Collisional Transitions Research Articles

Effect of collisional transitions on the population distributions of excited atoms in a plasma

The effect of collisional transitions on the population distribution of excited atoms has been studied by considering a plasma in which the electron density varies with time. It has been found that for highly excited states, for which the ionization rate is much larger than that of electron-density variations, the collisional transitions are not important compared with the collisional ionization and recombination processes. However, for intermediate and lower excited states, they play an important role in making the population distribution approach Saha equilibrium with the electrons.

Collisional ionization in the presence of intense laser radiation: Quantum-mechanical calculations for He* (3S)+Ar±h/ω→He+Ar+(2P)+e−

We present a quantum-mechanical coupled-channels approach for studying laser-assisted collisional processes leading to transitions between discrete and continuum electronic states. This coupled-channels approach is an approximate application of our more general quantum-mechanical formalism for treating bound-continuum collisional transitions assisted by intense laser radiation, and uses discretization procedures to deal with the continuum states, and so-called electronic-field representations to account for the presence of intense laser fields. We discuss in detail all approximations leading to our coupled-channels approach. We then use our approach to calculate laser-assisted Penning ionization of Ar by He* (1s2s,3S) for the specific cases of radiation from the 10.6 μm line of a CO2 laser of intensity near 1.6×108 W/cm2, and the 1.315 μm line of an iodine laser of intensity near 109 W/cm2. We obtain ionization cross sections (per unit emitted-electron energy ε) dσ/dε as a function of ε. dσ/dε reflects a spectrum of emitted-electron energies that exhibits pronounced laser-modified collisional effects. The usual field-free spectral region around ε0(?0.149 hartree) is augmented by new spectral regions governed by radiative coupling and located near ε+(−)?ε0+(−) h/ω, where h/ω is the laser’s photon energy. Compared to their field-free counterparts, these laser-modified spectral contributions are more sharply peaked and favor Penning ionization over associative ionization more strongly. In carrying out these calculations we treat the radiative coupling in the dipole approximation. By analyzing this radiative coupling for He* (1s2s,3S)+Ar Penning ionization, we show that it rigorously vanishes asymptotically. We use this fact to propose a physically reasonable functional form for such radiative coupling between bound and continuum electronic states.

The role of H + OH collisions in pumping the OH 18 centimeter maser lines

Cross sections, rate constants, and the steady-state distribution of the various OH sublevels are computed for collisions between H atoms and OH molecules. The results obtained demonstrate that H atoms preferentially excite the upper Lambda levels of the j = 5/2 manifold and that collisional transitions within the j = 3/2 manifold, although much faster than the rotational excitation rates, do not completely equilibrate the nonthermal state distribution induced by radiative decay from the j = 5/2 levels. This phenomenon is shown to be quite general and to exist for a large range of H atom densities and kinetic temperatures.

Two-photon transition in a colliding particle system

The first analysis is made of stimulated Raman scattering and two-photon absorption (emission) processes resulting in a change in the states of two colliding particles. These processes are caused the by interaction between dipole moments induced as a result of polarization of the particles by the radiation. Equations are derived to describe multiphoton transitions during a collision between two particles in a radiation field. Using the approximation of rectilinear trajectories, expressions are derived and analyzed for the cross section of collisional two-photon transitions in weak and strong fields. It is shown that in fields of ≈103 cgs esu, the cross section of these transitions may reach 10–4 cm2

Systematic trends in the inelastic cross sections and form factors for nl→n'l' direct collisional transitions

Certain theoretical predictions are presented for the preferential population of final states with angular momentum l' in collisions involving an initially excited atom. Varying l', the authors find that the maxima of both the inelastic form factors and cross sections for the nl to n'l' transitions in hydrogen, induced by collision with electrons and heavy particles, in general oscillate on a background which rises as l' is increased, until they both attain a pronounced peak at a unique value l'max which is strongly dependent on only the initial principal quantum number n and which is fairly insensitive to changes in l and n'. An expression for l'max is derived. For l'>l'max, the form factors and associated cross sections exhibit a dramatic decline, resulting in negligible population of those states. The predictions differ from those suggested by the Bethe high-energy asymptotic limit which favours dipole transitions, and assume significance in situations where excited states are important.

Stochastic theory of collisional energy transfer: nature of convergence of master equation transition probabilities and moments as a function of cumulant expansion order

Closed form expressions for collisional energy transfer transition probabilities and moments are determined for the forced oscillator model by solving cumulant expansion master equation in second, fourth, sixth and infinite order. This enables a study of convergence properties of the cumulant expansion.

Density-Dependent Lines of One- and Two-Electron Ions in Diagnostics of Laboratory Plasma. II. Intensity Line Ratios of Hydrogenlike, Heliumlike and Oxygenlike Multicharged Ions

Density-dependent spectral lines play an important role in the plasma physics research. Such lines can be used for determination of plasma density from the ratio of spectral line intensities. In the preceding paper [1] we have considered the inelastic collisional transitions between close ion levels which are responsible for excited state distribution when it changes from coronal model at low electron densities up to Boltzman distribution at high electron densities. This variation is the reason of the density dependence of spectral line intensity ratios.In this paper basing on the results obtained in [1] the ratios of several spectral lines of H, He and O-like ions are plotted as a function of electron density. The regions of the plasma parameters for which this ratio may be used as a diagnostics tool are found.The results of this work makes it possible to diagnose plasmas (laser and electron beam plasma, exploding wires, plasma focus etc.) for electron densities 1018 < Ne < 1023cm-3.

Excitation equilibrium for low lying levels in Cii, Niii, Oiv, Nevi, Mgviii, Six, and Siii

The populations of the excited state 2P3/2 relative to the ground state 2P1/2 have been investigated in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii by considering all the radiative and collisional processes including the collisional transitions to the higher states which cascade to the upper level. The relative populations are used for the calculation of the line emissivities. The intensities of 76 320, 30 258 and 14 302 lines of Nevi, Mgviii, and Six ions respectively in the chromosphere-corona transition region are also calculated.

Collisional transitions obeying nondipolar selection rules between M levels of FCN, a laser-microwave double-resonance study

Two Stark components of the R (2) transition of the ν1 band of FCN were tuned into resonance with the R (20) 9.4-μm CO2 laser line. From consideration of the collisionally induced double-resonance signals observed in the microwave spectrum, it was concluded that ΔJ=0, ΔM=±1 nondipolar collisional transitions occur with a frequency similar to that of the dipole-allowed ΔJ=±1, ΔM=±1 collisional transitions.

Recombination lasers utilizing vapors of chemical elements. II. Laser action due to transitions in metal ions

General requirements specifying the distribution of energy levels and discharge conditions are used as the basis of a discussion of the stimulated emission mechanisms in ionic recombination lasers utilizing strontium, calcium, beryllium, aluminum, tin, and lead vapors. The attention is concentrated on the strontium vapor laser. The population inversion which results in the emission of λ=430.5 nm radiation from Sr II is calculated allowing for radiative and collisional transitions between the levels. The role of deexciting collisions with slow electrons in the depopulation of the lower laser level is demonstrated. The results are given of theoretical and experimental determination of the electron temperature in the afterglow of gas discharges in mixtures of metal vapors with inert gases. The electron temperature is found to rise with the metal vapor pressure and this effect is explained. Average powers of 0.5 W at λ=373.7 nm from Ca II and 1 W at λ=430.5 nm from Sr II at the pulse repetition frequency 5 kHz and efficiencies of 0.1–0.14% are reported. The distributions of the active levels of new recombination laser transitions are considered briefly for beryllium, aluminum, tin, and lead ions.

Theory of thermal unimolecular reactions at low pressures. I. Solutions of the master equation

The master equation for a thermal unimolecular reaction in gases at low pressures is formulated. Steady-state solutions are derived in analytical form with an exponential model of collisional transition probabilities, (i) for vibrational energy transfer in molecules with variable densities of states, and (ii) for combined rotational and vibrational energy transfer in molecules with variable heights of the centrifugal barriers. Other models of transition probabilities are treated numerically. The diffusion limit of energy transfer is discussed. In all cases, the nonequilibrium populations of excited states and the weak collision efficiency factors βc are calculated.

Measurement of the collisional transition rate between the hydrogen levels n = 3 and n = 4

The ratio of the collisional transition rate C 43 and the total loss rate V 4 of the level n = 4 has been measured by means of dye-laser fluorescence in a rf hydrogen plasma. In a further experiment using a nitrogen-laser-pumped dye-laser (with pulses short compared to the life time of the n = 4 level), the total loss rate V 4 of this level has been determined by observing the exponential intensity decay of the H β line. From the results of these two experiments, the excitation rate C 43 is obtained. Agreement with theoretical values is found.

Time-resolved fluorescence decay and collisional transition rates of Cs-Xe molecules

The kinetics of formation and decay of Cs-Xe molecules in their $\ensuremath{\Sigma}$ and $\ensuremath{\Pi}$ bound states produced in Cs-Xe mixtures irradiated with a short-pulse dye laser tuned around the $7P\ensuremath{-}6S \mathrm{Cs}$ resonance transition has been investigated by fluorescence decay measurements. The time-resolved spectra of the 4600- and 5720-\AA{} molecular fluorescent bands were studied in relation to xenon density, which ranged from 0.9 to 6.9 amagats. Quantitative collisional cross sections are derived from the measured transition rates with peak values around ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$.

Calculation of the rotational transition probabilities of diatomic molecules when they collide with heavy particles

The collisional rotational transition probabilities for molecule-molecule and molecule-atom interactions in three-dimensional space are calculated. The quasiclassical approach developed in [1] is used. Expressions are obtained that are suitable for practical calculations of single-quantum and double-quantum rotational transitions in diatomic molecules. The collisional rotational transition probabilities are averaged over the Maxwell velocity distribution and their dependence on the gas temperature is obtained. To illustrate the method the results of a calculation of the probabilities for HCl-HCl, HCl-He, CO-CO interaction are presented.

Infrared-microwave double resonance in CF 3I; Microwave spectroscopy in emission and absorption

From the double resonance effects observed on the microwave spectrum of CF 3I it has been shown that the R(16) CO 2 laser line of the 9.4 μm band is coincident with the R(7), K = 2, F = 19 2 → 21 2 transition of the CF 3, symmetric stretch ( ν 1 band) of CF 3I. Using this laser line, 50 double resonance signals all with K = 2 were observed ranging from J = 4 → 5 to J = 12 → 13 transition. The fact that double resonance effects were observable over such a large range of J was explained as being caused by very strong Δ J = ± 1, Δ K = 0 collisional transitions. Extremely large pumping effects were produced using 6 W of laser radiation, which caused relative changes in intensity ( ΔI I ) in ground state lines of up to 25. The population transfer into the excited state was so large that many excited state lines, which had previously been undetectable, produced signals up to 30 times more intense than the corresponding undisturbed ground state lines (i.e., values of ΔI I of ∼6000 were achieved). Population inversions were produced by the laser pump in many of the K = 2 microwave transitions, not only in those with levels directly pumped by the laser but also in some connected only by collisional transitions. The results was that many of the signals were observed as stimulated emissions rather than absorptions. The rotational constant and quadrupole coupling constants of CF 3I in the v 1 excited state are calculated and an estimation of the center of the ν 1 band is made. The absolute population shifts produced by the laser pump are estimated and the rate constants of the collisional transitions are discussed.

Collisional-radiative recombination in cold plasmas

We have investigated the electron-ion three-body collisional-radiative recombination for electron temperatures below 4000 \ifmmode^\circ\else\textdegree\fi{}K using the Mansbach-Keck rates of electron-impact-induced transitions between hydrogenic energy levels of high principal quantum number. At such temperatures and for a wide range of electron densities, the statistical recombination process is simultaneously governed by collisional and radiative transitions between excited levels near the ionization limit, where many atomic and molecular systems possess a hydrogenic energy-level structure. In order to also take into account radiative transitions, we have numerically solved a system of coupled equations, describing the quasi-steady-state populations of 100 bound levels. These equations are expressed in, and solved for, the first differences between the reduced population densities $\ensuremath{\rho}(p)$, which improves the computing precision and establishes the location of the bottleneck in the recombination sequence. Our results are consistent with the following approximation for the collisional-radiative recombination rate coefficient (in ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$): ${\ensuremath{\alpha}}_{\mathrm{cr}}=1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{T}^{\ensuremath{-}0.63}+6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{T}^{\ensuremath{-}2.18}{[e]}^{0.37}+3.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{T}^{\ensuremath{-}4.5}[e]$, where the electron temperature $T$ is in \ifmmode^\circ\else\textdegree\fi{}K and the electron density [$e$] is in ${\mathrm{cm}}^{\ensuremath{-}3}$; the first and last terms describe purely radiative and collisional recombination, respectively, and the second term results from the complex interplay of collisional and radiative processes. Agreement with experimental data is reasonable.

Rotational relaxation in a line-selected continuous HF chemical laser

Rotational relaxation effects in a cw chemical laser amplifier or a line-selected oscillator are examined in terms of a simple analytical model. Three rotational levels in each vibrational level of the lasing transitionare taken to have finite rotational relaxation rates. The results.indicate the effects are likely to be significant, lowering the available power and leading to an optimum level of the line intensity for maximum power output, apart from consideration of optical losses at mirrors, windows, etc. While the model accounts for Deltaj = +/-1 transitions only, the collisional transition probability was augmented in the computations in an attempt to provide high estimates for the rotational relaxation rates, including factors such as Deltaj > 1 transitions.

Relaxation phenomena associated with microwave pumping of rotational transitions

It is shown that the Bloch equations describing a two-level system in a radiation field can be extended to take proper account of the degenerate magnetic sublevels. By solving the equation for a J = 0 to J = 1 transition in a linear rotor we show that the saturated linewidth depends on the realignment time T 4 as well as on the usual T 1 and T 2. Further we show that the ratio T 1 */ T 2 which enters the saturated linewidth is not exactly unity, and that its deviation from unity is related to the rate of collisional transitions between the two pumped levels. Thus it may be possible to measure collision rates of astrophysical interest which could not be obtained in any other way. Some approximate formulae are derived assuming that only long-range interactions are present.

Molecule-dust correlations in dark cloud Khavtassi 3

view Abstract Citations (24) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecule-dust correlations in dark cloud Khavtassi 3. Myers, P. C. Abstract Observations of microwave spectral-line transitions of H2CO, OH, and H I toward the dark cloud Kh 3 are reported which were made to determine quantitative relations among grain density, gas density, and gas temperature. For each species, correlations between excitation temperature and red extinction and between column density and red extinction are observed and discussed in terms of coupling parameters. Abundance and collisional transition models are used to predict values of the coupling parameters which are compared for consistency with the observations. The abundance prediction for H2CO based on ion-molecule reactions is found to be more consistent than that based on molecule formation on grain surfaces. The coupling of cloud temperature to extinction is found to vary significantly for the different species due to variations in spatial extent or model inaccuracies. Assumption of the same temperature coupling for each species is shown to yield the best results when the temperature does not exceed zero, indicating either an isothermal or core-cooled cloud. In this case, the results imply strong coupling of H2CO to extinction, moderate to zero coupling of H I, and zero to negative coupling of OH. Publication: The Astrophysical Journal Pub Date: June 1975 DOI: 10.1086/153608 Bibcode: 1975ApJ...198..331M Keywords: Abundance; Cosmic Dust; Interstellar Extinction; Interstellar Matter; Molecular Spectra; Nebulae; Absorption Spectra; Astronomical Models; Hydrogen Clouds; Hydrogen Ions; Line Spectra; Astrophysics full text sources ADS | data products SIMBAD (4)

Collisional transition probability in gaseous and liquid carbontetrachloride

In gaseous CCl4 at 20 to 30 °C, the probability that a binary collision effects an energy transfer between the translational and the vibrational degrees of freedom is 3.8 · 10−3, in liquid CC14 at the same temperature it is close to 2.6 · 10−3. The ratio of these experimental values corresponds exactly to that predicted from the theoretical relationship:P liquid/Pgas = (1 +S η/T) · exp(−ɛ/kT).