Heating of Low-Density Matter by Soft X-Ray Radiation and Radiation Waves

We investigate the nonlinear radiative thermal conductivity model, which is theoretically well substantiated and based on a small number of assumptions. We consider the spatial and temporal evolutions for a low-density polymer foam heated by radiation waves taking into account the dependence of the absorption coefficients on the quantum energy. The form of the radiation wavefront (its slope and speed of propagation) differs from the classical form by a low-temperature “tongue” penetrating deep into the plasma. The plasma in this segment of the wavefront is heated up to a temperature of 1–2 eV by photons in the hard part of the spectrum, with energies for which hν/kT?>?4. We simulate numerically the experiments at the PHELIX facility to heat a low-density cellulose triacetate (TAC, C₁₂H₁₆O₈) taking into account the radiation transfer. The energy and the spectral flux of radiation that passed through the TAC layer are satisfactorily consistent with the experimental results.     

Radiative opacity of plasmas studied by detailed term (level) accounting approaches

Detailed term and level accounting (DTA and DLA) schemes have been developed to calculate the spectrally resolved and Rosseland and Planck mean opacities of plasmas in local thermodynamic equilibrium. Various physical effects, such as configuration interaction effect (including core-valence electron correlations effect and relativistic effect), detailed line width effect (including the line saturation effect), etc., on the opacity of plasmas have been investigated in detail. Some of these physical effects are less capable or even impossible to be taken into account by statistical models such as unresolved transition arrays, super-transition-array or average atom models. Our detailed model can obtain accurate opacity of plasmas. Using this model, we have systematically investigated the radiative opacities of low, medium and high-Z plasmas under different conditions of temperature and density. For example, for aluminum plasma, in the X-ray region, we demonstrated the effects of autoionization resonance broadening on the opacity for the first time. Furthermore, the relativistic effects play an important role on the opacity as well. Our results are in good agreement with other theoretical ones although better agreement can be obtained after the effects of autoionization resonance broadening and relativity have been considered. Our results also show that the modelling of the opacity is very complicated, since too many physical effects influence the accuracy of opacity. For medium and high-Z plasmas, however, there are systematic discrepancies unexplained so far between the theoretical and experimental opacities. Here, the theoretical opacities are mainly obtained by statistical models. To clarify the discrepancies, efforts from both sides are needed. From the view-point of theory, however, a DLA method, in which various physical effects can be taken into account, should be useful in resolving the difference. Taking gold plasma as an example, we studied in detail the effects of core-valence electron correlation and line width on the opacity. Our DLA results correctly explained, for the first time, the relative intensity of the two strong absorption peaks located near the photon energy of 70 and 80 eV, which was experimentally observed by Eidmann et al. [Europhys. Lett., 1998, 44: 459].     

Opacity calculations for high-Z plasma in non-local thermodynamic equilibrium

Opacity of high-Z element plasma in non-local thermodynamic equilibrium (NLTE) is calculated. In the calculation, a collisional radiative model in detailed-configuration-accounting (DCA) is applied to population calculations for NLTE plasmas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based on the first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate the electron wave functions. The spin-orbit splitting is included in the spectrum calculations. The unresolved transition array method is used in the opacity calculations. The calculated frequency-dependent opacity of high-Z plasma Lu is presented. The comparison shows that the present spectrum agrees with another theoretical result well except that the present one shifts a little to the lower energy due to its lower mean ionization stage. The effects of highly doubly excited states on ionization balance and on the opacity are investigated. The mean ionization stage increases more than three stages when doubly excited states 5l6l′ and 5l5l′ are not included in the population calculations and the absorption spectrum also shifts to the higher energy.     

A numerical model for multigroup radiation hydrodynamics

We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model (M₁) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well-established kinetic model through Marshak wave tests with frequency-dependent opacities. Very good agreement between the multigroup M₁ and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.     

Decays of the mesons and the singlet photon

In an attempt to explain the recent measurements on the radiative decays of the vector-mesons (V→Pγ), we study the consequences of introducing a small admixture of SU (3) singlet piece in the electromagnetic current. We find that this leads to an excellent fit of the theory with the new measurements on theV→Pγ decays. However, this addition adversely affects the fit of the leptonic decays of the vector mesons (V→e ⁺ e ⁻) and of the radiative decay of the pion (π→2γ). We conclude that the overall fit to the available data does not favour a large (>10%) admixture of the SU(3) singlet. The decay rates have been calculated in the vectormeson dominance model. At the hadronic vertex (VVP), we assume asymptotic nonet symmetry. The electromagnetic couplings (V−γ) are the ones appropriate to vector-mixing.     

Circular polarization of gamma quanta in the radiative capture of polarized thermal neutrons by protons

An experiment to measure the circular polarization of γ-quanta, P_γ, in the reaction of radiative capture of polarized thermal neutrons by protons is described. Solid parahydrogen was employed as a proton target.The circular polarization was found to be P_γ = ?(2.90 ± 0.87) × 10^?3 which is in agreement with the theoretical estimates of P_γ arising from the admixture of the triplet capturing state to the ¹S₀ principal capturing state.     

A possible interpretation of enhanced raman scattering in the vicinity of a nanotip

Theoretical and experimental studies on Raman scattering in a space with small bodies are briefly reviewed. Probabilities of radiative transitions for atoms (molecules) in the vicinity of bodies with dimensions much smaller than a wavelength are calculated. It is shown that in the vicinity of a nanosphere with |ε|?1, the probability of a single-photon electric dipole transition increases by a factor of 9, and the probability of a two-photon transition—by a factor of 81. In the vicinity of a conical needle resting on a plane (the needle and plane have |ε|?1), the radiative transition probability increases by a factor (λ/R _in)² and (λ/R _in)⁴ for single-and two-photon transitions, respectively (R _in is the radius of the needle tip curvature). This theoretical result is offered to interpret the enhancement of radiative processes experimentally observed in the referenced studies.     

Influence of colloidal-gold films on the luminescence of Eu(TTFA)3 in PMMA

The effects of colloidal-gold layers on the luminescent properties of thin films of Eu(TTFA)₃(TTFA=thenoyltrifluoroacetonate) in PMMA (PMMA=poly(methyl methacrylate)) were investigated. Layers of spherical gold nanoparticles (12 nm) were formed by self-assembly on the surface of amino-derivatized glass slides. Eu(TTFA)₃-PMMA films were then spin-coated either directly onto the Au metal surfaces or onto spacer layers covering the gold. The luminescence properties were characterized both as a function of the density of Au particles in the colloidal layer, and as a function of the distance between the Au layer and the luminescent film. The distance between the metal and luminescent layers was controlled using polyelectrolyte spacer layers deposited on the colloidal-gold films by a spin-assisted, layer-by-layer (SA-LBL) method. It was found that the colloidal gold layer has a net quenching effect on Eu(TTFA)₃ luminescence under all conditions considered in this study. The luminescence intensities and lifetimes decrease with increasing density of Au nanoparticles and with decreasing separation (d) between the luminescent film and the gold layer. The measured luminescence intensity drops more quickly with decreasing distance than one would predict based solely on lifetime data, if one assumes a constant radiative relaxation rate. Fits of the luminescence decay kinetics to a model for non-radiative energy-transfer from Eu(TTFA)₃ to the gold layer yields a 1/d² dependence, where d is the distance from the gold layer to the nearest face of the luminescent film. It is suggested that there is no reasonable physical interpretation of this result within the constraints of the model and, therefore, the interaction between the luminescent and gold layer cannot be explained solely in terms of non-radiative energy transfer.     

Multipole effects to the opacity of hot dense gold plasma

The contributions of the multipole transitions to the opacity of hot dense gold plasma are taken into account by using an average-atom model. The influences of the E2, E3 and E4 transitions on the Rosseland opacity are studied, respectively. Comparisons with Miao’s calculation have been made. It shows that using the Taylor series to account for the multipole transitions is no longer valid since ik · r is not much smaller than the unit when the photon energy goes very high.     

平均原子模型中谱线位置偏移对辐射不透明度的影响

使用平均原子(average atom,AA)模型计算了局域热动平衡近似下的Fe,Br等离子体的辐射不透明度.对AA模型中跃迁系的跃迁能量引入了统计修正，修正后的谱线位置与细致谱项模型的结果更接近，但仍然存在差距，进一步减小这种差距需要考虑等离子体的价态分布.研究了AA模型中这种谱线位置移动对平均不透明度的影响，计算了Fe等离子体等温度系列的平均不透明度，发现这种修正对于平均不透明度的影响是比较明显的.     

Effect of focusing geometry on the continuous optical discharge properties

We studied the effect of the laser beam focusing geometry on the continuous optical discharge (COD) properties. We used a full two-dimensional radiative gas-dynamic model for the COD, maintained by a vertical CO₂ laser beam in free air atmosphere, in the Earth's gravitational field. The model takes into account all of the factors that are of importance in laser-sustained plasma processes, and uses realistic quasi optics to describe the laser radiation propagation. Results are presented for the optical discharge parameters as functions of applied laser power and degree (f-number) to which the laser beam is focused.     

Radiative Quark <Emphasis Type="Italic">p</Emphasis><Subscript>⊥</Subscript>-Broadening in a Quark–Gluon Plasma beyond the Soft Gluon Approximation

We study the radiative correction to p_⊥-broadening of a fast quark in a quark–gluon plasma beyond the soft gluon approximation. We find that the radiative processes can suppress considerably p_⊥-broadening. This differs dramatically from previous calculations to logarithmic accuracy in the soft gluon approximation, predicting a considerable enhancement of p_⊥-broadening.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Quantum chemical modelling of ‘green’ luminescence in self activated perovskite-type oxides

We discuss the origin of the intrinsic visible band emission of ABO₃ perovskite oxides (so-called ‘green luminescence’) which remains a topic of high interest during the last quarter of the century. We present a theoretical calculation modelling of this emission in the framework of a concept of charge transfer vibronic excitons [Phys. Solid State, 40 (1998) 834], i.e. as a result of radiative recombination of correlated (bound) self-trapped electron and hole polarons in the highly polarizable ABO₃-type matrix. The Intermediate Neglect of Differential Overlap method combined with the Large Unit Cell periodic defect model was used for quantum chemical calculations and theoretical simulation of the green emission for a series of model ABO₃ perovskites. The ‘green’ luminescence energies for PbTiO₃, SrTiO₃, BaTiO₃, KNbO₃ and KTaO₃ perovskite-type crystals agree well with those experimentally observed earlier.     

Electron tunneling in the presence of adsorbed molecules

We perform ab initio density functional theory calculations of the tunneling current through an electrode-molecule-electrode system with four different small organic molecules, benzenedithiol (BDT), benzenedimethanethiol (XYL), diethynylbenzene (DEB) and dodecanethiol (C12), sandwiched between two gold (1 1 1) electrodes. For the XYL molecule, we test the effect of alternate bonding types and sites. Although this reduces the current considerably, it does not account for the orders of magnitude differences between experimental and theoretical results in the literature. We also model a typical STM experimental setup with a gold nanoparticle absorbed on a self-assembled monolayer (SAM) of the molecule with a gap between the nanoparticle and probing tip and show that such a gap could account for these differences. Finally, we describe the effect that the gap has on the ability of STS measurements to distinguish between the i(V) characteristics and thicknesses of self-assembled monolayers of different molecules.     

Study of the recombination continuum in an SF6 arc discharge

An SF₆ plasma has been studied at atmospheric pressure. The radiation emitted in the range 400 < λ, nm < 600 is due to radiative recombination. The values obtained for the Biberman ξ-factor are in good agreement with theoretical values.     

Diagnosis of a low pressure capacitively coupled argon plasma by using a simple collisional-radiative model

This paper proposes a simple collisional-radiative model to characterise capacitively coupled argon plasmas driven by conventional radio frequency in combination with optical emission spectroscopy and Langmuir probe measurements. Two major processes are considered in this model, electron-impact excitation and the spontaneous radiative decay. The diffusion loss term, which is found to be important for the two metastable states (4s[3/2]₂, 4s'[1/2]₀), is also taken into account. Behaviours of representative metastable and radiative states are discussed. Two emission lines (located at 696.5 nm and 750.4 nm) are selected and intensities are measured to obtain populated densities of the corresponding radiative states in the argon plasma. The calculated results agree well with that measured by Langmuir probe, indicating that the current model combined with optical emission spectroscopy is a candidate tool for electron density and temperature measurement in radio frequency capacitively coupled discharges.     

Features of the microstructure of gold nanoparticles inside cavities of cucurbit[7]uril according to XAFS spectra

The Au L ₃ X-ray absorption fine structure (XAFS) spectra have been measured for gold nanoparticles with the calibrated size d _Au ～ 1 nm in the cavities of cucurbit[7]uril molecules. The features of their electronic structure and microstructure have been characterized. It has been found that gold clusters in cavities of cucurbit[7]uril are characterized by smaller (by 0.01–0.02 Å) interatomic distances and a noticeably larger (by a factor of 3 at 12 K) Debye-Waller factor as compared to bulk gold. Analysis of the experimental results and their comparison with the model calculations show that the special properties of atoms on the surface of small metal (gold) particles are not determined by their position at the vertices and edges of small clusters; they are likely attributed to the structural disorder, deformations, and stresses, which increase with a decrease in the size of the particles.     

Time resolved luminescence of CdS at high excitation

We report a study at low temperature of the time resolved luminescence of CdS, excited by two photon absorption. Concerning the so called EHP-LO and P bands, we confirm our results previously obtained on CdSe[1, 2]. (a) At high excitation a broad band (peak position at λ > 495 nm) occurs due to radiative recombination in an electron hole plasma, assisted by the emission of one LO phonon. (b) The simultaneous kinetics of the P line (λ ? 490.5 nm) and A-LO line (λ ? 492.5 nm) are conflicting with the interpretation of the P line as resulting of radiative exciton-exciton collisions. We interpret the P line as due to biexciton recombination.We have studied the luminescence in the (I₂-I₁) region (486nm < λ < 490nm) at low excitation. We observe clearly the following, (a) A broad band (488 nm < λ < 490.5 nm) which corresponds to the gain observed in previous experiments of pulse and probe spectroscopy and interpreted as direct recombination in an electron hole liquid (EHL). (b) After the disappearance of the EHL band, one single line (M_D), which shifts continuously towards the I₂ position during the time resolved kinetics. We suggest it to be connected with the high excitation effect on donor impurities (bound polyexcitons).     

Leed and Aes study of the Au/AuPb2 interface

The adsorption of lead on gold at room temperature in UHV conditions has been studied by LEED and AES. We review some of the data obtained on the Au(100), (111), and (110) faces, published elsewhere, and we give some new experimental results on the stepped Au(S) [n(100) × (111)] (with n = 3, 4, 5, and 6) faces. On all these faces, as lead is deposited on the gold substrate it first forms a monolayer of lead, then a compound AuPb₂. Using the LEED and Auger data we give a model of the epitaxy with a layer-by-layer growth mechanism. We propose a model which involves a transition alloy wich forms at the interface Au/AuPb₂. This model is in agreement with the LEED diagrams observed before the one corresponding to bulk AuPb₂. In the case of the epitaxy of lead on gold (100), we calculate the Auger peak-to-peak ] heights of the gold (72 eV) and lead (93 eV) transitions versus coverage. We obtain good agreement with the experimental data, assuming that the first and last layers of the alloy are lead monolayers and diffusion of lead in gold as well as gold in lead.