Low Atomic Density Research Articles

The fine structure emission of thermospheric atomic oxygen

The fine structure emission of thermospheric atomic oxygen at 63 μm wavelength was measured for the first time globally by the CRISTA (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere) experiment in November 1994. The data indicate low atomic oxygen densities throughtout the lower thermosphere during the time period of the experiment. A first analysis of the latitudinal distribution of oxygen atoms at 150 km altitude shows deviations from the respective CIRA-86 model predictions at mid to high northern latitudes. These deviations vary from day to day, and are possibly the result of changes in the geomagnetic activity.

Enhancement of nondegenerate four-wave mixing based on electromagnetically induced transparency in rubidium atoms

We report an experimental observation of the enhancement of nondegenerate four-wave mixing (NDFWM) based on electromagnetically induced transparency (EIT) in a lambda-type three-level system of rubidium atoms. We measured both the linear susceptibility Im chi(D)((1)) (absorption) and the third-order nonlinear coefficient chi(D)((3)) separately for the NDFWM process at a low atomic density. We found that, owing to the EIT effect, the linear absorption term Im chi(D)((1)) is greatly reduced, while the nonlinear generation term chi(D)((3)) is resonantly enhanced, permitting us to observe a significant enhancement of the NDFWM signal in an optically dense medium.

Laser cooling of cesium atoms in gray optical molasses down to 1.1 microK.

We have studied the behavior of cesium atoms cooled in six-beam gray optical molasses. Cooling occurs for a laser detuned to the blue side of the $6{S}_{\frac{1}{2}}$, $F=3\ensuremath{\rightarrow}6{P}_{\frac{3}{2}}$, ${F}^{\ensuremath{'}}=2$ transition, and a Sisyphus-type effect accumulates the atoms in states not coupled to the light. We measure a minimum temperature of 1.1\ifmmode\pm\else\textpm\fi{}0.1 \ensuremath{\mu}K at low atomic density. The typical cooling time is on the order of 1 ms. A linear dependence of the temperature versus atomic density is found with a slope of \ensuremath{\sim}0.6 \ensuremath{\mu}K/(${10}^{10}$ atoms/${\mathrm{cm}}^{3}$).

Measurement of the potassium 4P excited state diffusion coefficient in xenon gas using degenerate four-wave mixing.

We present a method for measuring the diffusion coefficients for excited atoms in buffer gases that uses degenerate four-wave mixing. We show that, for low beam intensities and atomic densities, the relative angular response of the degenerate four-wave mixing signal is dependent on the excited state diffusion and decay rate only, and that it is independent of the ground state diffusion. We have experimentally measured the diffusion of potassium atoms in the $4P$ state in xenon and found that the diffusion coefficient $D$ at 155 \ifmmode^\circ\else\textdegree\fi{}C and 760 torr is 0.084(4) c${\mathrm{m}}^{2}$ ${\mathrm{s}}^{--1}$.

Observation of a single atom in a magneto-optical trap

Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap.

Modification of multiphoton ionization spectra via third harmonic generation in focused laser beams

The modification of multiphoton ionization spectra through third harmonic generation in focused laser beam geometries is reported. A strong enhancement of the ionization peaks corresponding to the decay of thenf' autoionizing series of Kr and the observation of a double resonance in Xe, which is missing in low atomic density spectra, is shown to originate from excitation channels involving absorption of third harmonic photons. This excitation is demonstrated to occur in a significantly extended volume outside the focal region, thus strongly increasing the ionization yield.

Light-pressure force in N-atom systems.

An analytical description of long-range collisions between atoms in a laser cooling field is developed. We begin by considering an N-atom master equation. In the regime of low atomic densities (i.e., where the mean distance between two atoms is much larger than the laser wavelength) it is possible to treat the atom-atom interactions in perturbation theory. Furthermore we assume temperatures which allow a semiclassical treatment of the cooling process. The effect of the presence of other atoms can be separated analytically into two parts; an attenuation force due to the absorption of the laser beams in the atomic cloud similar to the results of Dalibard [Opt. Commun. 68, 203 (1988)], which tends to compress the atomic cloud, and a two-atom force due to photon emission and absorption cycles between different atoms. This force proves to be repulsive for the configurations studied and prevents the cloud from collapsing. The result for the first-order perturbation expansion in collision strength generalizes the model proposed by Walker, Sesko, and Wieman [J. Opt. Soc. B 8, 946 (1991)] by including additional terms, such as those associated with Raman couplings.

Electron scattering of low-Z high-density materials in X-ray mask patterning

The scattering properties of low- Z high-density materials are discussed, through Monte Carlo simulation, in view of their utilization as membranes for X-ray masks. The interplay between low atomic number and high atomic density is discussed and a comparison with silicon is carried out, both in case of bulk targets and membranes. The low Z causes smaller proximity effects, but the decisive factor relative to silicon is constituted by the higher mechanical stiffness, which, in principle, allows to greatly reduce membrane thickness. However, we show that, in spite of the more favourable intrinsic scattering properties, the choice of these materials, in actual systems, is not always a real advantage. In the analysis of the single-layer resist process for X-ray mask fabrication, no significant difference relative to Si is found in the two limits of low and high energy. On the contrary, the choice of low- Z high-density materials is found to be advantageous at intermediate energy. In particular, we demonstrate that, while in the case of low- Z high-density materials, 0.15 μm resolution is successfully obtained already at 30 keV, in case of Si, this same resolution is only reached at e-beam energies of at least 40 keV.

Positron annihilation studies of the Al-Ca-Zn superplastic alloy: Thermal and thermomechanical contribution

Positron annihilation spectroscopy (PAS) is an established method for the study of electronic structure and defect properties in metals and alloys. The application of this technique to the study of positron trapping in grain boundaries and related phenomena, however, is relatively scarce. The physical basis for the application of PAS to the study of grain boundaries is the fact that grain boundaries are regions of low atomic density which result in attractive sites to the trap positions. The superplastic alloys are particularly suitable materials to be studied with PAS; they have a fine-grained structure, and therefore a high density of grain boundaries. Moreover, in the annealed condition, they have a low density of other types of defects capable of trapping positrons, such as dislocations. This type of polycrystalline material can undergo extremely high deformations (up to hundreds and thousands percent) in a certain temperature-strain rate range without macroscopic failure. This paper is part of a whole study of the thermal and thermomechanical effects on the positron lifetime parameters and their relation with microstructural changes and the phenomenon of structural superplasticity in a Al-Ca-Zn alloy.

Utilisation combinée des deux sources de rayonnements laser et synchrotron : caractéristiques et résultats

The first experiments combining the use of cw dye lasers with synchrotron radiation for inner-shell photoionization in laser-excited atoms, were carried out more than 10 years ago. Since that time, a number of resonant inner-shell photoexcitation processes have been measured. The results of these measurements will be summarized. The use of new synchrotron radiation sources such as the third generation storage ring Super ACO in Orsay or the undulator installed at BESSY, in Berlin, enhanced the photon flux available, making feasible new experiments. At high atomic densities, partial photoionization cross sections or angular distribution of inner-electro ns were measured in laser-excited alkali- and alkaline earth atoms, revealing strong enhan­ cements of the intensity of correlation satellites in inner-shell photoionization of excited atoms. At low atomic densities, synchrotron radiation studies of laser-aligned atoms were started, allowing to demonstrate the influence of the laser polarization on the angular distribution of electrons emitted in the decay of core-excited autoionizing states. These new results will be described. Finally, the perspectives open for present and future uses of third generation storage rings in combination with laser(s) will be discussed.

Extension of the dynamic range by using an atom formation function in electrothermal atomic absorption spectrometry

A method of extending the dynamic range of a calibration graph by calculating the initial mass of an analyte on a tungsten strip heater using an atom formation model in electrothermal atomic absorption spectrometry is proposed. The model is based on the assumption that the rate of atom formation, given by a simple Arrhenius-type expression, is a function of the number of atoms in a sample on the heater and the heater temperature. The absorbance for the initial mass of iron chosen as a test analyte was obtained by applying the absorbance at the initial stage of the signals to the model equation, and the calculated values of the mass were plotted against the amount of iron in the test solution to give a calibration graph. To extend the dynamic range further, the light beam from a hollow-cathode lamp was adjusted so as to pass through a region of low atom density above the heater.

Applications of slow positron beams to study point defect distributions near solid surfaces

Injection of monoenergetic “slow” (0 to 30 keV) positrons into solid materials can be used to obtain unique information on depth distribution of open-volume lattice defects near solid surfaces. This is due to the tendency of thermalized positrons to become localized into regions of low atomic density. These include vacancies, their agglomerates and dislocations in metals, and negatively-charged vacancies and impurities in semiconductors. Specific information on the type and density of defects can be obtained starting from the outermost atomic layer down to a few μm depths. Applications to studies of damage associated with ion-implanted semiconductors, sputtered surfaces and growth processes of thin epitaxial heterostructures are presented.

Extended x-ray-absorption and electron-energy-loss fine-structure studies of the local atomic structure of amorphous unhydrogenated and hydrogenated silicon carbide.

Extended x-ray-absorption (EXAFS) and electron-energy-loss fine-structure (EXELFS) measurements have been performed on amorphous unhydrogenated silicon carbide, a-SiC, and amorphous hydrogenated silicon carbide, a-SiC:H. Two hydrogenated samples with hydrogen concentrations corresponding, respectively, to H flows of 4 sccm (20% of argon flow) and 8 sccm (40% of argon flow) during the reactive sputtering process, were analyzed (sccm denotes standard cubic centimeters per minute at STP). It is found that short-range order (SRO), consisting of the same tetrahedrally coordinated units present in cubic crystalline c-SiC (zinc-blende structure), where a Si atom is surrounded by nearly four C atoms and vice versa, does exist in all the amorphous samples. This SRO, however, is detected only at a level of the first C and Si coordination shells in a-SiC and a-SiC:H. The structural disorder of the first Si and C coordination shells in all forms of amorphous SiC is somewhat greater than c-SiC, and it decreases appreciably as hydrogen is added. The a-SiC sample exhibits large Si and C coordination numbers, almost identical to c-SiC, a low atomic density, and virtually the same Si-C bond length as c-SiC. These results indicate that a relatively small concentration of large voids exist in a highly disordered a-SiC matrix. The a-SiC:H samples, on the other hand, exhibit a decrease in the C coordination number relative to a-SiC, which is independent of H concentration, low Si and C atomic densities, comparable to a-SiC, and virtually the same Si coordination number as a-SiC. These EXAFS-EXELFS results are consistent with a model where part of the H is substituting for Si in the local tetrahedra surrounding C atoms, while the rest is located inside internal voids in the a-SiC:H samples. The surface of the voids is composed of C atoms which have at least one bond to H, and of Si atoms. Finally, a straightforward computational procedure is applied to estimate the size of these voids. It is found that their average size \ensuremath{\ge}21 A\r{}, thus excluding the possibility that the voids might be point defects.

Microscopic diffusion model for calculating orthopositronium lifetimes in micellar solutions

A microscopic diffusion model was formulated to describe the orthopositronium (o-Ps) lifetime density function in micellar solutions with the following main assumptions: the o-Ps lifetime in the solvent is different from that in the micelles; o-Ps and micelles diffuse in the solvent; the low atomic density region of the micellar boundary serves as an effective o-Ps trap. A nonexponential o-Ps lifetime density function has been derived by approximately solving an o-Ps diffusion problem with randomly distributed spherical absorbers. The parameters of the function are the formation and annihilation probabilities of o-Ps in the solvent and in the micellar pseudo-phase, the diffusion coefficients of o-Ps and the micelles in the solvent, as well as the structural data of the micellar solution, viz. the surfactant concentration and the micellar aggregation number and radius. 44 references, 6 figures.

State of the art of coherent forward scattering spectroscopy

Magneto-optic rotation produced by the Faraday or the Voigt configuration has been employed to determine trace elements in an atomic vapour under the name of coherent forward scattering spectroscopy (CFS). Theoretically and experimentally, at low atomic density, the forward scattering radiation intensity is proportional to the square of the atomic density. Therefore, highly sensitive detection can be expected. However, from the opposite point of view, CFS signal drops very rapidly with the concentration near the detection limit, and CFS degrades sensitivity when it is utilized to detect atomic species at low concentration. The alleviation of this unattractive feature and that of small dynamic range were attempted by the experimental set-up of the Voigt configuration: a transverse magnetic field of 10 kOe around a graphite furnace with Rochon prisms as polarizer and analyzer. Then the analytical applications of CFS for the determination of trace elements in several metals following simple sample dissolving procedure were carried out. One of the advantage of the CFS: capability of simultaneous multielement analysis with simpler instrumentation could be ascertained using a xenon lamp as a radiation source.

Influence of Sputtering Ar Pressure on Characteristics of Tb-Co Amorphous Thain Films

Amorphous Tb-Co films were prepared by diode RF sputtering under various Ar gas pressures and without applying a bias voltage, which is known to induce a large perpendicular magnetic anisotropy. The coercive force, saturation magnetization, and perpendicular anisotropy energy were measured as functions of the Ar pressure, and it was found that at higher pressures a perpendicular magnetic anisotropy was induced. Measurements of the film density and oxygen-content indicated that a large perpendicular magnetic anisotropy is induced for high oxygen contents and low atomic packing densities.

Degenerate four-wave mixing in sodium vapor in the Rabi regime

Degenerate four-wave mixing in sodium vapor is investigated using a flashlamp-pumped dye laser synchronized on a cw dye laser. At low atomic density, the reflectivity versus the pump intensity and the line shapes are in good agreement with theoretical predictions in the case of both plane waves and Gaussian beams. At high atomic density, reflectivities much higher than unity have been observed. The influence of the pump imbalance has been studied as well as the deformation of the wave front of the conjugated beam introduced by the pump beams. In the high-atomic-density case, the theoretical problem is more complex, and our experimental results exhibit only qualitative agreement with theory.

Laser induced collisional energy transfer processes in a rubidium-potassium mixture

Two laser induced collisional energy transfer processes in alkali vapours are investigated. For process K(4 2S 1/2)+ Rb(5 2P 3/2)+ℏω T→K(6 2S 1/2)+Rb(5 2S 1/2), a population inversion between the final state K(6 2S 1/2) and lower states, leading to a superradiant emission has been observed. Comparison between experiment and theory shows a qualitative agreement in the limit of low atomic densities and laser power. However the transfer profile width is about two times larger than the calculated one for both processes.

Ion formation in sodium vapor containing Rydberg atoms

The formation of ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_{2}^{+}$ in sodium vapor (\ensuremath{\sim}${10}^{11}$-${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) containing laser-selected Rydberg atoms $(15l~nl~35)$ has been studied. At low atom densities photoionization, collisional ionization, and associative ionization dominate, but more-complicated mechanisms contribute to the ion yield at higher densities.

Rigid lattice proton NMR study of the constitutive water of titanium oxides (rutile, anatase, amorphous oxide)

The superficial constitutive water of high specific area titanium oxides in the rutile, anatase, and X-ray amorphous forms has been compared on the basis of results obtained by calculating the shape function of the “rigid lattice” 1H NMR spectra. After treatment at 22°C and 10 −4 Torr the amorphous oxide is less hydrated per unit area than the crystalline phases. The ratio of the number of water molecules to the number of OH groups is 1 for the amorphous oxide and 0.5 for rutile and anatase. The totality of the OH groups and the water molecules can occupy the amorphous oxide surface, whereas the OH groups are enough to cover that of rutile and anatase. These differences can be explained by assuming that the amorphous oxide surface is comparable to crystalline phase planes of low atomic density. When the treatment temperature at 10 −4 Torr is increased all the molecular water is eliminated shortly about 200°C and the elimination of OH groups slows down in all the compounds. The hydration of the anatase surface is more stable than that of the other phases.