Resonant coupling in the van der Waals interaction between an excited alkali atom and a dielectric surface: an experimental study via stepwise selective reflection spectroscopy

We present a detailed experimental study of the evaluation of the van der Waals (vW) atom-surface interaction for high-lying excited states of alkali-metal atoms (Cs and Rb), notably when they couple resonantly with a surface-polariton mode of the neighbouring dielectric surface. This report extends our initial observation [Phys. Rev. Lett. 83, 5467 (1999)] of a vW repulsion between Cs(6D_3/2) and a sapphire surface. The experiment is based upon FM selective reflection spectroscopy, on a transition reaching a high-lying state from a resonance level, that has been thermally pumped by an initial one-photon step. Along with a strong vW repulsion fitted with a blue lineshift, -160±25 kHz μm³ for Cs(6D_3/2) in front of a sapphire surface (with a perpendicular c-axis), we demonstrate a weaker vW repulsion (-32±5 kHz μm³) for Cs(6D_3/2) in front of a YAG surface, as due to a similar resonant coupling at 12 μm between a virtual atomic emission (6D_3/2-7P_1/2) and the surface polariton modes. A resonant behaviour of Rb(6D_5/2) in front of a sapphire surface exists also because of analogous decay channels in the 12 μm range. Finally, one demonstrates that fused silica, nonresonant for a virtual transition in the 12 μm range and hence weakly attracting for Cs(6D_3/2), exhibits a resonant behaviour for Cs(9S_1/2) as due to its surface polariton resonance in the 8-9 μm range. The limiting factors that affect both the accuracy of the theoretical prediction, and that of the fitting method applied to the experimental data, are discussed in the conclusion. Received 16 January 2003 / Received in final form 25 March 2003 Published online 5 May 2003     

Van der Waals interactions between atoms and dispersive surfaces at finite temperature

The long-range interactions between an atomic system in an arbitrary energy level and dispersive surfaces in thermal equilibrium at non-zero temperature are revisited within the framework of the quantum-mechanical linear response theory, using generalized susceptibilities for both atom and electromagnetic field. After defining the observables of interest, one presents a general analysis of the atomic level shift valid for any number and form of dielectric surfaces. It is shown that, at zero temperature, one recovers well-known results previously obtained in the linear response regime. The case of a plane dispersive surface is elaborated on in the non-retarded regime. Calculations are given in detail for a dielectric surface exhibiting a single polariton resonance. Theoretical predictions are presented within a physical viewpoint allowing one to discriminate between the various interaction processes: on one hand, the level shift induced by non-resonant quantum fluctuations, on the other hand, two potentially resonant atom-surface couplings. The first resonant process appears for excited-state atoms and originates in an atomic de-excitation channel resonantly coupled to the surface polariton mode. It exists also at zero temperature, and has been studied and observed previously. The second physical process, which exists at non-zero temperature only, corresponds to the reverse process in which a thermal quantum excitation of a surface polariton resonantly couples to an atomic absorption channel. This novel phenomenon is predicted as well for a ground state atom, and can turn the ordinary long-range van der Waals attraction of atoms into a surface repulsion at increasing temperatures. This opens the way to the control and engineering of the sign and amplitude of van der Waals forces via surface temperature adjustment.     

Shifts of one-electron ions lines from n = 0 interactions with electrons in hot and dense plasma

This paper is concerned with lineshifts of hydrogen-like ions due to electron collisions in dense and hot plasmas. These collisions are treated by including all effects due to monopole, dipole, and quadrupole interactions between radiator and electron perturbers. The latter follow exact hyperbolic trajectories with a possible penetrating part inside atomic orbits. A simple closed form for the line shift has been derived. Comparison between our semi-classical results and the quantum mechanical ones shows good agreement for a large range of high electron densities and temperatures. Received: 30 June 1997 / Revised: 28 April 1998 / Accepted: 29 April 1998     

Line-shape of dark line and maser emission profile in CPT

The paper is concerned with the line shapes of resonance phenomena observed in Coherent Population Trapping (CPT) applied to alkali atoms in a cell containing a buffer gas. Significant asymmetries and departures from a Lorentzian shape have been observed in connection with the measurement of dark lines and CPT maser emission profiles. Measurements are reported as a function of the power and frequency tuning of the laser used to create the CPT phenomenon. The paper reports on different experimental conditions and a comparison between theory and experiments is made for the cases of cesium and rubidium in a buffer gas. Received 3 March 2000 and Received in final form 10 April 2000     

Spontaneous emission of an excited two-level atom without both rotating-wave and Markovian approximation

The spontaneous emission of an excited atom is analyzed by quantum stochastic trajectory approach without both rotating-wave approximation and Markovian approximation. The atom finite size effect is also taken into account. We show by an example that the correction due to the counter-rotating wave term is rather small, even for the largest atomic number of real nuclei. Received 10 July 2002 / Received in final form 12 November 2002 Published online 4 February 2003     

Variation of Kβ/ Kα X-ray intensity ratio and lineshape with the effects of external magnetic field and chemical combination

In this work, the effects caused by different chemical combination and external magnetic field in several copper and zinc compounds (Cu, CuBr, Cu₂O, CuI, CuCl, Cu₂Te, Cu₅Si, CuSO₄, CuSeO₄.5H₂O, CuCl₂, Cu(NO₃)₂, CuS, CuSe, CuF₂, CuF2.3H₂O, CuBr₂, Cu(ClO₄)₂.6H₂O, Zn, ZnSO₄.5H2O, Zn(C₂H₃O₂)₂, ZnF₂, Zn(NO₃)₂.6H₂O, ZnO, ZnS, ZnSe, ZnTe and ZnF₂.4H₂O) were studied using a Si(Li) detector. The samples were excited by 22.69 keV X-rays from ¹⁰⁹Cd point radioactive source of strength 10 mCi in the external magnetic field of intensities 0.6 T and 1.2 T. The shift, asymmetry, FWHM and Kβ/Kα intensity ratio values were determined. For B = 0, the present experimental results were compared with the experimental and theoretical data in literature. The results have shown that the atomic parameters such as energy shifts, asymmetry indices, FWHM and Kβ/Kα intensity ratios can change when irradiation is conducted in a magnetic field.     

Advanced simulations of spectroscopic signatures of charge exchange in laser-produced plasmas

We present an advanced theory of charge-exchange-caused dips (also called X-dips) in spectral lines from laser-produced plasmas. We compare predictions of this advanced theory with our previously published experimental results where, in the process of a laser irradiation of targets made out of aluminum carbide, we observed two X-dips in the L _γ line of Al XIII perturbed by fully stripped carbon. We show that our advanced theory is in excellent agreement with our experimental results. From the practical point of view, our results open up a way to experimentally produce not-yet-available fundamental data on charge exchange between multicharged ions, virtually inaccessible by other experimental methods. From the theoretical viewpoint, the results are important because the X-dips are the only one signature of charge exchange in profiles of spectral lines emitted by plasmas and they are the only one quasimolecular phenomenon that could be observed at relatively “low” densities of laser-produced plasmas. Received 25 March 2002 Published online 19 July 2002     

Stark broadening of the hydrogen Paschen <Emphasis Type="Bold">γ</Emphasis> transition at electron densities of the order of cm

Stark broadening measurements and calculations of the Paschen γ spectral line of hydrogen ( λ = 1.094[: MU :] m) are reported. Investigations have been performed at plasma electron densities between 1.4×10 ¹⁵ cm ^-3 and 3.7×10 ¹⁵ cm ^-3 . As the light source a wall-stabilized arc operated in a helium-hydrogen gas mixture at atmospheric pressure has been applied. The radiation of the plasma emitted from nearly homogeneous plasma layers in end-on direction, was measured with the use of a grating spectrometer equipped with a charge coupled device (CCD) detector. The radiance calibration was carried out against light outputs originating from a tungsten strip radiation standard. The measured FWHM are compared with results of our calculations based on computer simulation techniques (full computer simulation method -- FCSM). Our broadening data are also compared with results of other theoretical approaches (MM-method, quasi-static approximation) and with experimental data obtained at electron densities about one order of magnitude larger than ours. Received 21 January 2003 Published online 24 April 2003 RID="a" ID="a"e-mail: wujec@uni.opole.pl     

On the Stark broadening of the He I 447.1 nm spectral line

Characteristics of the Stark broadened and overlapping 447.1 nm He I spectral line and its forbidden 447.0 nm components have been measured at electron densities between 4.4×10²² m^-3 and 8.2×10²² m^-3 and electron temperatures between 18 000 K and 33 000 K in plasmas created in five various discharge conditions using the low pressure pulsed arc as an optically thin plasma source operated in helium-nitrogen-oxygen gas mixture. Good agreement was found among our measured line characteristics and their existing calculated values, based on the quasistatic approximation. Possible influence of the singly ionized oxygen impurity atoms (O II) on the intensity values of the dip between allowed and forbidden components was found that can explain the disagreement among some existing experimental and calculated line characteristics data, at higher electron temperatures and densities. On the basis of the observed asymmetry of the 447.1 nm spectral line profile we have obtained the ion contribution parameter at 10²² m^-3 electron density and 8 000 K electron temperature. Received 20 February 2001 and Received in final form 25 April 2001     

The He I 706.52 nm line shape characteristics in the plasma diagnostics

On the basis of the precisely recorded 706.52 nm He I line shape we have obtained the basic plasma parameters i.e. electron temperature (T) and electron density (N) using our new line deconvolution procedure in the case of five various plasmas created in a linear, low-pressure, pulsed arc discharge. Plasma parameters have been also measured using independent experimental diagnostical techniques. Excellent agreement was found among the two sets of the obtained parameters. This enables our deconvolution procedure to recommendation for plasma diagnostical purposes, especially in astrophysics where direct measurements of the plasma parameters (T and N) are not possible. Besides, on the basis of the observed asymmetry of the Stark broadened line profile we have obtained its ion broadening parameter (A) caused by influence of the ion microfield to the line broadening mechanism and also the influence of the ion dynamic effect (D) to the line shape. Our A and D parameters represent the first data obtained experimentally using the line profile deconvolution procedure. We have found stronger influence of the ion contribution to the 706.52 nm He I line profile than the existing theoretical approximations estimate. This can be important for plasma modeling or for diagnostics. Received 30 October 2002 Published online 15 April 2003 RID="a" ID="a"e-mail: vladimir@ff.bg.ac.yu     

Asymmetry of Stark profiles

The theoretical basis is presented that allows to compute the Stark broadened line shapes of atomic ions up to the quadrupole terms in the interaction potential between the radiator and the plasma electric microfields and their gradients. The nature of the corrections due to the plasma polarization effects associated with the electron distribution around ion perturbers are carefully analyzed. The relevant universal plasma functions are evaluated in a cluster expansion or by Monte Carlo simulations, and the line shape is calculated with ion dynamic effects by the Model Microfield Method. The asymmetry of the Lyman line of hydrogenic ions is then studied. Received 21 January 2000 and Received in final form 27 April 2000     

Static van der Waals interactions in electrolytes

Using a field-theoretic formalism, we calculate the static contribution to the van der Waals interaction between two dielectric semi-infinite half-spaces in the presence of mobile salt ions. The ions can be located in the slab, in one, or in both half-spaces. We include an excess polarizability of the salt ions, i.e., each (spherical) ion has a dielectric constant which in general is different from the surrounding medium. This leads to a modification of the effective dielectric constant of the medium hosting the ions. This shift can be large for high salt concentrations and therefore influences the Hamaker constant decisively. Salt ions in the slab screen the static van der Waals interaction, as was shown by Davies and Ninham. The salt-modified van der Waals interaction also contains salt-confinement and salt-correlation effects. This is clearly demonstrated by the fact that the interaction is non-zero even in the case when the dielectric constant is homogeneous throughout the system, in which case salt correlations are solely responsible for the interaction. If the salt ions are in one or both of the two half-spaces (and no ions in the slab), the van der Waals interaction is not screened but the effective Hamaker constant approaches a universal value for large slab thickness which is different from the value in the absence of salt ions and which is independent of the salt concentration and of the effective electrolyte dielectric constant. If both half-spaces contain salt, the asymptotic value of the Hamaker constant for large separation between the half-spaces is the one obtained for the interaction between two metallic half-spaces through an arbitrary dielectric medium, which is given by A≃ - 1.20. As is explicitly demonstrated, ion enrichment or depletion at the interfaces due to image-charge effects is already included on the one-loop level and therefore does not lead to a change of the screened van der Waals interaction as given by Davies and Ninham. For positive and negative ions with different valencies or different excess polarizabilities, we obtain different adsorbed surface excesses of positive and negative ions, leading to a non-vanishing surface potential, which is computed explicitly. All of these results are obtained on the linear one-loop level. For a special case we extend the calculation of the dispersion interaction to the two-loop level. We find the corrections to the one-loop results to be quite large for high salt concentrations or multivalent ions. Received 17 February 2000     

Possibility of plasma density diagnostics using Langmuir-wave-caused dips observed in dense laser plasmas

A systematic study of the Langmuir-wave-caused dips (L dips) observed in profiles of the Al Ly γ line emitted from laser irradiated sandwich targets indicates that these fine spectral features can be used as a tool for density diagnostics in intermediately coupled plasmas. The spectroscopic data required for a reliable identification of L dips were collected by a vertical-geometry Johann spectrometer providing high spectral and spatial resolution. The electron densities deduced from the evolution of the L dips along the laser target axis compare well with those derived from hydrosimulations and from measurements of the line widths and shifts.     

Coupling and ionization effects on hydrogen spectral line shapes in dense plasmas

A study of hydrogen lines emitted in dense and low temperature plasmas is presented. Coupling and ionization effects in a transition from impact to quasi-static broadening for electrons are analyzed with the help of the Frequency Fluctuation Model (FFM). Electron broadening of Balmer series lines is studied for different densities and temperatures spanning a wide domain from impact to quasi-static limit. It is shown that electronic broadening makes a transition from impact to quasi-static limit depending on plasma conditions and principal quantum number. Even for the Balmer alpha line, at a density equals 10¹⁸ cm^-3 and a temperature equals 1 eV, this transition occurs both in the wings and the core of the line.     

A collision-induced satellite in the Lyman profile due to H-H collisions

We present a theoretical profile of the Lyman line of atomic hydrogen perturbed by collisions with neutral hydrogen atoms and protons. We use a general unified theory in which the electric dipole moment varies during a collision. A collision-induced satellite appears on Lyman , correlated to the asymptotically forbidden transition of H₂. As a consequence, the appearance of the line wing between Lyman and Lyman is shown to be sensitive to the relative abundance of hydrogen ions and neutral atoms, and thereby to provide a temperature diagnostic for stellar atmospheres and laboratory plasmas. Received 15 January 2000 and Received in final form 17 May 2000     

The shapes of Auger decay lines in photoelectron satellite spectra

The theory of the shapes of Auger decay lines of satellite two-hole-one particle states accompanying photoionization based on the Green's function method is developed. The lineshapes of Auger decay of satellite states [2 s 2 p ]( ^1,3 P )3 s ( ² P ), [2 s 2 p ]( ¹ P )4 s ( ² P ) and [3 s 3 p ]( ³ P )4 s ( ² P ) in valence p-photoelectron spectra of Ne and Ar atoms are calculated (hole states are indicated by square brackets throughout). It is shown that in some cases the Auger lineshapes reproduce the shape of the photoelectron satellite line, but in other cases Auger line may be narrower then the photoelectron line and may have opposite direction of asymmetry. The theoretical results are in agreement with experimental low-energy Auger spectra. Received: 25 May 1998 / Accepted: 2 October 1998     

Spectrum of helium at high pressures

The effect of high pressures on an atom is frequently simulated by enclosing the atom in an impenetrable spherical box. The spectrum of such a confined helium atom placed at the centre of a spherical box is investigated. A model potential is used to calculate the energies of twelve excited states and thereby the transition wavelengths for a range of values of the radius of the confining sphere. Applications of results are discussed. Received 29 August 2002 Published online 10 December 2002 RID="a" ID="a"e-mail: ypvsj@uottawa.ca     

Persistent spectral hole-burning in a -doped aluminosilicate glass from 8 to 295 K: Study of the burning and refilling kinetics, and of optical dephasing

High-temperature persistent spectral hole-burning (PSHB), up to room temperature, has been observed in a Eu ³⁺ -doped aluminosilicate glass using a high peak-power nanosecond dye laser. Spontaneous refilling as well as thermal cycling measurements show that at least two mechanisms, a fast and a slow one, are involved in our sample. We suggest that the fast or “easy” component may correspond to a non-photochemical local rearrangement of the host or to photoreduction of the Eu ³⁺ ions and that the second one leading to very stable photoproducts may correspond to transfer of an electron over a sizable distance through a several-step process. The mechanisms we suggest agree with light-induced hole refilling measurements. Line broadening mechanisms are discussed and the temperature-dependent part of the homogeneous width and of the spectral shift is interpreted in terms of a two-phonon (Raman) process involving pseudo-local phonons. Received 28 July 1999     

Impact broadening and shift rates for the 6p2 3PJ′ → 7s 3P J o transitions of lead induced by collisions with argon and helium

The collisional broadening and shift rate coefficients of the 283.39 nm (6p ^{2 3}P₀ → 7s ³P₁ ^o ), 364.06 nm (6p ^{2 3}P₁ → 7s ³P₁ ^o ), 368.45 nm (6p ^{2 3}P₁ → 7s ³P₀ ^o ) and 405.90 nm (6p ^{2 3}P₂ → 7s ³P₁ ^o ) Pb lines by He and Ar have been measured by fitting the experimental absorption line shapes to theoretical Voigt profiles. The absorption measurements were performed in a resistively heated, Pb loaded oven with an integrated dc noble gas discharge to produce also Pb atoms in the 6p ^{2 3}P¹ and 6p ^{2 3}P² metastable states. The diffusion of the metastable atoms out of the discharge zone into the neutral noble-gas atmosphere enabled the line-shape and shift measurement of the lines involving the metastable states without the influence of the discharge plasma.     

Quantum phase expectation values of a mesoscopic Josephson junction from quantum current measurements

A simple way to acquire information on the mean values of the phase operators sinϕ and cosϕ of an ultrasmall Josephson junction prepared in an arbitrary pure or not state is reported. Our proposal exploits the recently predicted occurrence of current spikes in the I-V characteristic of a mesojunction irradiated by a quantum single-mode low-intensity coherent electromagnetic field. A necessary condition for the validity of our treatment is presented and discussed. Received 11 April 2001