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A concise summary of out-of-equilibrium dark matter production during post-inflationary reheating is presented. We show that the dark matter relic abundance is in general sensitive to the thermalization rate of the inflaton decay products, and the evolution of the temperature of the subsequently thermalized radiation. We discuss how smoking-gun signals, such as monochromatic neutrinos or gamma-ray lines, or Lyman-$\alpha$ data, can help constrain out-of-equilibrium DM models., Comment: 6 pages, 4 figures. Contribution to the IWARA 2020 conference proceedings

Liquid water being the major constituent of the human body, is of fundamental importance in radiobiological research. Hence, the knowledge of electron-water interaction physics and particularly the secondary electron yield is essential. However, to date, only very little is known experimentally on the low energy electron interaction with liquid water because of certain practical limitations. The purpose of this study was to gain some useful information about electron emission from water using a Monte Carlo (MC) simulation technique that can numerically model electron transport trajectories in water.In this study, we have performed MC simulations of electron emission from liquid water in the primary energy range of 50 eV-30 keV by using two different codes, i.e., a classical trajectory MC (CMC) code developed in our laboratory and the Geant4-DNA (G4DNA) code. The calculated secondary electron yield and electron backscattering coefficient are compared with experimental results wherever applicable to verify the validity of physical models for the electron-water interaction.The secondary electron yield vs. primary energy curves calculated using the two codes present the same generic curve shape as that of metals but in rather different absolute values. G4DNA underestimates the secondary electron yield due to the application of one step thermalization model by setting a cutoff energy at 10 eV so that the low energy losses due to phonon excitations are omitted. Our CMC code, using a full energy loss spectrum to model electron inelastic scattering, allows the simulation of individual phonon scattering events for very low energy losses down to 10 meV, which then enables the calculated secondary electron yields much closer to the experimental data and also gives quite reasonable energy distribution curve of secondary electrons.It is concluded that full dielectric function data at low energy loss values below 10 eV are recommended for modeling of low energy electrons in liquid water.

Singlet fission has emerged as a promising strategy to avoid the loss of extra energy through thermalization in solar cells. A family of dimers consisting of nitrogen‐doped pyrene‐fused acenes that undergo singlet fission with triplet quantum yields as high as 125 % are presented. They provide new perspectives for nitrogenated polycyclic aromatic hydrocarbons and for the design of new materials for singlet fission., Come Together: By introducing dibenzodiazahexacene moieties in a dimeric architecture, a new type of singlet fission material was obtained. Solvent‐dependent triplet quantum yields of up to 125 % were achieved.

A computational Monte Carlo simulation approach for modeling the thermalization of low-energy electrons is presented. The simulation methods rely on, and use, experimentally based cross sections for elastic and inelastic collisions. To demonstrate the different simulation options, average numbers of interactions and the range of low-energy electrons with initial energies ranging from 1 to 20 eV are calculated for density normalized gaseous water. Experimental gas-phase cross sections for (subexcitation) electrons of energies in the range of 1-20 eV were taken from the compilation of Hayashi. The ballistic collision-by-collision simulations provide information on the intricacies of the thermalization processes not available experimentally. © 2018 Wiley Periodicals, Inc.

The aim of this paper is to study the efficiency of excitonic and activator recombination channels in activated alkali halides depending on the activator concentration. Both theoretical considerations and experimental verification of competition of these channels were carried out. For a model system a CsI-based scintillator was chosen. It is shown that at low temperature (20 K) the excitonic luminescence significantly drops in a hyperbolic law with increase of activator (Tl or In) emission. A corresponding increase of the activator-related recombination channel at 20 K was found to be only 20% of the excitonic yield of pure crystals, reaching a saturation at about 0.04 mol% of the activator. The energy loss observed is attributed to trapping of electron excitations at different centers. This is connected with the competition of electron–hole recombination and trapping of electrons on activators. Estimation of recombination efficiency was done based on the generalized Onsager model. Activator-induced screening, broad spatial distribution of thermalized electrons (mean thermalization length in CsI is about 180 nm) and strongly inhomogeneous electron track structure were taken into account in the model. Energy transfer from the excitons to the activator was suggested to be the main cause of the activator emission observed in doped CsI crystals at 20 K.

We have revealed that a new type of solar cells, intermediate-band-assisted hot-carrier solar cells using indirect-bandgap absorbers, can solve serious issues involved in intermediate-band solar cells (IB-SCs) and hot-carrier solar cells (HC-SCs). We have analyzed energy dissipation processes in these cells using the detailed balance models. In an HC-SC, energy dissipation related to entropy generation associated with hot-carrier extraction is a major issue when the thermalization rate of hot carriers is too rapid, although energy dissipation caused by the thermalization is sufficiently reduced. An IB-SC requires “photon selectivity,” where the spectral absorption ranges for the different interband transitions have no overlap with each other; otherwise, additional energy dissipation due to thermalization occurs. When hot carriers are extracted from an IB absorber, the feature that the current density is decreased because of the two-step excitation via the IB significantly reduces the entropy generation and consequently relaxes the requisite for the thermalization time. Even though the photon selectivity is broken, the increasing carrier energy in excess of each bandgap is efficiently converted to electricity by extracting hot carriers. The current density is further decreased using an indirect-bandgap absorber, leading to even higher efficiency. The limiting conversion efficiency of this new type with no photon selectivity has been calculated to be higher than those of IB-SCs with perfect selectivity by around 10%, assuming the thermalization time being 1 ns. This improvement is contrasting to the fact that the efficiency of HC-SCs is only slightly higher than the Shockley–Quisser limit at 1 sun. Copyright © 2012 John Wiley & Sons, Ltd.

We present a microscopic approach on ultrafast relaxation dynamics of optically excited carriers in graphene. In agreement with experimental differential transmission spectra, we find the dynamics to be characterized by two timescales: the ultrafast femtosecond thermalization and the carrier cooling on a picosecond timescale. The framework of the density matrix theory allows to separately analyze the strength of different relaxation channels, such as carrier–carrier and carrier–phonon scattering, resolved in time, momentum, and angle. Our calculations reveal that the initial thermalization is governed by the Coulomb interaction, however the carrier–phonon scattering also considerably contributes to the process of forming a hot Fermi distribution. The energy dissipation and the cooling of carriers is determined by phonon-induced intra- and interband scattering.

Laser-matter interaction is defined by an electronic band structure of condensed matter and frequency ωL of electromagnetic radiation. In the range of moderate fluences, the energy absorbed by electrons from radiation finally thermalizes in the ion thermal energy. The thermalization processes are different for optical as compared with X-ray quanta and for metals relative to semiconductors and dielectrics, since the light absorption and electron-electron, electron-ion dynamics are sensitive to the electron population in a conduction band and the width of a forbidden gap. Although the thermalization processes are different, the final state is simply a heated matter. Laser heating creates powerful stresses in a target if duration of a laser pulse τL is short in acoustic time scale. Nucleation and material removal take place under such stresses. Such way of removal is called here the spallative ablation. Thus the spallative ablation is an ablation mechanism universally important for qualitatively different materials and quanta. c

Ultrafast non-equilibrium carrier dynamics and many body effects have been investigated in GaN beyond the Mott density. The transient (∼1 ps) induced absorption was observed near the band edge due to band gap renormalisation; collision broadening and temporal evolution of the mean carrier energy were also obtained experimentally. The development of the longitudinal optical (LO)-phonon induced non-thermal electron distribution observed in the pump-probe at early times (≤1 ps) was in excellent agreement with a Monte-Carlo simulation, where the mean carrier energy loss rate of screened carrier-phonon interactions was found to be further reduced due to the hot phonon effect. Schematic diagram of non-equilibrium carrier dynamics.

We report on recent progress toward bridging the gap between the fields of ab initio time-dependent electronic structure theory on the one hand, and quantum statistics/thermodynamics on the other hand. It will be shown that Boltzmanns definition of temperature is more suitable, then the attempt to extract temperature information from a fit of the Fermi–Dirac distribution to populations of Hartree–Fock single particle states. However, the discrepancy between the two leads to the still open question of how to define an appropriate single particle picture. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

This paper considers intrinsic loss processes that lead to fundamental limits in solar cell efficiency. Five intrinsic loss processes are quantified, accounting for all incident solar radiation. An analytical approach is taken to highlight physical mechanisms, obscured in previous numerical studies. It is found that the free energy available per carrier is limited by a Carnot factor resulting from the conversion of thermal energy into entropy free work, a Boltzmann factor arising from the mismatch between absorption and emission angles and also carrier thermalisation. It is shown that in a degenerate band absorber, a free energy advantage is achieved over a discrete energy level absorber due to entropy transfer during carrier cooling. The non-absorption of photons with energy below the bandgap and photon emission from the device are shown to be current limiting processes. All losses are evaluated using the same approach providing a complete mathematical and graphical description of intrinsic mechanisms leading to limiting efficiency. Intrinsic losses in concentrator cells and spectrum splitting devices are considered and it is shown that dominant intrinsic losses are theoretically avoidable with novel device designs. Copyright © 2010 John Wiley & Sons, Ltd.

There is a growing interest in understanding how size-dependent quantum confinement affects the photoluminescence efficiency, excited-state dynamics, energy-transfer and thermalization phenomena in nanophosphors. For lanthanide (Ln 3+ )-doped nanocrystals, despite the localized 4f states, confinement effects are induced mostly via electron-phonon interactions. In particular, the anomalous thermalization reported so far for a handful of Ln 3+ -doped nanocrystals has been rationalized by the absence of low-frequency phonon modes. This nanoconfinement may further impact on the Ln 3+ luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd 2 O 3 :Eu 3+ and Gd 2 O 3 :Yb 3+ ,Er 3+ nanotubes, exhibiting up to one order of magnitude larger than previously reported for similar materials, is reported. This anomalous thermalization induces unexpected energy transfer from Eu 3+ C 2 to S 6 crystallographic sites, at 11 K, and 2 H 11/2 → 4 I 15/2 Er 3+ upconversion emission; it is interpreted on the basis of the discretization of the phonon density of states, easily tuned by varying the annealing temperature (923-1123 K) in the synthesis procedure, and/or the Ln 3+ concentration (0.16-6.60%).

The pressure acts on the sputtered species resulting in their thermalization. Space-resolved velocity distributions of titanium atoms sputtered in a direct-current magnetron discharge were investigated using the tunable diode-laser induced fluorescence technique (TD-LIF). The titanium 3d24s2 − 3d2(3F)4s4p(1P0) transition centred at λ0 = 398.289 nm was used to excite neutral Ti atoms and to measure their velocity distribution profile through the TD-LIF signal measured at λTD-LIF = 736.31 nm. Two groups of metallic particles, namely energetic and thermalized atoms, could be clearly distinguished and the corresponding flux distributions along the laser beam direction were deduced. The paper discusses the relative importance of the energetic atoms versus the discharge pressure especially in the vicinity of the target.

Ultrafast intraband carrier relaxation in carbon nanotubes can be described by Carbon Nanotube Bloch Equations (CNBE) treating the Coulomb interaction beyond the Hartree-Fock level to include carrier-carrier scattering. We obtain equations of motion for carrier densities and two-particle carrier correlations that are solved numerically. The complex non-Markovian relaxation dynamics of hot electrons is calculated for various carbon nanotubes of different diameter and chirality. All calculated nanotubes show an ultrafast relaxation to equilibrium within approximately 100 femtoseconds. The observed relaxation is only weakly dependent on chirality, but depends on the diameter of the nanotube. With increasing diameter of the CNT the relaxation is slowed down, because of the weaker Coulomb interaction in larger diameter CNT. Additionally, we find oscillations of the electron plasma that are due to memory effects in the electron equilibration.

Polariton quantum degeneracy is obtained in a GaAs Microcavity by pulsed resonant excitation of the excitonic reservoir. Time resolved emission is monitored in this stimulated regime along the polariton branch. Analyzing the polariton population distribution for different time delays, we conclude that polariton thermalization is not achieved and that the dynamics is governed by the scattering kinetics from the reservoir into the polariton trap. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new Age-Momentum Correlation apparatus has been built for the purpose of studying Positronium-Gas scattering. This apparatus uses a HPGe detector to perform time-resolved Doppler Broadening Studies on positronium in a gas environment. With this information, the thermalization process can be studied by observing the energy as a function of time. Modelling the rate of thermalization can then be used to extract a momentum transfer cross-section. Improved data rate and energy resolution over previous measurements that used Doppler broadening methods have been obtained. More importantly, the simultaneous acquisition of both time and energy spectra allow for systematic checks that were not available to previous experiments. Preliminary results are presented. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The experimental ionized physical vapor deposition (IPVD) system with an external antenna is described, and experimental characterization of the plasma was performed: plasma density was up to 5 × 1012 cm−3, the argon ionization ratio was ∼0.2–0.4%, and copper ionization ratio was ∼28% in bulk plasma, corresponding to ionized Cu+ flux ratio in range the 73–95%. To aid further development and IPVD technology extendibility to the future nanoscale fabrication, the 2D plasma fluid model for IPVD system was developed. The model comprised the Cu + Ar collisional mechanism including charge exchange and Penning ionization, considering gas rarefaction effect, variable ion mobility approach, and analytical approach in description of the thermalization of sputtered Cu. The simulation results were validated through experimental measurements of the plasma characteristics by electrical probe and provided reasonable results in 2 to 15 Pa pressure range. The implementation of the model for optimized design of scaled-up IPVD system was demonstrated.

Algorithmic Cooling (AC) of Spins is potentially the first near-future application of quantum computing devices. Straightforward quantum algorithms combined with novel entropy manipulations can result in a method to improve the identification of molecules. We introduce here several new exhaustive cooling algorithms, such as the Tribonacci and k-bonacci algorithms. In particular, we present the ``all-bonacci'' algorithm, which appears to reach the maximal degree of cooling obtainable by the optimal AC approach., Comment: 20 pages, 5 figures

We present an analytical model that combines the complementary experimental evidence of spatial dispersion (DAP recombination) and energetic dispersion (band tails). The model describes the competition between thermalization and recombination of excess carriers trapped in exponentially distributed (in energy), discrete localized (in space) states. We use the energy dependence of the relaxation rates to derive the energy and time dependence of sub gap photoluminescence. The model predicts that the yellow luminescence band (YLB) and blue luminescence band (BLB) commonly observed in GaN are not separate entities, but reflect the competition of thermalization and recombination. A distinct kink is observed in transient PL in the microsecond range, in the limiting cases of strong tailing and/or low temperatures, indicating the transition from thermalization-limited to (radiative) recombination-limited excess carrier relaxation. Both prediction are in line with experiment, and able to resolve interpretational difficulties. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton dynamics in nonpolar (11-20) InxGa1–xN/GaN multiple quantum wells (QWs) grown on GaN templates prepared by lateral epitaxial overgrowth were studied. Due to the absence of the polarization fields normal to the QW plane, the photoluminescence (PL) peak energy moderately shifted to the higher energy and the radiative lifetime did not change remarkably with the decrease in the well thickness. Similar to the case for polar InGaN QWs, time-resolved PL signals exhibited the nonexponential decay shape, which can be explained by thermalization and subsequent localization of excitons. Although the growth conditions were not fully optimized, values of the PL intensity at 300 K divided by that at 8 K were 25 and 17% for the peaks at 2.92 and 2.60 eV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In-plane spatial diffusion of indirect excitons in GaAs/AlGaAs coupled quantum wells (QWs) is studied theoretically. In spatial photoluminescence (PL) diagrams obtained from experiments, two rings of PL appear around the excitation spot, and their size and intensity depend upon the lattice temperature Tb and the optical pump intensity. We explain and model numerically the first PL ring, by using coupled nonlinear equations for the diffusion, thermalization and optical decay of the excitons. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We review different applications of quasinormal modes to black hole physics, string theory and thermalisation in quantum field theory. In particular, we describe the the relation between quasi normal modes of AdS black holes and the time scale for thermalisation in strongly coupled, large $N$ conformal field theory. We also discuss the problem of unitarity within this approach., Comment: Talk given at 36th International Symposium Ahrenshoop on the Theory of Elementary Particles: Recent Developments in String/M-Theory and Field Theory, Berlin, Germany, 26-30 Aug 2003, to appear in Fortsch.Phys

The energy transfer to Er3+ ions in a-Si1−xCx:H films has been studied by absorption, luminescence and magnetic resonance spectroscopy. Erbium is incorporated either by magnetron assisted co-sputtering of Si and Er in a silane plasma or by conventional plasma deposition of a metalorganic compound which yields films of very different defect density and Er environment. The data confirm Er ions as efficient radiative recombination centres in a-Si:H, which are excited by resonant energy transfer from electron hole pairs in the host and disprove that dangling bonds are needed for emission at 1.5 μm wavelength by Erbium ions. We propose a Forster energy transfer, which is based on resonant dipole coupling. Resonance to excited states of Er3+ is achieved as electrons thermalise in tail states and form pairs with localized holes and enables rapid energy transfer to Erbium ions while back transfer is inefficient due to lattice relaxation. Thermalisation and recombination rates in the host material control the excitation of Er ions and can be modified by alloying silicon with carbon. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a theoretical model of the time-resolved excitonic photoluminescence in quantum wires. We study the role of disorder on the temperature dependence of the photoluminescence lifetime. We find that at low temperature the exciton population is not thermalized in disordered quantum wires and that the lifetime is mainly determined by the localization length of the exciton. However for large localization lengths or high enough temperatures the exciton population thermalizes within its lifetime and the temperature dependence becomes square root like.

We investigated the hot carrier dynamics in InGaN multi-quantum wells using photoluminescence, time-resolved photoluminescence (TRPL) and white-light pump and probe methods at room temperature. The pump and probe spectra or ΔOD spectra have not reflected the screening of electric field by photoexcited carriers but the distribution of hot carriers. It reflects the band filling of the tail state in lower energy side and Maxwell-Boltzmann-like tail of hot carriers on the higher energy side. ΔOD spectra in 1.5 ps rose in the observed spectral region, which indicates that thermalization among carriers is very fast. It is because of carrier-LO phonon and carrier-carrier interaction. After 2 ps ΔOD spectra are red shifted as increasing the time delay. At 40 ps time delay, the Maxwell-Boltzmann-like tail shows still the high temperature of carriers. This slow cooling is likely due to the hot phonon effects where the nonequilibrium phonon population may lead to a reabsorption of phonons by photoexcited carriers.

Time-resolved measurements of the photo-induced magneto-optical Kerr effect were performed in p-doped and intrinsic InP at 20 K in the spectral range nearby the spin-orbit split-off exciton transition. Three relaxation components with different temporal and spectral behavior could be distinguished and their origin is discussed, supported by theoretical calculations. The observed processes are attributed to the exciton thermalization, hole spin and electron spin relaxation.

In order to model the very recent experimental data on the relaxation kinetics and photoluminescence (PL) dynamics of indirect excitons in high-quality GaAs/AlGaAs coupled quantum wells (QWs) at extremely low bath temperatures T b = 0.05 K, we analyze, both analytically and numerically, the relevant quantum Boltzmann equation. We show that at large delay times after initial optical excitation, thermalization of QW excitons occurs through the nonequilibrium states, weakly correlates with the initial conditions, and depends only upon the two control parameters, T b and the degeneracy temperature To. We present a generic solution of the quantum Boltzmann equation and compare it with the numerical simulations. Finally, we report the experimental and theoretical study on the radiative lifetime τ opt of quantum degenerate indirect excitons at low temperatures, 50 mK < T b < 2 K.

We present numerical evidence for the occurrence of coherent backscattering in the Fock space of a small disordered Bose-Hubbard system consisting of four sites and containing five particles. This many-body interference phenomenon can most conveniently be seen in time evolution processes that start from a Fock state of the Bose-Hubbard system. It manifests itself in an enhanced detection probability of this initial state as compared to other Fock states with comparable total energy. We argue that coherent backscattering in Fock space can be experimentally measured with ultracold bosonic atoms in optical lattices using state-of-the-art single-site detection techniques. A synthetic gauge field can be induced in order to break time-reversal symmetry within the lattice and thereby destroy coherent backscattering. While this many-body interference effect is most prominently visible in the presence of eigenstate thermalization, we briefly discuss its significance in the opposite regime of many-body localization.

We study the nature of and approach to thermal equilibrium in isolated quantum systems. An individual isolated macroscopic quantum system in a pure or mixed state is regarded as being in thermal equilibrium if all macroscopic observables assume rather sharply the values obtained from thermodynamics. Of such a system (or state) we say that it is in macroscopic thermal equilibrium (MATE). A stronger requirement than MATE is that even microscopic observables (i.e., ones referring to a small subsystem) have a probability distribution in agreement with that obtained from the micro-canonical, or equivalently the canonical, ensemble for the whole system. Of such a system we say that it is in microscopic thermal equilibrium (MITE). The distinction between MITE and MATE is particularly relevant for systems with many-body localization (MBL) for which the energy eigenfuctions fail to be in MITE while necessarily most of them, but not all, are in MATE. However, if we consider superpositions of energy eigenfunctions (i.e., typical wave functions $\psi$) in an energy shell, then for generic macroscopic systems, including those with MBL, most $\psi$ are in both MATE and MITE. We explore here the properties of MATE and MITE and compare the two notions, thereby elaborating on ideas introduced in [Goldstein et al., Phys.Rev.Lett. 115: 100402 (2015)]., Comment: 36 pages LaTeX, 2 figures; v3 minor improvements

The dynamics of resonant photoluminescence (PL) of quantum well (QW) excitons is studied within the thermodynamic description of the LA-phonon-assisted relaxation kinetics, taking into account Bose-Einstein statistics of excitons. Nonclassical statistics strongly influences the effective radiative lifetime τopt of quasi-equilibrium QW excitons. For a strongly degenerate gas of excitons, the law τopt ∝ T (T is the effective temperature of excitons) changes and τopt approaches 2τR, where τR is the intrinsic radiative lifetime of QW excitons. Our approach describes self-consistently both the rise of PL, due to thermalization, and the decay of PL, due to the radiative recombination of thermalized QW excitons. Finally, we model numerically the PL kinetics of statistically-degenerate indirect excitons in coupled GaAs/AlGaAs quantum wells.

At room temperature the ortho-positronium (oPs) lifetime in porosities is well described by the rectangular Tao-Eldrup model (RTE) [1]. Recent experiments [2-5] at low temperature have shown unexpected deviations from the RTE predictions. In Ref. [2] a pick-off increase is observed below 200 K in pores with size less than 2 nm, at the contrary a decrease of pick-off is found in pores with size around 3nm. An increase of the oPs lifetime (decrease of pick-off) at low temperature was also observed by the authors of Ref. [3-5] in porosities with sizes from 2.3 to 4 nm. In pores with size larger than 5 nm an increase of pick-off was again pointed out [3]. The authors of the present communication, have indicated [6] that these experimental deviations from RTE model could be due to oPs confining in small pores and to a longer oPs thermalization time in largest pores. Both the effects give a longer permanence of oPs in excited states where the pick-off rate is higher. In this work we discuss and further develop the hypothesis reported in Ref. [6] taking also into account the effect of the temperature on the interaction of oPs with the walls of the pores. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The strong exciton–LO phonon coupling in ZnSe QWs gives a direct access to the relaxation dynamics of nonthermal, free heavy-hole and light-hole excitons. Narrow hot-exciton distributions can be generated by LO-phonon assisted exciton formation. The thermalization of these excitons is monitored by cw or time-resolved phonon-sideband spectroscopy. A thermal exciton distribution evolves on a 100 ps timescale at low exciton densities when only scattering with acoustic phonons is possible. Exciton–exciton scattering accelerates the relaxation dynamics at high densities. Filling of the free heavy-hole exciton states and thermal population of the light-hole exciton states is observed. Measurements of the polarization degree after excitation with a σ+-polarized picosecond pulse show that light-hole excitons with zero in-plane momentum change sign of their spin with each emission of an LO phonon.

Using the thermal lens experimental configuration, many types of fast molecular dynamics besides the density variation (the normal thermal lens signal) are reflected by the lens signal. Various transient lens techniques in a fast timescale are reviewed. By using the population lens signal, excited state dynamics can be studied with a probe light which has less restricted wavelength range over a wide timescale from a few femotoseconds to seconds. From the optical Kerr lens signal, molecular reorientational dynamics in solution can be revealed. The temporal development of the temperature lens signal provides unique information such as the thermalization time after nonradiative transition from the excited state, which is usually very difficult to determine by other spectroscopies. Identification of the lens components and the advantages of the time-resolved transient lens method are discussed.

We investigate the exciton formation process from free carriers in a single GaAs quantum well. We focus on the formation processes assisted by emission and absorption of acoustic and optical phonons. The formation rates are evaluated and compared to the experimental results. The exciton dissociation rates are also calculated as a function of the lattice temperature. Finally, these rates are used in a rate equation model in order to reproduce the dynamics of excitons and free carriers in a photoluminescence experiment. We show that for lattice temperature lower than 30K, the thermalization between free carriers and excitons is very slow, and deviations from the action mass law persist for several nanoseconds.

The study and control of magnetic materials using ultrashort laser pulses is of great interest both from a fundamental point of view and for potential applications in the technology of information and communication. In this review the state of the art experimental and theoretical works in the field of Ultrafast Magnetization Dynamics is described, with a particular emphasis on the dynamics of nanostructures. The elementary physical mechanisms involving the spin dynamics when exciting magnetic nanostructures with femtosecond optical pulses are considered. The variety of experimental methods and theoretical approaches used to study the magnetic properties of the materials on a broad range of temporal and spatial scales are examined. The concepts of coherent photon-spin coupling, spin thermalization, precession dynamics and damping, magnetoacoustic interactions, are discussed. Some general trends and prospective works are also envisaged.

We examine the exact equation of motion for the relaxation of populations of strongly correlated electrons after a nonequilibrium excitation by a pulsed field, and prove that the populations do not change when the Green's functions have no average time dependence. We show how the average time dependence enters into the equation of motion to lowest order and describe what governs the relaxation process of the electron populations in the long-time limit. While this result may appear, on the surface, to be required by any steady-state solution, the proof is nontrivial, and provides new critical insight into how nonequilibrium populations relax, which goes beyond the assumption that they thermalize via a simple relaxation rate determined by the imaginary part of the self-energy, or that they can be described by a quasi-equilibrium condition with a Fermi-Dirac distribution and a time-dependent temperature. We also discuss the implications of this result to approximate theories, which may not satisfy the exact relation in the equation of motion.

An ultrafast redistribution time of less than 40 fs for nonequilibrium photoexcited electron densities as low as 9×10 16 cm -3 is reported. The electron populations thermalize within 100 to 200 fs after the end of the pump pulse and the cooling of the thermalized electrons to the lattice is observed. These results are obtained from femtosecond pump-probe measurements yielding differential absorption signals directly proportional to the electron distribution function and independent of the hole distribution. This is achieved by using transitions from the unoccupied split-off valence band to test the electron populations in the conduction band

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface. We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.

Pump and probe techniques with ultrafast laser pulses represent a well established tool for the measurements of electron-electron and electron-phonon thermalization processes. It is shown that expanding these techniques to magnetic systems for spin-lattice relaxation measurements requires a careful selection of the thermal characteristics of the samples. More precisely, a simulation of the thermal behavior of thin metal films heated by short laser pulses shows that film thickness and substrate thermal properties are crucial parameters.

The various forms (granular, liquid, gum) of lecithin can be heated under certain conditions of time and temperature to greatly improve their properties as emulsifiers for water-in-oil emulsions. Viscosity, discontinuous phase-holding capacity, stability and water retention were greatly enhanced in emulsions containing thermalized lecithins as the emulsifier compared to those prepared with corresponding amounts of nonthermalized lecithins. The improved emulsification properties of the thermalized lecithins appeared to be due, at least in part, to an increase in diglycerides and free fatty acids resulting from the thermal degradation of phosphatides.

Time-resolved luminescence spectra of highly excited CdS-microcrystallites are studied at room temperature in relation to the hot electron–hole plasma (EHP) effect. When the EHP thermalization is complete the luminescence kinetics shows a steady increase in the recombination rate with decreasing microcrystallite average radius (from 1000 A to 54 A). The obtained size dpendence is attributed to surface recombination.

We analyze the process of dissipation of energy in excess of equilibrium in photoexcited carriers (electrons and holes) during their second kinetic stage of relaxation (SKS). Quantum kinetic equations obtained from Zubarev's nonequilibrium statistical operator are used, and different initial effective temperatures for electrons and holes are taken. It is shown that holes relax their excess energy faster than electrons for all levels of photoexcitation, and that the mutual thermalization of electron and hole sub-systems depends on the degree of photoexcitation. The lapse of time for electrons and holes to attain mutual equilibrium is estimated to be of the order of 6ps for a density of photoinjected carriers in GaAs of the order of n = 2.0 × 1018 cm − 3, when the initial excess of kinetic energy per pairis taken as 2.4 eV.

The formation of electronically excited S(1D) atoms was studied during the thermal decomposition of carbonyl sulfide behind reflected shock waves by using atomic resonance absorption spectroscopy (ARAS). The experiments were performed in highly diluted mixtures of Ar with relative concentrations of COS between 8 to 100 ppm in the temperature range 2130 K≤T≤3020 K at pressures of about 1.2 bar. The measured rate of the linear S(1D) increases divided by the initial reactants was found to obey the Arrhenius law: . Assuming a fast thermalisation between the (3P) and (1D) states of S atoms this result is in agreement with previous measurements of the S(3P) formation during the thermal decomposition of COS reported recently [1].

Beam-induced voltage contrast has been demonstrated in a superconducting thin film of YBa2Cu3O7-x (YBCO) grown on MgO. Copper-rich inclusions in the material contribute to spatially varying resistive behaviour but do not account for all the observed details. A new method of scanning in which the beam was moved away from the sample between each data acquisition point was found to increase the resolution by limiting thermalisation, to allow imaging of resistive features as small as 0.5 μm. Direct comparisons of beam-induced contrast and micro-structure at 9000 times magnification were possible using simultaneous image acquisition via a purpose-built image store. Mounting a preamplifier within the scanning electron microscope chamber, to give a variable overall gain of 1000–100,000, was also found to significantly improve the signal-to-noise ratio.

Time- and spatially resolved luminescence or GaAs/AlGaAs quantum wires (QWRs) grown by organometallic chemical vapor deposition on nonplanar substrates is reported. The effective width or these nanoscopic wires is as small as 9.5 nm, giving rise to a separation or 45 meV for the quasi-one-dimensional conduction subbands. The QWRs exhibit high luminescence efficiency and the radiative recombination lifetime or the quantum confined carriers is round to be larger than 310 ps. Combined spectral-time-resolved experiments directly visualize the carrier capture process and the thermalization or the 1D carriers in the QWR structures