Relaxation Of Electron Energy Research Articles

Influence of the ionization process on characteristics of spatial relaxation of the average electron energy in inert gases in a uniform electric field.

Theoretical studies of the characteristics of the spatial relaxation of the average energy of electrons in inert gases (He, Ne, Ar, Kr, Xe) in a uniform electric field had been carried out. Conditions were considered when spatial relaxation has the character of damped oscillations. The calculations were performed using the Monte Carlo technique both without and taking into account the secondary electrons that appeared due to the ionization of atoms by electron impact. It was shown that the inclusion of secondary electrons leads to a noticeable decrease in the spatial relaxation length even in the case when the contribution of the ionization process to the electron energy balance is relatively small. For these gases, the upper boundary of the the electric field strength was determined, where the spatial relaxation of the average energy has the character of damped oscillations.

Peculiarities of spatial relaxation of the mean electron energy in inert gases and their mixtures in a uniform electric field

A theoretical study of the spatial relaxation characteristics of the mean electron energy in a uniform electric field in He, Ne, Ar, Kr, Xe and a mixture of He:Xe was performed. Calculations were carried out with the use of the Monte Carlo technique. Electrons were launched at some point z = 0, and the electron energy distribution function and the mean electron energy u m(z) were calculated at various distances from the source. According to calculations, for pure gases in a wide range of the electric field strength E, the dependence u m(z) has the form of damped oscillations. At that, the relaxation length L and the oscillation period Λ are almost independent of the form of the energy spectrum of the electron source. It is shown that for all pure gases the calculated dependences L(E) have the shape of a hump, and for each gas the position of the maximum corresponds to such E values at which the energy lost by electrons in elastic collisions is about 4%–8%. For the case of Xe we also studied the variation of L(E) depending on the set of cross sections used in the calculations. It was also shown that for the same E value, the relaxation length in He:Xe mixture differs significantly from that in pure He or Xe. Moreover, u m(z) in a certain range of parameters (E and percentage of Xe in the gas mixture) is characterized by the sum of two damped oscillations with different spatial periods.

Electron energy relaxation in disordered superconducting NbN films

We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product q_Tl(q_T is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14-30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/\tau_{e-ph} \sim T^n with the exponents n = 3.2-3.8. We found that in this temperature range \tau_{e-ph} and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9-17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.

Kinetics of intrasubband electron energy relaxation in quantum wells in a quantizing magnetic field

Herein we report the intrasubband relaxation of electron energy in the Landau level system of quantum well as a function of magnetic field strength. The relative role of scattering processes contributing to relaxation kinetics, the electron-electron scattering which redistributes the electrons between Landau levels and the optical phonon emission by the electrons reaching the Landau levels lying near and above the optical phonon energy is revealed. It was shown the most important factor determining the kinetics of energy relaxation is the electron-electron scattering processes, delivering electrons to Landau levels close to optical phonon energy. The flux of these electrons depends on the number of the Landau levels lying below the energy of the optical phonon and increases substantially with a decrease in this number. On the contrary, the electron-phonon scattering rate dependence of the energy relaxation time was found to be much weaker.

Mechanism of energy relaxation in the system of Landau levels in quantum wells

The kinetics of intersubband relaxation of electron energy has been studied in the system of Landau levels lying below the optical phonon energy. The relaxation character in the considered system is revealed to differ qualitatively from that in the two-dimensional continuous subband of the quantum well. In particular, the mechanisms of electron subsystem thermalization and energy relaxation in the system of Landau levels are qualitatively different, and the electron subsystem relaxation time exceeds the thermalization time by several orders of magnitude.

Experimental investigation of plasma dynamics in dc and short-pulse magnetron discharges

The spatiotemporal evolution of the electron energy distribution function (EEDF) and of plasma parameters such as the electron density, the electron temperature and the plasma and floating potentials has been investigated using spatially and temporally resolved single Langmuir probe measurements in dc and mid-frequency, short-pulse magnetron discharges with a repetition frequency of 10 kHz and a duty cycle of 10%. In the pulsed discharge of the short duty cycle, a peak electron temperature higher than 10 eV was observed near the cathode fall region during the early phase of the pulse-on, which is about three times higher than the steady-state value of the electron temperature in the dc discharge. The temporal evolution of the measured EEDFs showed the initial efficient electron heating during the early phase of the pulse-on and the subsequent relaxation of electron energy by the inelastic collisions and the diffusive loss. The high-energy electrons generated during the pulse-on phase diffused the downstream region toward the grounded substrate, resulting in a bi-Maxwellian EEDF consisting of the background low-energy electrons and the high-energy electrons. The results of the spatially and temporally resolved probe measurements will be presented and the enhanced efficiency of the electron heating in the short-pulse discharge will be explained on the basis of the global model of a pulsed discharge.

Collisions and Langmuir waves in nonideal plasmas

Langmuir waves and particle-particle collisions in an equilibrium two-component plasma consisting of nondegenerate electrons and singly charged ions are studied by molecular dynamics simulation. Dispersion of frequency and damping rate is determined for Langmuir waves. A method is outlined for extending the theory of Langmuir waves developed for ideal plasmas to nonideal plasmas. Conductivity and dynamic collision frequency are determined as functions of perturbation frequency. Relaxation of electron and ion energy in nonequilibrium plasmas is analyzed.

Relaxation of electron energy in the polar semiconductor double quantum dots

Electron energy relaxation in the asymmetric double quantum dot system is calculated numerically using the material parameters of GaAs. The double dot consists of two tunneling coupled quantum dots each having a single nondegenerate electronic orbital. The effect of the intradot multiple scattering of the electron on the longitudinal optical phonons is considered. The inter-dot interaction is provided by the electronic tunneling mechanism. The dependence of the relaxation rate on the thickness of the tunneling barrier and on the temperature is calculated.

Relaxation of electron energy in the polar semiconductor double quantum dots

Electron energy relaxation in the asymmetric double quantum dot system is calculated numerically using the material parameters of GaAs. The double dot consists of two tunneling coupled quantum dots each having a single nondegenerate electronic orbital. The effect of the intradot multiple scattering of the single electron on the longitudinal optical phonons is considered. The interdot interaction is provided by the electronic tunneling mechanism. The dependence of the relaxation rate on the thickness of the tunneling barrier, on the temperature and on the electron energy-level separation is calculated.

Relaxation of Electron Energy in the Coupled Polar Semiconductor Quantum Dots

The zero-dimensional semiconductor nanostructures belong to the candidates for the realization of the quantum bits. They are expected to be scalable for the purpose of the tuning their physical properties. In these structures the quantum bit could be realized in the form of a single quantum dot with two electronic energy levels, with only one electron in the dot. As the basic states of the quantum bit, realized in this way, the two orbital states of the electron in the dot could be used. It appears however that usually the relaxation of the energy of the electron from the excited energy level is often rather fast in the polar semiconductor quantum dots. It is the purpose of this paper to present calculations of the relaxation rate of the electron in an asymetric pair of tunneling coupled quantum dots, in which the two electronic orbitals of the quantum bit are located each in a separate dot. The calculation of the electronic energy relaxation is based on the multiple electron-LO-phonon scattering processes, implemented to the theory via the electronic self-energy taken in the self-consistent Born approximation. The dependence of the relaxation rate on the geometry of the pair of the coupled dots and on the lattice temperature is presented for a realistic model of this nanostructure.

New mechanism of heterogeneous relaxation of electron energy in the active medium of an oxygen—iodine laser

A mechanism of relaxation of electron energy is proposed: it includes the transfer of electron excitation to the walls of a laser cell, fast resonant transfer of excitation from singlet oxygen to iodine atoms, and heterogeneous relaxation of excited iodine atoms on the walls. It is shown that the rate of heterogeneous relaxation of electron energy in an oxygen—iodine laser is comparable with the rate of relaxation in the gaseous phase. The effective probability of the heterogeneous relaxation of singlet oxygen can reach ~0.01.

Electron degradation and thermalization in CH4 gas

The relaxation to equilibrium of an ensemble of electrons dilutely dispersed in a large excess of CH4 is studied with solutions of the Boltzmann equation. Elastic and vibrationally inelastic collision processes are included in the analysis. The relaxation time for the approach to equilibrium defined for the relaxation of the average electron energy is determined for two different cross section sets. The kinetic theory formalism, based on the Boltzmann equation, is compared with the formalism used in radiation chemistry and physics and based on the Spencer–Fano equation.

Memory function approach to electron thermalization

A linear closed equation of the form ε̇(t)=ωε(t) +∫t0Λ(t−t′) ε (t′)dt′ is proposed to describe the nonlinear relaxation of electron energy ε(t) in the Lorentz gas. Higher-order memory functions associated with Λ(t) decay rapidly, and are easily amenable to approximation. Good numerical results for ε(t) suggest a useful extension of our memory function approach to many nonlinear problems for calculating various integral quantities.