Nonstationary Absorption Research Articles

Ultrafast dynamics and mechanisms of non-stationary absorption in thin gallium selenide samples

Herein, the dynamics and mechanisms of induced absorption in thin samples of gallium selenide under various excitation conditions are studied using femtosecond kinetic spectroscopy. We have registered several types of induced changes including induced absorption on free charge carriers (“hot” and thermalized electrons), bleaching and absorption due to the population of near-edge trap or exciton states, as well as rapid changes in the absorption of probing radiation in the region of the overlap of the exciting and probing pulses due to two-quantum two-frequency interband transitions. The time ranges of the relaxation processes are estimated. It is shown that when using relatively low-intensity long-wave excitation (790 nm), the resonant excitation of the near-edge states occurs mainly due to two-quantum two-frequency transitions followed by the formation of the dynamic equilibrium between bound and free electrons in the time range up to 5 ps. When electrons are excited deeply into the conduction band with the formation of hot free electrons and their subsequent thermalization to the bottom of the conduction band in the time range up to 1 ps, the population of the near-edge states and the establishment of the dynamic equilibrium between bound and free electrons is realized in the same time range (5 ps) as when they are excited “from below”.

Invariant submodels of model of nonlinear diffusion in inhomogeneous medium with nonstationary absorption or source

We carried out a complete group classification of the nonlinear differential equation of the model, describing the nonlinear diffusion process (or the process of heat propagation) in an inhomogeneous medium with nonstationary absorption or source. We found all models whose equations admit a nontrivial continuous Lie group of the transformations. We researched invariant submodels of the obtained models. Some invariant solutions, describing these submodels were found either explicitly, or their search was reduced to solving of the nonlinear integral equations. In particular, we obtained an invariant solution that describes the nonlinear diffusion process (or heat distribution process) with two fixed “black holes”, and invariant solution that describes the nonlinear diffusion process (or heat distribution process) with a fixed “black hole”, and with a moving “black hole”. The solutions with “black holes” can be used to describe the process of destruction of glaciers under the influence of the external heat source, for example a solar energy. Using other invariant solution, we studied a diffusion process (or heat distribution process) for which at the initial moment of the time at a fixed point a concentration (a temperature) and its gradient are specified. The solution of the boundary value problem describing this process reduces to the solution of nonlinear integral equations. We have established the existence and uniqueness of solutions of this boundary value problem under certain conditions. The mechanical significance of the obtained solutions is as follows: 1) they describe specific nonlinear diffusion processes (or heat distribution processes), 2) they can be used as tests in numerical calculations in studies of nonlinear diffusion (or heat distribution) in an inhomogeneous medium with nonstationary absorption or source, 3) they allow to assess the degree of adequacy of a given mathematical model to real physical processes, after carrying out experiments corresponding to these decisions, and estimating the arising deviations.

Submodels of model of nonlinear diffusion with non-stationary absorption

We study the model, describing a nonlinear diffusion process (or a heat propagation process) in an inhomogeneous medium with non-stationary absorption (or source). We found tree submodels of the original model of the nonlinear diffusion process (or the heat propagation process), having different symmetry properties. We found all invariant submodels. All essentially distinct invariant solutions describing these invariant submodels are found either explicitly, or their search is reduced to the solution of the nonlinear integral equations. For example, we obtained the invariant solution describing the nonlinear diffusion process (or the heat distribution process) with two fixed "black holes", and the invariant solution describing the nonlinear diffusion process (or the heat distribution process) with the fixed "black hole" and the moving "black hole". The presence of the arbitrary constants in the integral equations, that determine these solutions provides a new opportunities for analytical and numerical study of the boundary value problems for the received submodels, and, thus, for the original model of the nonlinear diffusion process (or the heat distribution process). For the received invariant submodels we are studied diffusion processes (or heat distribution process) for which at the initial moment of the time at a fixed point are specified or a concentration (a temperature) and its gradient, or a concentration (a temperature) and its rate of change. Solving of boundary value problems describing these processes are reduced to the solving of nonlinear integral equations. We are established the existence and uniqueness of solutions of these boundary value problems under some additional conditions. The obtained results can be used to study the diffusion of substances, diffusion of conduction electrons and other particles, diffusion of physical fields, propagation of heat in inhomogeneous medium.

Self-similar Shape Mode of Optical Pulse Propagation in Medium with non-stationary Absorption

We discuss laser pulse propagation with the self-similar shape in a medium with instantaneous nonlinear absorption. We consider two cases of the laser pulse propagation. First one corresponds to problem of laser-induced plasma generation in silica under action of TW laser pulse. The second one corresponds to femtosecond laser pulse propagation in medium with nanoparticles of noble metals. In both cases the mode of the self-similar shape of pulse is of interest. We discuss also a physical mechanism of non-linear acceleration or slowing-down for laser pulse propagation in a medium with nanoparticles. The last phenomena are important, in particular, for a problem of data processing of all optical method. We used analytical approach for considered problem as well as computer simulation.

Stationary and nonstationary absorption in lead silicate glasses with short-range order inversion

The methods of stationary and pulsed absorption spectroscopy were used to study the optical properties of xPbO·(1– x)SiO 2 glasses produced by cooling of a molten mixture of chemically pure oxides. The spectral dependence of the absorption in the range of the short-wavelength edge obeys the Urbach rule. As the PbO concentration increases, a red shift of the optical transparency cutoff is observed. At x = 0.45–0.50 the amorphous matrix undergoes a structural inversion, which is due to a transition from a silicate to a lead–oxygen glass-forming network. This transition shows up as an abrupt change in the type of optical transitions, the width of the optical gap E g, and the Urbach energy E U. The short-range order inversion in the glass is accompanied by an increase in the atomic correlation radius R 0 characterizing the size of the medium-range order in the system. It was found empirically that R 0 has a linear relationship with a continuum-disorder parameter E U. It was found that pulsed electron irradiation produces short-lived color centers, which absorb at 1.65 and 2.30 eV. The relaxation of unstable absorption centers is characterized by microsecond kinetics. The nature of unstable absorption centers and their relationship with a short-range order inversion and the structure function of lead atoms have been discussed. The kinetic dependences have been interpreted in the context of a mechanism responsible for diffusion-controlled tunneling recombination of radiation-induced electronic and hole states of the matrix.

Transient magnetoresponse of photoexcited electrons: Negative cyclotron absorption and evolution of Faraday angle

The transient magnetooptical response of electrons with partly inverted initial distribution produced by an ultrashort optical pulse near the optical phonon energy is studied theoretically. Transient cyclotron absorption and Faraday rotation of polarization plane are considered for bulk semiconductors (GaAs, InAs, and InSb) as well as for a GaAs-based quantum well. Damping of the response due to electron momentum relaxation associated with elastic scattering from acoustic phonons is taken into account in calculations, as well as the evolution of the electron distribution due to quasi-elastic energy relaxation at acoustic phonons and effective inelastic transitions accompanied by spontaneous emission of optical phonons. Nonstationary negative absorption in the cyclotron resonance conditions and peculiarities of Faraday rotation of the polarization plane associated with partial inversion of the initial distribution are considered. The possibility of transient enhancement of the probe field under cyclotron resonance conditions is indicated.

Amplification and generation of radiation at the 42Γ → 1,22Γ transition of the Kr2F molecule in an electron-beam-pumped wide-aperture laser

The radiation amplification and oscillation are obtained in an electron-beam-pumped Berdysh laser with an active volume of 10 L in the visible range from 430 to 470 nm at the broadband 42Γ → 1,22Γ transition in the triatomic Kr2F molecule of interest for amplification of ultrashort laser pulses. It is shown that, along with absorption of laser radiation in the active medium, the amplification dynamics is considerably determined by short-lived absorption induced in the amplifier windows by bremsstrahlung X-rays. For the specific pump power 0.6 — 0.7 MW cm-3, the gain, corrected for nonstationary absorption, was ∼10-3 cm-1. A scheme of a Kr2F amplifier is proposed for amplification of ultrashort pulses up to multiterawatt peak powers in the active volume ∼10 L.

Nonstationary Absorption of Methane by a Quiescent Aqueous Phase

Nonequilibrium dissolution of methane in unstirred water was investigated. The experiments were carried out in the ranges of temperatures, pressures, and concentrations corresponding to the existence of methane hydrate in an equilibrium state. The experimental data were satisfactorily described by a model taking into account the methane transfer both through the gas–liquid interface layer and in the liquid phase. Calculations using the model and the experimental data showed that the methane diffusivity in water is lower than in the interface layer. This can be associated with the formation of methane hydrate in the aqueous phase, since methane hydrate molecules are larger than methane molecules.

Electromagnetically induced transparency and evolution of a two-level system under chaotic field of arbitrary intensity

The electromagnetically induced transparency (EIT) with a (near-)resonant chaotic (amplitude-phase fluctuating, Gaussian-Markovian) coupling field is studied theoretically. The Fourier transform of the steady-state EIT spectrum, which determines a nonstationary probe absorption, is also considered. This quantity equals the average diagonal element of the (reduced) evolution operator of the coupled transition (the evolution function). The exact solution in the form of a continued fraction is obtained and used to perform numerical calculations. Moreover, a number of approximate analytical results are obtained, which, together with the results of previous publications, describe the EIT and the evolution function in all possible regimes. In particular, in the constructive-interference case the EIT increases with the coupling-field bandwidth ν at sufficiently small ν. For a strong field, the maximum of the transparency as a function of ν is less than that for a monochromatic field of the same average intensity. In contrast, for a weak field, there is a range of ν values, where the field fluctuations do not affect the EIT. The latter result is shown to hold for a broad class of stochastic fields.

On the mechanism and kinetics of the transport processes in system with intensive interphase mass transfer. 4. Effect of the interface concentration

A comparative analysis of the non-stationary absorption rate of pure (concentrated) gases in a stagnant liquids has been performed. The results obtained indicate that the rate depends on the equilibrium concentration at the interface. A theoretical relationship has been developed. The relationship confirms the experimental data and explains the differences between the absorption and desorption rates as well.

Absorption and fluorescent properties of pyrylium compounds: II. Spectra and cross sections for absorption from the ground and excited states of the structurally rearranged form

The nonstationary absorption of solutions of 2,4,6-substituted pyrylium salt is studied by the pump-probe method. The solutions in methylene chloride are studied upon picosecond pumping and probing with different time delays. The solutions in methyl ethyl ketone are studied upon pumping and synchronous probing by nanosecond pulses. It is shown that excitation of the initially nonplanar molecule causes its structural rearrangement. As a result, substituent 2 enters into the pyrylium ring plane and two nonstationary conformers are formed that differ in the position of substituent 6. The motion upon structural rearrangement has a small amplitude, and localization of molecular orbitals changes only slightly. The temperature dependence of the nonstationary absorption is studied in the range from 293 to 223 K. The spectra and cross sections are measured for transitions from the S0 and S1 states. In addition, the activation energies and kinetic parameters of the rearrangement in the ground and excited states are determined. Experimental data are compared with theoretical calculations of the transitions from the S1 state. It is shown that a partial flattening of conformer molecules results in the long-wavelength shift of the bands and an increase in the cross sections for transitions localized on a flat part of the molecule and does not change parameters of the band localized on the molecular part that did not change upon the rearrangement but remained nonplanar.

On the mechanism and kinetics of the transport processes in systems with intensive interphase mass transfer. 1.: Heat and mass transfer

A theoretical analysis of the mechanism and the kinetics of non-stationary absorption is proposed in an immobile liquid at big concentration gradients, when the mass transfer may be a result of a natural convection, non-linear mass transfer and the Marangoni effect. It is shown that the Marangoni effect is negligible under these conditions and the flow rate is limited by the non-linear mass transfer. The rate of the mass transfer is determined.

On the mechanism and kinetics of the transport processes in systems with intensive interphase mass transfer. 2.: Stability

This research suggests a linear analysis of the stability of non-stationary absorption of concentrated gases in an immobile liquid, where a basic flow is induced as a result of a natural convection and a non-linear mass transfer. A specter of disturbances that lead to a stable dissipative structure with a high mass transfer coefficient has been found. The disturbance amplitude is determined by experimental data for the mass transfer rate in absorption of pure CO 2 in a stagnant water column. It has been shown that conditions for the occurrence of the Marangoni effect do not exist. The theoretical results have a good agreement with the experimental data.

Rapidly progressing photophysical and photochemical relaxation processes in complex organic molecules

Using methods of high-speed kinetic laser spectroscopy, we investigated the dynamics and mechanisms of rapidly progressing relaxation processes in multiatomic molecules. We separated intra- and intermolecular channels of relaxation of the vibrational energy of excitation by the rate of transformation of nonstationary absorption spectra in a picosecond range of times. The role of highly excited electronic states in the process of electronic-vibrational relaxation in molecules of the class of phenazines was ascertained. Applying femtosecond light pulses in a real time scale, we recorded the dynamics of the decay of optically induced anisotropy in rarefied vapors of organic compounds. Several mechanisms of transfer of a hydrogen atom in the process of formation of free radicals in photoreduction of ketones were established. The dynamics of formation of inter- and intramolecular exciplexes in binary gas-phase systems was investigated. We determined the mechanisms of intramolecular dissociation of a C–O chemical bond in spiropyrans and xanthene dyes. Photodissociation of S–S bonds in molecules of disulfides was studied. The possibility of the existence of states with intramolecular charge transport with a twisted configuration for organic free radicals is shown.

Non-stationary Two-photon Doppler-free Coherent Spectroscopy in an Atomic Gas in the Presence of Depolarizing Collisions

Abstract Non-stationary two-photon absorption coherent spectroscopy free of Doppler broadening in an atomic gas has been considered in detail. Correct account has been taken of the level degeneracy and depolarizing collisions. This two-photon Doppler-free spectroscopy has been developed for determining the relaxation rates of the atomic multipole moments in gas. A criterion to test the adopted model of the depolarizing collisions has been developed. A possible method for the identification of the atomic transitions in the angular momentum has been formulated. A technique of precisely determining the separation between the energy levels of the two-photon transition has been proposed. The conditions under which the Stokes wave is conjugated with respect to the probe pulse in amplitude, phase and polarization have been determined for the atomic gas in the presence of depolarizing collisions.

Observation of the intermediate energy state of the DX centre in AlxGa1-xAs:Te in non-stationary absorption experiments

Detailed studies of the absorption kinetics of the DX centre are presented. These studies were performed on unique thick AlGaAs:Te layers and clearly demonstrated the participation of the neutral charge state of DX as an intermediate state in the photoionization process. The neutral state of the DX observed here is the localized relaxed energy state similar to the ground state of the DX centre.

Theory of femtosecond time-resolved infrared absorption spectroscopy with application to ligand dissociation in haemoglobin

In this paper we present a theory of femtosecond time-resolved infrared absorption spectroscopy utilizing the theory of nonstationary time-dependent absorption. We treat the pump-probe absorption with nonoverlapping pulses using the stochastic Liouville equation. In this case, we deal only with the linear susceptibility and obtain the generalized linear response theory due to the fact that the system is initially nonstationary because of the femtosecond pulse duration. The adiabatic approximation is introduced to derive the theoretical expressions for the time-resolved IR absorption. Theoretical results are applied to interpret the experimental fs time-resolved IR spectra reported by Hochstrasser and co-workers for the direct observations of ligand dynamics in haemoglobin.

Plasma heating by an ultrashort light pulse

Nonstationary absorption of the energy of a short electromagnetic-radiation pulse in a superdense semibounded plasma is considered. An approach is proposed for finding the electron distribution function, wherein the region of the skin layer (in the anomalous skin-effect regime) is described as a boundary condition for the distribution function over the entire plasma volume. A self-similar electron-distribution function is obtained and is used to analyze the dynamics of the electromagnetic-field absorption, of the heat transfer, and of the bremsstrahlung of the heated plasma.

Nonstationary absorption spectra from aminophenylthiyl radicals

Nonstationary absorption spectra from aminophenylthiyl radicals

The nonlinear theory of electromagnetic wave attenuation in plasmas

The absorption of a circularly polarized electromagnetic wave which propagates in a plasma along a magnetic field is analyzed. The exact equations of particle motion in the resonance region are solved with aid of elliptic functions. It is shown that the nonlinear damping constant has an oscillatory form. For t→0, it coincides with the constant obtained on the basis of linear theory, while for t→∞, in the absence of collisions, it tends to zero. The influence of collisions on wave absorption is studied. It is shown that with allowance for collistions, the damping constant depends on the amplitude of both the H1 and H1 −3/2 waves. The analysis of slowly decaying waves may be based on a model proposed by Dawson [1] and later modified in [2,3]. According to this model, all plasma particles are grouped into resonant and nonresonant ones. The velocity distribution function of the nonresonant particles is assumed to be the same as in the case of undamped waves. The distribution function of resonant particles at the initial instant is assumed to be Maxwellian. The nonlinear equations of motion of the resonant particles are integrated exactly. The damping constant is defined as the ratio of the energy expended by the wave at the resonant particles to the total energy of the wave. In nonlinear formulation, resonant absorption appears to be nonstationary. After a time lapse on the order of several vibrational period of a particle captured by the wave, nonstationary absorption ceases, and stationary absorption, created by infrequent collisions, becomes essential. It is noteworthy that absorption of this type has been studied by V. E. Zakharov and V. I. Karpman [4] for the case of plasma waves.