Interpretation of measured spectral line shapes using a square-well potential: The absorption lines of cesium broadened by rare gases

A simple square-well potential is used as an aid in understanding the complicated evolution of the absorption lines and their associated satellites for cesium perturbed by the rare gases. New experimental halfwidth data for cesium perturved by Xe, Ar, and He are presented. In the case of Xe, data for the second, third and fourth members are presented to 30, 7, 1 rd (relative density), respectively. For Ar, data are presented for the third through the ninth member. The pressure range for the third and fourth members is from 0 to 20 rd. For the other members, it ranges to 8 rd for the fifth and to 2 rd for the ninth. Third and fourth member helium data are presented to about 20 and 8 rd, respectively. Several theoretical computer profiles are used to illustrate the evolution of the line and satellites with foreign gas pressure and these are compared with the experimental data.     

Equivalent step-index profile for graded W-fibres: application to TE01 mode cut-off

Snyder's equivalent step-index method is extended to the case of graded profiles with a depressed cladding. This method is then used to determine the limit of monomode operation. Comparison with the results obtained by an exact numerical technique is made for a large variety of simulated profiles, including those with ripples in the cladding. Conclusions are drawn concerning the sensitivity of various profile-shapes to such ripple structure. The method is applied also to measured profiles, reconstructed by a well-known tomographic procedure. In this latter case a comparison is made with experimentally determined values.On leave from CNET, Lannion, France.     

Laboratory studies of spectral features of stimulated electromagnetic emission during the ionospheric heating experiments

We present the results of laboratory studies of the formation of a number of spectral components of stimulated electromagnetic emission, which are related to the excitation of small-scale irregularities in the heated ionosphere. In the laboratory experiment, the small-scale irregularity was formed as a result of thermal self-channeling of short-wavelength quasielectrostatic oscillations in a magnetoplasma. Using the method of probing waves, it is experimentally shown that the trapping and waveguide propagation in a small-scale plasma irregularity are exclusively due to Langmuir waves, whereas the upper-hybrid waves with anomalous dispersion are not trapped into the irregularity. It is found that satellites shifted by about 1–2 MHz from the carrier frequency (700 MHz under the experimental conditions) are formed in the Langmuir wave spectrum during the thermal self-channeling. Two mechanisms of generation of spectral satellites have been detected. The first (dynamic) mechanism is observed during the formation of a small-scale irregularity with rapidly increasing longitudinal size. In this case, one low-frequency satellite is excited in the trapped-wave spectrum. The mechanism of the formation of this satellite is apparently related to the Doppler shift of the frequency of the Langmuir waves trapped inside the irregularity. The second (stationary) mechanism is observed in the case of a developed irregularity where its shape is close to cylindrical. In this regime, the trapped-wave spectrum has two symmetric spectral satellites, namely, high- and low-frequency ones. It may be hypothesized that the generation of these satellites is due to scattering of trapped Langmuir waves from drift oscillations of the irregularity.     

Measuring the one-particle excitations of ultracold fermionic atoms by stimulated Raman spectroscopy

We propose a Raman spectroscopy technique which is able to probe the one-particle Green function, the Fermi surface, and the quasiparticles of a gas of strongly interacting ultracold atoms. We give quantitative examples of experimentally accessible spectra. The efficiency of the method is validated by means of simulated images for the case of a usual Fermi liquid as well as for more exotic states: specific signatures of, e.g., a d-wave pseudogap are clearly visible.     

Experimental investigation of light storage of diffraction-free and quasi-diffraction-free beams in hot atomic gas cell

In this article we report on the experimental investigation of light storage for several types of diffractionfree beams (Bessel and Airy beams) and quasi-diffraction-free beams by utilizing electromagnetically induced transparency (EIT) technique in a hot atomic gas cell. The experimental results show that the diffraction-free and quasi-diffraction-free beams have better storage performances when compared with ordinary images possessing similar spatial profiles. Meanwhile, the Bessel beams and the quasidiffraction-free images are able to maintain their spatial profiles with a long storage time while the sidelobes of the Airy beam are gradually depleted with the increment of the storage time. We quantitatively analyze the storage results and give physical explanations behind these phenomena. Furthermore, the self-healing of the retrieved diffraction-free beams is verified, signifying that their characteristics preserve well after storage.     

Electron momentum spectroscopy of valence satellites of neon

Electron momentum spectroscopy (EMS) study of the neon valence satellites is reported. The experiments were performed at impact energies of 1250, 1450 and 1670 eV using a multichannel spectrometer that features high sensitivity. Binding energy spectra up to 100 eV and momentum profiles for the 2p⁻¹ and 2s⁻¹ primary transitions as well as the satellites are presented. The results are used to examine impact energy dependence of the relative intensities and shapes of the satellite momentum profiles. The results are also used to determine symmetries and spectroscopic factors of the satellites, and are compared with the previous experiments by EMS and photoelectron spectroscopy and sophisticated theoretical calculations. The present study has largely resolved controversies in the previous studies.     

A model of the kinetics and equilibria of surface segregation in the monolayer regime

A kinetic model of surface segregation based on foreign atom movement in an appropriate potential is developed. It is used to calculate the time and temperature dependence of surface concentration and the concentration distribution perpendicular to the surface for the case of structure limited surface enrichment. The results give an explanation for Auger electron spectroscopy obervations of the segregation of tin at the surface of copper single crystals. In-depth profiles of composition obtained by sputtering are in agreement with predictions from the model. Deviations from other models of segregation kinetics are briefly discussed.     

高n双电子复合伴线对类氦镁离子Ka共振线贡献的理论计算

利用多组态Dixac-Foek方法计算了类氯镁离子的双电子复合截面．分析了类氨镁离子的高n双电子伴线结构及其对类氯镁离子Ka共振线的影响．结果表明类氨镁离子的高n双电子伴线几乎都分布于Ka共振线的长波一侧．随着n的增加，DS强度呈现明显减小的趋势．虽然来自KLM（n=3）的发射线最强．但n〉3的部分依然给出了非常重要的贡献．即便是来自n〉10的共振的贡献也是不可忽略的．并且高n双电子伴线随着n的增加不断接近Ka共振线，导致和Ka共振线的强烈混合．在实验测量中会使Ka共振线峰位的测量值向长波方向移动，使谱形加宽，并且增强了Ka共振线测量值强度．     

Appearance of resonant Γ-X mixing in gradient GaAs/AlAs short-period superlattices

We have experimentally investigated the low-temperature (10 K) luminescence and reflection spectra of a gradient GaAs/AlAs superlattice. We have examined the behavior of phonon satellites in the vicinity of the X-Γ resonance. Smooth passage through the resonance was achieved by scanning an exciting light beam along the surface of a gradient sample. Based on our experimental results, we have determined the functional dependence of the Γ-X mixing potential on the resonance detuning. Fiz. Tverd. Tela (St. Petersburg) 40, 822–823 (May 1998)     

Off-resonance effects of the radiofrequency pulses used in spectral editing with double-quantum coherence transfer

Spectral editing using gradient selected double-quantum (DQ) coherence transfer is often used for the selective observation of metabolites in vivo. In attempting to optimize the detection sensitivity of a conventional DQ spectral editing sequence, the effects of using radiofrequency (RF) pulses that are not at the resonance frequency of the observed peaks were investigated both theoretically and experimentally. The results show that spectral editing using pulses at the frequency of the observed resonance does not necessarily give the optimal detection sensitivity. At 7 T, the detection sensitivity of lactate observed using a DQ editing method can be increased by up to 30% by setting the RF pulses off resonance at the proper frequency. The results also suggest that slice selective RF pulses used in DQ spectral editing combined with PRESS localization may have slice profiles different from those when the same pulses are used for standard PRESS spatial localization.     

The active minimization of harmonic enclosed sound fields,part III: Experimental verification

The principle objective of this paper is to compare the measured results of active minimization experiments in an enclosed sound field with those predicted from theory. The enclosure used was essentially two dimensional over the frequency range of interest and was only lightly damped. A practical control system was built which minimized the sum of the squares of a number of microphone outputs by adjusting the outputs of a number of secondary loudspeakers at a single frequency. Various approaches to designing the algorithm which controls such a system are discussed, including matrix inversion, gradient descent methods, and pattern search methods. Although some problems with coupling between the acoustic and structural modes were initially encountered, the response of the experimental enclosure was very close to that predicted by the computer model when these problems were overcome. The pressure field inside the enclosure was measured at 200 points when excited both on resonance and off resonance, and the form of the pressure field was also found to be very similar to that predicted by the computer model. The conditions under which significant reductions in the total acoustic potential energy in the enclosure could be achieved by the action of a number of secondary sources were experimentally investigated. It was found that, in general, large reductions can be achieved only when the enclosure is excited on resonance. The secondary source does not have to be within half a wavelength of the primary to give good reductions, provided it is able to couple in to the most strongly excited modes.     

Laser heating of optically thin samples

Summary A mathematical model able to describe the temperature profiles generated in a thin film by the steady-state illumination by a Gaussian laser beam is presented. The film is supposed to be made by a weakly absorbing liquid sample bounded by two parallel transparent plates, the cell walls, whose thermal exchange to the surrounding ambient may be assumed to be linear with the temperature difference. An analytical solution of the problem is presented in form of Hankel transform and a matrix numerical approach to the computation of the temperature profiles is reported. It is particularly well suited to a computer implementation and allows one to get very accurate results in very short computing times. The influence of the heat exchange coefficient to the ambient is shown in an example. A check of the method accuracy is performed by comparison with literature results in the particular case of infinite heat exchange coefficient. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Multiple Andreev-reflection in superconducting weak-links in the interaction with external microwave-fields

We present calculated current-voltage characteristics (CVCs) of NS- and SNS-contacts, where the carrier transport is characterised by single and multiple Andreev-reflection (AR) in the presence of an external microwave field. Based on the Tien-Gordon model of multiphoton- assisted electron tunneling [1] we describe the microwave field by an ac-voltage potential. This additional potential is added in the time-dependent Bogoliubov-de Gennes equations (BdGE) which describe inhomogeneous superconductors and in this way NS-phase boundaries. These modified BdGE are solved by single particle wave-functions, which contain the multiphoton emission and absorption processes of the quasi-particles (QPs). With this ansatz we obtain an extension of the BTK-theory for NS-contacts [2] and the OTBK-theory for SNS-contacts [3,4]. We show, that the calculated CVCs for NS-contacts are in the case of a vanishing probability for AR identical with the characteristics obtained with the Tien-Gordon model. For SNS-contacts we show theoretically that features in the CVCs appear at voltages $V = \frac{{2\Delta }} {{ne}} + \frac{m} {n}\frac{{\hbar \omega }} {e}$ (n,m integers). This equation was initially found experimentally by Hoffmann-Soerensen et al. [5]. We are now able to explain their results and show that investigations of the interaction of multiple AR with an external microwave field give clear evidence for the occurrence of multiple AR in the junction.     

On an equivalent Hermitian Hamiltonian for the wrong-sign quartic potential

In this paper I present the results of a calculation with J. Mateo, in which, by a judicious choice of the contour on which the Schrödinger equation for the potential ?gz ⁴ is posed, we were able to give an explicit construction of an equivalent Hermitian Hamiltonian with the same spectrum. I also discuss the functional-integral approach to constructing equivalent Hamiltonians. In many cases this gives the simplest derivation. However, in this particular case it only gives the classical Hamiltonian, without a linear term, which is in fact an anomaly that can only be obtained by a careful discretization of the functional integral.     

Wall-enhanced convection in vibrofluidized granular systems

An event-driven molecular dynamics simulation of inelastic hard spheres contained in a cylinder and subject to strong vibration reproduces accurately experimental results [R. D. Wildman et al., Phys. Rev. Lett. 86, 3304 (2001)] for a system of vibrofluidized glass beads. In particular, we are able to obtain the velocity field and the density and temperature profiles observed experimentally. In addition, we show that the appearance of convection rolls is strongly influenced by the value of the sidewall-particle restitution coefficient. Suggestions for observing more complex convection patterns are proposed.     

Application of the electrostatic mean value theorem to electrostatic sensor electrodes

We present a method for approximating the potential of conducting objects due to a known electrostatic source. The method involves averaging the incident potential over the conductor surface or volume region, which is known to give the exact value for a perfectly conducting sphere. The method is extended to spheroidal geometries, both prolate and oblate, to study the error incurred for deviations from the spherical case. Exact values for the spheroid potentials are derived and compared with those obtained by the mean value approximations. The result for the oblate spheroid is extended to the case of a two-dimensional electrostatic disk. The approximations are proposed as a method for predicting the potential of conducting electrodes used with electrostatic sensors for the measurement of electrostatic field disturbances. In this regard, the mean value approximation is applied to determine the source to electrode mutual capacitance, which is implemented in the model for the sensor system. Electrostatic disk electrodes are used with an electrostatic disturbance sensor to experimentally validate the application of the mean value approximation.     

Higgs interchange and bound states of superheavy fermions

Hypothetical superheavy fourth-generation fermions with a very small coupling with the rest of the Standard Model can give rise to long enough lived bound states. The production and the detection of these bound states would be experimentally feasible at the LHC. Extending, in the present study, the analysis of other authors, a semirelativistic wave equation is solved using an accurate numerical method to determine the binding energies of these possible superheavy fermion-bound states. The interaction given by the Yukawa potential of the Higgs boson exchange is considered; the corresponding relativistic corrections are calculated by means of a model based on the covariance properties of the Hamiltonian. We study the effects given by the Coulomb force. Moreover, we calculate the contributions given by the Coulombic and confining terms of the strong interaction in the case of superheavy quark bound states. The results of the model are critically analysed.     

Ground state properties of a spinless Falicov-Kimball model; additional features

We have studied some energetic and structural ground state properties of a spinless Falicov-Kimball model. Using a method based on “Tchebycheff-Markoff inequalities”, we have calculated sharp upper and lower bounds for the ground state energy. These calculations lead to rigorous results for a special range of parameters, where we are able to give the exactf-level occupation pattern for a square lattice. The results for the “symmetric” case, where a superstructure occurs leading to a metal-insulator transition, have already been published. In this paper we present some additional results also for the “unsymmetric case” and study valence-transitions for fixed particle-number.     

Zur formelmäßigen Darstellung der Ionisierungsquerschnitte gegenüber Elektronenstoß

An empirical analytical expression for the ionisation cross sections is given which is in very good agreement with experimentally measured cross sections, and which is able to give a large part of the ionisation curves as a function of the energy of the impacting electrons. For high electron energies the formula is in agreement with the quantum mechanical expression for the ionisation cross section (Born-Bethe-formula).     

Trapped and free electrons in the near-anode region of a striated discharge

Spatial potential profiles and electron energy distribution functions are measured in the near-anode region of a striated neon glow discharge. It is discovered that potential wells of small depth adjacent to the anode appear on the spatial potential profiles at certain moments in time. The distribution functions measured in the potential wells have a pronounced maximum of slow electrons, which sharply distinguishes them from the distribution functions in the striation phases where there are no wells. The mechanism which shapes the electron distribution function for electrons trapped in a potential well is analyzed. A perturbing effect of the anode on the electron distribution function as the anode is approached is discovered experimentally, and an interpretation of this effect is given. Zh. Tekh. Fiz. 68, 25–32 (March 1998)