Light-induced Drift Research Articles

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.

Light-induced heat flux and drift of one-component gas in a flat channel

Processes of heat transfer and drift of rarefied gas in a flat channel due to the resonance absorption of the optical radiation of atom selectively on their velocities have been studied. The flux profiles and kinetic coefficients determining the surface and collision mechanisms of light-induced phenomena of transfer in the one-component gas in a flat channel have been calculated on the basis of the numerical solution of the kinetic equations depending on the values of the Knudsen number and the ratio of the frequency of the radiation decay of the excited level of atoms to the frequency of the molecular collisions.

Surface light-induced drift, including number flux and heat flux, in flat-plate and circular-cylindrical geometries

With standard forms of the approximate Maxwell–Boltzmann rate equations, some of the author’s previous work is used to obtain exact results for the gas particle velocity distribution functions for the general case of laser excitation function, and, for use in calculations of the fluxes, a very simple effective distribution function is derived. The work is confined to the limit of low radiation absorption by the gas and to the limit of large Knudsen number. Where possible, the exact results for the fluxes are compared with recent corresponding approximate results of Chermyaninov, Chernyak, Khinkina, and Vilisova (CCKV), with interesting conclusions. The results allow useful assessment of approximation schemes such as those in CCKV.

Photosystem I Bio-Photosensor Integrated with Complementary Metal–Oxide–Semiconductor Source–Drain Follower on a Chip

A new bio-photosensor where photons are converted to electrons by photosystem I (PSI) of Thermosynechococcus elongatus is described; in this bio-photosensor, the converted electronic charge is sensed by a complementary metal–oxide–semiconductor (CMOS) source–drain follower circuit fabricated by the 1.2 µm standard CMOS process. Thin Au/Ti films are deposited and patterned with a 4.255×4.255 µm2 extended-gate electrode, and an SU-8 layer is formed, with the exception of the electrode, to reduce the light-induced drift of silicon nitride. As a result, only the Au electrode and the SU-8 layer come into contact with the electrolyte. The PSI is immobilized on the electrode, and the photoresponse due to the photovoltaic effect of PSI is measured as the output voltage of the CMOS source–drain follower. The action spectrum from the light source is consistent with the PSI absorption spectrum. In addition, the selectivity of the photoresponse between two adjacent sensor cells is confirmed, which is promising for ultrasensitive bioimage sensing.

Modelling of mercury isotope separation in CP stellar atmospheres: Results and problems

Formation of anomalous isotope abundances in the atmospheres of chemically peculiar (CP) stars can be explained by light-induced drift (LID). This effect is additional to the radiative acceleration and appears due to systematic asymmetry of radiative flux in partly overlapping isotopic spectral line profiles. LID causes levitation of an isotope with a red-shifted spectral line and sinking of an isotope with a blue-shifted line, generating thus diffusive separation of isotopes. We have studied diffusion of mercury as a typical well-studied isotope-rich heavy metal. Our model computations show that in mercury-rich quiescent atmospheres of CP stars LID causes levitation of the heavier mercury isotopes and sinking of the lighter ones. Precise quantitative modelling of the process of isotope separation demands very high-resolution computations and the high-precision input data, including data on hyperfine and isotopic splitting of spectral lines, adequate line profiles and impact cross-sections. Presence of microturbulence and weak stellar winds can essentially reduce the effect of radiative-driven diffusion.

Theory of surface light-induced drift with forced specularity

Waxman has recently proposed new mechanisms for surface light-induced drift (SLID), which he called “SLID with forced specularity” (christened SLIDWFS here). SLIDWFS was proposed for its potential use in modelling several important physical processes; examples are selective control of spatial distributions of gases, including isotope separation, probing the molecule-surface interaction potential in gas-surface scattering, heterogeneous chemical reactions in catalysis, and laser cooling. Waxman’s cell geometry consisted of a single flat plate; the usual approximations were made in the analysis, and substantial effects (larger than in ordinary SLID) were reported. In the present contribution, we study the more realistic double-flat-plate and circular-cylindrical geometries. We get exact analytical solutions to the model rate equations, with results quite different from Waxman’s. For example, for circular-cylindrical geometry, which is clearly preferable in ordinary SLID, we predict a negligible SLIDWFS effect.PACS Nos.: 05.60.–k, 34.35.+a, 34.50.Rk

Isotopic anomaly and stratification of Ca in magnetic Ap stars

We have carried out an accurate investigation of the Ca isotopic composition and stratification in the atmospheres of 23 magnetic chemically peculiar (Ap) stars of different temperature and magnetic field strength. With the UVES spectrograph at the 8m ESO VLT, we have obtained high-resolution spectra of Ap stars in the wavelength range 3000-10000 A. Using a detailed spectrum synthesis calculations, we have reproduced a variety of Ca lines in the optical and ultraviolet spectral regions, inferring the overall vertical distribution of Ca abundance, then we have deduced the relative isotopic composition and its dependence on height using the profile of the the IR-triplet Ca II line at lambda 8498 A. In 22 out of 23 studied stars, we found that Ca is strongly stratified, being usually overabundant by 1.0-1.5 dex below log tau_5000 ~ -1, and strongly depleted above log tau_5000 = -1.5. The IR-triplet Ca II line at lambda 8498 A reveals a significant contribution of the heavy isotopes 46Ca and 48Ca, which represent less than 1% of the terrestrial Ca isotopic mixture. We confirm our previous finding that the presence of heavy Ca isotopes is generally anticorrelated with the magnetic field strength. Moreover, we discover that in Ap stars with relatively small surface magnetic fields (<=4-5 kG), the light isotope 40Ca is concentrated close to the photosphere, while the heavy isotopes are dominant in the outer atmospheric layers. This vertical isotopic separation, observed for the first time for any metal in a stellar atmosphere, disappears in stars with magnetic field strength above 6-7 kG. We suggest that the overall Ca stratification and depth-dependent isotopic anomaly observed in Ap stars may be attributed to a combined action of the radiatively-driven diffusion and light-induced drift.

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.

Theory of light-induced drift. VI. Roles of accommodation of normal and tangential momenta in surface light-induced drift with positive spontaneous relaxation parameter

Surface light-induced drift of a rarefied gas in cells with flat-plate and circular-cylindrical geometries is studied, and exact analytical solutions to the model rate equations are obtained in the limit of large Knudsen number. The work is a continuation of part V of this series of papers, in which the model rate equations were tailored specifically in order to study the roles played by both the tangential and normal momentum molecule-surface accommodation coefficients. In part V, the ``spontaneous decay'' parameter, $\ensuremath{\gamma}$, was set equal to zero, and the case of general ``laser excitation'' parameter, $q$, was studied. In the present paper, we get exact analytical results for the case of arbitrary $\ensuremath{\gamma}$ in the limit of small $q$; we also get accurate numerical results for arbitrary $\ensuremath{\gamma}$ and $q$. In passing, one serious, and other less serious, typographical errors in part V are corrected.

Diffusion in stellar atmospheres

A review of some of the most important studies of the past several decades pertaining to the study of atomic diffusion in stellar atmospheres is presented. A brief description of various studies of radiative accelerations and diffusion calculations in stellar atmospheres is given. Several physical phenomena such as ambipolar diffusion, light-induced drift and the effect of magnetic fields on diffusion are discussed. Results from recent self-consistent model atmospheres including elemental stratification are also reviewed. Particularities of radiative acceleration calculations in stellar atmospheres as well as difficulties and future challenges related to diffusion calculations in stellar atmospheres are also presented.

Light-induced heat and mass transfer of gas in a plane channel

The paper deals with the processes of heat and mass transfer of a rarefied gas in a plane channel in the field of resonance radiation. The problem is solved using linearized kinetic equations with a first-order model integral of collisions. The surface and collision mechanisms of light-induced drift are investigated, as well as the mechanism of heat flux at arbitrary values of the Knudsen number. Analytical expressions for channel cross section-averaged flows are obtained at high and low values of the Knudsen number.

The role of light-induced drift in diffusion of heavy metals and their isotopes in CP stars: an example of mercury

Main theoretical formulae and some computational results of evolutionary segregation of Hg isotopes due to light-induced drift in the atmospheres of chemically peculiar stars have been represented.

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.

Theory of light-induced drift. V. Roles of accommodation of normal and tangential momenta in surface light-induced drift

Surface light-induced drift (SLID) of a rarefied gas in cells with flat-plate and circular-cylindrical geometries is studied, and exact analytical solutions to the model rate equations are obtained in the limit of large Knudsen number. The model rate equations, which have not appeared before, have been tailored specifically in order to study the roles played in SLID, in a physically realistic setting, by both the tangential momentum and the normal momentum molecule-surface accommodation coefficients. Because the model equations are new (to the best of our knowledge), the results are generally different from those of previous work, although emphasis is placed on obtaining relations between the present and previous results; applications to experiments may be made by means of those relations.

The pH-sensing and light-induced drift properties of titanium dioxide thin films deposited by MOCVD

Titanium dioxide thin films were deposited on thermally oxidized silicon wafers by metal organic CVD (MOCVD). Then the resulting TiO2/SiO2/Si test structures were annealed in dry oxygen for 60min at temperatures 700, 800, 900, and 1000°C, respectively. Refractive index and relative permittivity of the as-deposited and annealed titanium dioxide layers were investigated by spectral ellipsometry and high frequency capacitance–voltage (HF-CV) measurements. The pH-sensing properties of the titanium dioxide layers were investigated by capacitance–voltage measurements on electrolyte/insulator/silicon (EIS) structures. In addition, light-induced drift of the test structures was investigated by illumination using 12V light bulb for 60min in a measurement chamber. The titanium dioxide layers revealed refractive index of 2.38–2.58 (λ=550nm) and relative permittivity of 31.28–36.27. Integral pH-sensitivities (pH ranging from 3 to 11) of the titanium dioxide layers were 57.4–62.3mV/pH (T=32°C). All the titanium dioxide layers showed no light-induced drift. After long-term measurement, the titanium dioxide layer annealed at 900°C revealed a better stability of the pH-sensing properties than other titanium dioxide layers investigated in this study.

Theory of light-induced drift. III. Models of surface and bulk light-induced drift in one dimension

Light-induced drift (LID) of a rarefied gas in a cell is studied, and exact analytical closed-form solutions to the model rate equations, which model the gas motion in one dimension, are obtained for cases of both surface LID (SLID) and bulk LID (BLID); the special case of the limit of low radiation absorption by the gas is given particular attention. Similarities and differences among the results for SLID and BLID are discussed. This is part III of a series of papers, parts I and II having studied LID, but concentrating on SLID, with flat-plate and circular-cylindrical cell geometries, respectively [F. O. Goodman, Phys. Rev. A 65, 064309 (2002); 65, 064310 (2002)].

Theory of light-induced drift. IV. Models of bulk light-induced drift in three dimensions

Light-induced drift (LID) of a rarefied gas in a cell is studied, and exact analytical closed-form solutions to the model rate equations are obtained for bulk LID (BLID) in three dimensions, emphasis being placed on the limit of low radiation absorption by the gas. Comparisons are made with an existing model of BLID in one dimension [F. O. Goodman, preceding paper, Phys. Rev. A 67, 013410 (2003)], and we examine whether there are any conditions under which that model is satisfactory. Comparisons are also made with the author's models of surface LID in one dimension (above reference) and in three dimensions [F. O. Goodman, Phys. Rev. A 65, 063409 (2002); 65, 063410 (2002)].

Flux measurements of alkali atoms diffusing via multiple surface scattering in asilanated glass tube

Alkali atoms that are diffusing away from a source via multiple surface scatteringin a nonstick coated glass tube are monitored by laser fluorescence. Densitiesare measured by fluorescence intensity and weak-field absorption, andsmall asymmetries in the line shape provide a measurement of the flux.Average velocities as small as 0.3% of the thermal velocity can be resolvedusing the technique presented. Model calculations are presented and fitto the data, providing approximate values for the sticking probability,and the probability of specular scattering versus diffuse scattering(related to momentum accommodation). For the silane-based coatingSurfaSilTM,we find that the scattering is mostly diffuse, and that the sticking probability is ofthe order of 10−4 or 10−3. With refinement, the technique can be used to measurethe variation in the sticking probability along the length of the tube, which couldbe useful for studying and improving gas separation columns. We presentnumerical calculations of surface light-induced drift (SLID) effects, but noevidence for SLID is observed experimentally.

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.