Collisional Relaxation Research Articles

Laser-induced multiphoton and collisional ionizations in sodium-argon mixture

It is attractive to investigate the interaction mechanism between metal and rare gas atoms within ionization process, especially for the mixture of metal vapor and rare gas. In this paper, laser-induced breakdown spectroscopy is used to study the ionization and recombination kinetic processes in sodium-argon mixture. Sodium-argon excimer is formed by the atomic collision, and it plays an important role in the ionization process, sodium atoms on 3P state are produced after the dissociation of excimer, then multiphoton ionization happens, the electrons in plasma are accelerated to collide with argon atoms, electron-impact ionization of argon will take place, identified with time-resolved atomic emission spectrum. Population accumulate on 3D,4P, and 5S states after the recombination process of Na+ with electron, then most population will transit to 3P state mainly by collisional relaxation.

Potential energy surface of interaction of two diatomic molecules for air flows simulation at intermediate temperatures

The paper studies the potential energy surface (PES) of interaction between two rigid diatomic molecules (N2–N2, N2–O2, O2–O2). The dependence of energy on the angles of mutual orientation of the two molecules is carefully described (18000 DFT calculations for each pair) so that the obtained PES would be suitable for detailed simulation of the rotational excitation and relaxation in intermolecular collisions in air. The DFT results were fitted by proposed multi-particle interaction model with good accuracy. Corresponding force field was calculated and tested on two problems (simulation of the equilibrium state and the adiabatic rotational relaxation), its software implementation is attached to the paper and one can use it for molecular dynamics calculations.

An atomic-level study of the N2–N2 collision process at temperatures up to 2000 K

This article studies the mechanics of the N2–N2 collision process at temperatures up to 2000 K through an extensive set of classical trajectory calculations of binary collisions. It is found that key postcollision characteristics, namely, the deflection angle and the rotational–translational energy exchange rate, are significantly affected by precollision values of the rotational energies of the molecules, which is not addressed in commonly used collision models. On the macroscopic scale, such a behavior will lead to viscosity collision cross section and relaxation rate becoming dependent on both translational and rotational temperatures, as well as on the form of the nonequilibrium rotational energy distribution.

Investigation of dual-wavelength pump schemes for optically pumped rare gas lasers.

Optically pumped rare gas lasers (OPRGLs) have shown great potential to generate high energy laser radiation with high beam quality. As an alternative to the diode-pumped alkali vapor lasers (DPALs), they have similar working principles and characteristics, but OPRGLs have the advantage that the gain medium is chemically inert and is appropriate for closed-cycle operation. One of the challenges OPRGLs are faced with is the bottleneck caused by the slow 1s4-1s5 collisional relaxations at room temperature. A 1s4-2p10 dual-wavelength pump method had been proposed to transfer the populations pooled on the 1s4 level to the lasing cycle using a steady-state laser model. We explored this method further through 1s4-2p8 and 1s4-2p7 dual-wavelength pump schemes. The enhancement efficiencies at room temperature for a repetitively pulsed discharge, CW dual-wavelength pump system were examined using a dynamic model, and an experiment with a pulsed secondary pump was conducted for qualitative evaluations.

Analysis of the C2 Swan bands as a thermometric probe in CO2 microwave plasmas

The optical emission spectra of high pressure CO2 microwave plasmas are usually dominated by the C2 Swan bands. In this paper, the use of the C2 Swan bands for estimating the gas temperature in CO2 microwave plasmas is assessed. State by state fitting is employed to check the correctness of assuming a Boltzmann distribution for the rotational and vibrational distribution functions and, within statistical and systematic uncertainties, the C2 Swan band can be fitted accurately with a single temperature for rotational and vibrational levels. The processes leading to the production of the C2 molecule and particularly its state are briefly reviewed as well as collisional relaxation times of the latter. It is concluded that its rotational temperature can be associated to the gas temperature of the CO2 microwave plasma and the results are moreover cross-checked by adding a small amount of N2 in the discharge and measuring the CN violet band system. The 2.45 GHz plasma source is analyzed in the pressure range 180–925 mbar, for input microwave powers ranging from 0.9 to 3 kW and with gas flow rates of 5–100 l min−1. An intense C2 Swan bands emission spectrum can be measured only when the plasma is operated in contracted regime. A unique temperature of about 6000 ± 500 K is obtained for all investigated conditions. A spectroscopic database is constructed using the recent compilation and calculations by Brooke et al (2013 J. Quant. Spectrosc. Radiat. Transfer 124 11–20) of the line strengths and molecular constants for the C2 (d−a) Swan bands system and made available as supplementary material in a format compatible with the open source MassiveOES software.

Ultrafast collisional dissipation of symmetric-top molecules probed by rotational alignment echoes

We experimentally and theoretically investigate the ultrafast collisional dynamics of a symmetric-top molecule (${\mathrm{C}}_{2}{\mathrm{H}}_{6}$) in pure gas and mixtures with He at high density by employing the rotational alignment echo created by a pair of time-delayed intense laser kicks. The decrease of the amplitude of the echo when increasing the delay between the two laser pulses, reflecting the collisional relaxation of the system, is measured by probing the transient birefringence induced in the medium. The theoretical predictions, carried using purely classical molecular dynamics simulations, reproduce well the observed features, as demonstrated previously for a linear molecule. The analysis shows that the dissipation of the ethane alignment, despite the fact that this species has an extra rotational degree of freedom as compared to a linear molecule, barely involves more complex collisional relaxation channels due to characteristics of the ${\mathrm{C}}_{2}{\mathrm{H}}_{6}\ensuremath{-}{\mathrm{C}}_{2}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}\ensuremath{-}\mathrm{He}$ interactions. However, our findings reveal that the dissipative dynamics of a symmetric-top molecule can be properly approached using the recently discovered rotational alignment echoes, which, so far, have been only tested for probing rotational decoherence of simpler (linear) molecules.

Fast collisional electron heating and relaxation in thin foils driven by a circularly polarized ultraintense short-pulse laser

The creation of well-thermalized, hot and dense plasmas is attractive for warm dense matter studies. We investigate collisionally induced energy absorption of an ultraintense and ultrashort laser pulse in a solid copper target using particle-in-cell simulations. We find that, upon irradiation by a$2\times 10^{20}~\text{W}\,\text{cm}^{-2}$intensity, 60 fs duration, circularly polarized laser pulse, the electrons in the collisional simulation rapidly reach a well-thermalized distribution with${\sim}3.5~\text{keV}$temperature, while in the collisionless simulation the absorption is several orders of magnitude weaker. Circular polarization inhibits the generation of suprathermal electrons, while ensuring efficient bulk heating through inverse bremsstrahlung, a mechanism usually overlooked at relativistic laser intensity. An additional simulation, taking account of both collisional and field ionization, yields similar results: the bulk electrons are heated to${\sim}2.5~\text{keV}$, but with a somewhat lower degree of thermalization than in the pre-set, fixed-ionization case. The collisional absorption mechanism is found to be robust against variations in the laser parameters. At fixed laser pulse energy, increasing the pulse duration rather than the intensity leads to a higher electron temperature.

Ionization and acceleration of multiply charged gold ions in solid film irradiated by high intensity laser

In this paper, we present the mechanisms of ionization of a thin gold film irradiated by a high-intensity, short-pulse laser in the range of I=1020−22 W/cm2 and the associated acceleration of multiply charged gold ions. A numerical one-dimensional simulation using an extended particle-in-cell code, which includes atomic and collisional relaxation processes, indicates that two types of acceleration, hole-boring radiation pressure acceleration (RPA) and target normal sheath acceleration (TNSA), contribute to the generation of highly charged ions with kinetic energies on the order of 10 MeV/u. In each acceleration, a longitudinal electrostatic field excited by different mechanisms dominantly ionizes atoms to higher charge states and accelerates them to the vacuum region from the rear surface, which is opposite the front surface irradiated by the laser field. The field ionization process dominantly ionizes high energy ions to the high charge state, while a large number of ions with energy <1 MeV/u are ionized by an electron impact ionization process. In TNSA, a multiply charged ion generated at the rear surface is accelerated to the maximum energy although the ion with the highest charge state is generated at the front surface in RPA. However, the existence of contamination, such as water vapor, suppresses the ion energy of TNSA to less than that of RPA since the sheath field readily accelerates the protons and oxygen prior to the acceleration of the gold ions. Our derived theoretical scaling describes the maximum ion energy for each charge state in the cases with and without contamination using the relationship between the longitudinal electrostatic field profile near the rear surface and the classical tunnel field ionization model.

Striation phenomena in a low temperature atmospheric pressure neon plasma jet by optical emission spectroscopy

Striation phenomena in a plasma flare produced by an atmospheric pressure plasma jet had been observed. In this study, we measured the striation structure in a neon plasma flare by optical emission spectroscopy. As a result, it shows that the emission from the plasma flare is synchronized with the plasma current. We discuss the temperature estimated from the Boltzmann plot using the two emission lines of Ne I at 692.9 and 724.5 nm whose upper levels are Ne [3p (2p6)] and Ne [3p (2p10)], respectively. This temperature has peaks with a spatial interval of about 0.5 mm, and the peak positions correspond to the spatial variation of the emission intensity. We also discuss the mechanism of the striation phenomena with the estimated temperature, and collisional relaxation between excited and neutral atoms.

Anomalous Humidity Dependence in Photoacoustic Spectroscopy of CO Explained by Kinetic Cooling

Water affects the amplitude of photoacoustic signals from many gas phase molecules. In quartz-enhanced photoacoustic (QEPAS) measurements of CO excited at the fundamental vibrational resonance of CO, the photoacoustic signal decreases with increasing humidity, reaches a pronounced minimum at ~0.19%V, and increases with humidity for higher water contents. This peculiar trend is explained by competing endothermal and exothermal pathways of the vibrational relaxation of CO in N2 and H2O. Near-resonant vibrational–vibrational transfer from CO to N2, whose vibrational frequency is 188 cm−1 higher than in CO, consumes thermal energy, yielding a kinetic cooling effect. In contrast, vibrational relaxation via H2O is fast and exothermal, and hence counteracts kinetic cooling, explaining the observed trend. A detailed kinetic model for collisional relaxation of CO in N2 and H2O is presented. Simulations using rate constants obtained from literature were performed and compared to humidity dependent QEPAS experiments at varying pressure. Agreement between the experiments and simulations confirmed the validity of the model. The kinetic model can be used to identify optimized experimental conditions for sensing CO and can be readily adapted to include further collision partners.

Strong enhancement of electron-ion recombination induced by free-bound and bound-bound resonance transitions

SynopsisThe role of free-bound and bound-bound resonant nonadiabatic transitions on electron-ion recombination in plasma of noble gas mixtures is studied. A kinetic model of recombination is proposed which includes the energy relaxation in collisions with electrons, resonant and non-resonant electron capture to Rydberg states through collisions of atomic ions with neutral particles, and resonant n → n’ transitions. It is shown that the resonant processes considered may strongly enhance total recombination coefficient even at relatively high degrees of ionization of plasma.

The importance of cascade emission and metastable excitation in modeling strong atomic oxygen lines in laboratory plasmas

A collisional-radiative model for optically accessible atomic oxygen lines in laboratory plasmas has been developed to identify dominant mechanisms and assess the validity and utility for plasma diagnostics. A rate sensitivity analysis of the common 777 nm/844 nm line ratio was carried out and used to determine the importance of often ignored mechanisms including cascade emission and metastable excitation. Commonly used basic models for emission of these lines is compared to the model developed here and the mechanisms responsible for disagreement are identified. At low pressures metastable excitation from the state becomes important and at high pressures collisional relaxation of to is also non-negligible. At all pressures cascade emission from higher lying states was found to be crucial in accurately estimating the 777 nm/844 nm line ratio. Effective cascade excitation cross sections were calculated and used to include a cascade rate in the basic analytic models, which improves overall agreement with the collisional radiative model, particularly in the medium pressure range. Finally, the 777 nm/844 nm line ratio is calculated and presented for a variety of common laboratory plasma parameters.

Observing collisions beyond the secular approximation limit

Quantum coherence plays an essential role in diverse natural phenomena and technological applications. The unavoidable coupling of the quantum system to an uncontrolled environment incurs dissipation that is often described using the secular approximation. Here we probe the limit of this approximation in the rotational relaxation of molecules due to thermal collisions by using the laser-kicked molecular rotor as a model system. Specifically, rotational coherences in N2O gas (diluted in He) are created by two successive nonresonant short and intense laser pulses and probed by studying the change of amplitude of the rotational alignment echo with the gas density. By interrogating the system at the early stage of its collisional relaxation, we observe a significant variation of the dissipative influence of collisions with the time of appearance of the echo, featuring a decoherence process that is well reproduced by the nonsecular quantum master equation for modeling molecular collisions.

The Linearized Vlasov and Vlasov–Fokker–Planck Equations in a Uniform Magnetic Field

We study the linearized Vlasov equations and the linearized Vlasov-Fokker-Planck equations in the weakly collisional limit in a uniform magnetic field. In both cases, we consider periodic confinement and Maxwellian (or close to Maxwellian) backgrounds. In the collisionless case, for modes transverse to the magnetic field, we provide a precise decomposition into a countably infinite family of standing waves for each spatial mode. These are known as Bernstein modes in the physics literature, though the decomposition is not an obvious consequence of any existing arguments that we are aware of. We show that other modes undergo Landau damping. In the presence of collisions with collision frequency $\nu \ll 1$, we show that these modes undergo uniform-in-$\nu$ Landau damping and enhanced collisional relaxation at the time-scale $O(\nu^{-1/3})$. The modes transverse to the field are uniformly stable and exponentially thermalize on the time-scale $O(\nu^{-1})$. Most of the results are proved using Laplace transform analysis of the associated Volterra equations, whereas a simple case of Yan Guo's energy method for hypocoercivity of collision operators is applied for stability in the collisional case.

Narrowing of the Absorption Line of Light Alkali Metal Atoms in an Atmosphere of Heavy Inert Gases at Growing Radiation Intensity

The effect of narrowing of the absorption line of light alkali metal 7Li and 23Na atoms present in an atmosphere of heavy inert gas (xenon) with an increase in the external radiation intensity has been investigated. For 7Li atoms at temperature T = 300 K and buffer gas (Xe) pressure $${{p}_{{{\text{Xe}}}}} = 0.002$$ Torr, the linewidth at half maximum decreases by a factor of 1.20 with an increase in the radiation intensity from 1 µW/cm2 to 2.5 mW/cm2. For 23Na atoms at T = 600 K and $${{p}_{{{\text{Xe}}}}} = 0.01$$ Torr, the linewidth at half maximum decreases by a factor of 1.29 with an increase in the radiation intensity from 1 µW/cm2 to 6 mW/cm2. The effect of field narrowing of the absorption line is due to the following factors. First, the collisional relaxation of the velocities of light resonance particles in an atmosphere of heavy buffer particles is divided into two stages with significantly different durations: relaxation in the velocity direction (fast stage) and relaxation in the velocity magnitude (slow stage). Second, there are no collisional transitions between hyperfine components of the ground state.

CO2 IR radiation modelling with a multi-temperature approach in flows under vibrational nonequilibrium

The aim of this paper is to develop a line by line model for CO2 vibrational nonequilibrium radiation and to investigate nonequilibrium effects in the case of some applications involving high temperature expanding flows. A vibrational specific collisional relaxation model is developed and is incorporated in a multi-temperature thermodynamic description of the gas mixture, in order to compute vibrational level populations along the flow. The HITEMP-2010 spectroscopic database is employed with a model for level energy splitting to provide line by line absorption and emission total and per-vibrational mode specific spectra. The specific spectra allow us to derive the radiative source terms to be used in the multi-temperature model if a coupled approach is required. The model is applied first to a simple conical nozzle flow and then to the plume of a high altitude solid propellant rocket engine. It is shown that, for the considered applications, the partial freeze of vibrational excitation in the expanding flow increases significantly the radiative intensity escaping from the mixture.

Sub-Doppler Spectroscopy of Excited Atoms in a Multilayer Gas Cell

The potential for spectroscopy in a multilayer cell with rarefied gas medium is theoretically investigated where said cell can be used as a compact analog of a large number of plane-parallel beams of atoms (or molecules) in an optically excited quantum level. In particular, this situation occurs for metastable and highly excited long-lived Rydberg states of atoms and molecules. Velocity selection of atoms (molecules) excited to said quantum level by broadband optical pumping was found to be efficient because of their characteristic transit and collisional relaxation in said cell. Sub-Doppler resonances at the optical transition frequencies from a given level were narrow as a result of the selection when the monochromatic probe light beam was absorbed.

Structure of Butyl Carbamate and of Its Water Complex in the Gas Phase.

The structure of butyl carbamate and of its complex with water generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. Up to 13 low-energy conformations of the monomer have been predicted that differ in the relative orientation of the butyl chain and the amide group. However, only three conformations have been observed experimentally. The remaining low-energy conformers are expected to interconvert into the observed rotamers through collisional relaxation processes in the supersonic jet. The values of the C-O-Cα-Cβ dihedral angle observed for the two most stable conformers of butyl carbamate, with extended configurations, can be directly correlated with the values of this angle in the two experimentally observed conformers of the shorter-chain molecule, ethyl carbamate. The less stable form shows a weak C-H···O═C intramolecular hydrogen bond from the terminal methyl group to the carbamate C═O group, stabilizing a folded configuration. For the most stable butyl carbamate monomer the complex with one molecule of water has been observed. In that complex the water molecule attaches to the amide group in a cyclic arrangement using two hydrogen bonds. The results indicate that water does not substantially alter the conformational behavior of butyl carbamate.

Collisional relaxation of a strongly magnetized ion-electron plasma

The relaxation of an ion-electron plasma due to collisions, in a uniform background magnetic field, is studied. Based on the recently found ion velocity transfer by a binary collision, the diffusion coefficients are derived. The Fokker-Planck equation is treated in the framework of the diffusion approximation, from which the drag term is extracted. The diffusion and drag coefficients are written appropriately in parallel and perpendicular components, relative to the direction of the magnetic field. Then, the relaxation times of the ion parallel and perpendicular temperatures are calculated. Finally, the energy loss rate of a test ion moving in an electron plasma is obtained, and a specific case in the conditions of the ALPHA experiment is presented.

Polyatomic degrees of freedom and their temporal evolution extracted from the damping of micro-oscillators

This paper investigates for the first time the influence of molecular degrees of freedom (DOF) on the damping behavior of micro-oscillators in the molecular flow regime. None of the existing theories of damping does take this aspect into account. The damping of piezoelectrically driven bending oscillators is determined by measuring the pressure dependent quality factor Q in an atmosphere consisting of noble gases or polyatomic gases. A neighboring plate with an adjustable gap width ranging from 150 to 3500 μm was mounted above the oscillator, limiting the time of flight of the gas molecules in between the delimitation to several microseconds. Taking the behavior of the noble gases with three translational DOF as a reference, the development of additional rotational and vibrational DOF of polyatomic gas molecules is determined as a function of the gap width, which can be correlated to the time of flight. The determined relaxation times and collision numbers of the DOF are compared to existing theories and to experimental results explored by other methods. Based on the experimental evidence, additions to the existing molecular damping theory are derived by including the higher DOF and adapting an existing thermodynamic excitation model.