocity versus laser frequency curve shows a strong deviation from the dispersion curve predicted by the standard theory of light-induced drift based on velocity-independent collision rates. The observed drift velocity versus frequency curves have three zero-crossings. For a qualitative explanation of the results, a strong collision model with velocity-dependent collision rates is used. The partial pressures that cancel normal light-induced drift also provide accurate ratios of cross section for potassium collisions with the various buffer gases. The observation of anomalous light-induced drift in a simple atomic system offers the opportunity to relate the results to potential curves. \textcopyright{} 1996 The American Physical Society.

Generalized analytical description of light-induced drift and an experimental test using C2H4.

It is demonstrated that both standard and anomalous light-induced drift (LID) profiles as a function of laser frequency can be analytically simulated using the description of LID in terms of transport mean paths. This description allows one to directly link arbitrary LID frequency profiles to derivatives of the unsaturated absorption profile. In order to test this approach, LID experiments have been performed on gaseous ethylene ${(}^{13}$${\mathrm{C}}^{12}$${\mathrm{CH}}_{4}$) for the rovibrational transition (7,1,6) \ensuremath{\rightarrow}${\ensuremath{\nu}}_{7}$(8,0,8) in a background of Kr, Xe, ${\mathrm{SF}}_{6}$, ${\mathrm{CO}}_{2}$, or ${\mathrm{CH}}_{3}$Cl. Although the observed LID frequency profiles show a large variation, they are well simulated using this approach. \textcopyright{} 1996 The American Physical Society.

Spectral anomalies in light-induced drift in alkali-noble gas mixtures

The spectral shape of a light-induced drift (LID) signal in alkali-noble gas mixtures has been investigated. Realistic calculations of the velocity dependence of the transport cross sections in different quantum states have been made with the help of the semiempirical potentials of Pascale and Vandeplanque (1974). The anomalous LID spectral profile for the Li-Ne system has been obtained theoretically.

Description of light-induced drift in terms of transport mean paths

Light-induced drift is often described in terms of a change Delta nu in the collision rate (or of the transport collision rate) caused by optical excitation of atoms or molecules immersed in a buffer gas. One assumes (at least tacitly) that the collision rates are independent of velocity, though this is true only for heavy particles in a light buffer gas. The recently observed anomalous light-induced drift of molecular gases suggests using a speed-dependent Delta nu which, however, is difficult to justify. The formulation in terms of speed-dependent transport mean paths, proposed here, is free of such ambiguities. For molecules a Wang-Chang-Uhlenbeck-de Boer multilevel formalism leads to extended transport mean paths, whereupon anomalous light-induced drift can be interpreted in a plausible way.

Velocity-dependent collision rates from light-induced drift experiments: C2H4-noble-gas mixtures.

Light-induced drift arises in a binary mixture when velocity-selective optical excitation is combined with a state-dependent kinetic collision frequency (or cross section). This phenomenon is studied experimentally for rovibrationally excited ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$ immersed in the atomic buffer gases Ne, Ar, Kr, or Xe at temperatures between 275 and 500 K. In several cases, the laser-frequency dependence of the drift is observed to be anomalous and to change dramatically with temperature. This means that the relative change in collision frequency upon excitation depends strongly on velocity. To evaluate the data, an analytic model description is presented and applied to the experimental data. It is found that for each particular buffer gas, all data are well described with a relative difference in collision rate depending on a single parameter only, i.e., the average relative velocity. This dependence is found to be different for the various noble gases. This suggests that the attractive part of the interaction potential is crucial for the occurrence of anomalous light-induced drift.

Influence of molecular rotation on light-induced drift of CH3F

Experimental results on light-induced drift of (ro)vibrationally excited CH3F immersed in the noble buffer gas Kr or in the polar buffer gas CH3Cl are presented. For pure vibrational excitation, the relative change in collision rate is found to be essentially velocity independent. For rovibrational excitation, this quantity can have a significant velocity dependence, as can be concluded from the detuning behaviour of light-induced drift for two transitions of CH3F immersed in Kr. In combination with earlier observations of anomalous light-induced drift in C2H4, these results demonstrate that a sizable velocity dependence of the change in collision rate caused by rovibrational excitation is a general feature for molecular systems. Additionally, while the transport collision rate generally increases with vibrational quantum number, the data suggest that it decreases with increasing rotational quantum number. Finally, the data for CH3F in CH3Cl indicate that rotational-state-changing collisions are accompanied by a significant velocity change.

Observation of anomalous light-induced drift.

In a two-component gas consisting of light-absorbing particles immersed in a buffer gas, velocity-selective excitation combined with a state-dependent kinetic collision frequency results in light-induced drift. Standard theory predicts that the drift velocity as a function of frequency detuning should have a dispersion-curve-like behavior. This was indeed found experimentally for the molecular species studied so far. We report a dramatic deviation from this behavior for C{sub 2}H{sub 4}, excited into its {nu}{sub 7} vibrational mode by a CO{sub 2} laser, with Kr as a buffer gas. The extent of the deviation is found to depend strongly upon the rotational sublevels involved. A possible explanation in terms of a three-level model is attempted but found to be unlikely.