Collisional Redistribution Research Articles

Kinetics of thermalization in shock waves

The recently developed interlaced system which can be employed for the numerical calculation of flow problems with arbitrary cell-Knudsen numbers, Knc=λ/Δx (λ=mean free path length, Δx=typical computational cell dimension), has been extended by introducing a Lennard–Jones (LJ) interaction potential. The scattering behavior of the LJ-model is quite different from that of the previously used hard-sphere (HS) model: The HS-model has a symmetric dipole-type scattering lobe, whereas the scattering lobe of the LJ-model shows a strong forward scattering, due to the large cross sections for low energy collisions. The influence of this scattering behavior on the collisional loss frequency ν and the gain function fg=G/ν (with G being the gain rate) in the kinetic equation is considerable and may be roughly stated as follows: For the HS-model the loss rate is small but the collisional redistribution is very efficient, for the LJ-model the situation is reversed. The kinetics of thermalization controlled by the loss and gain terms is studied, in a first step, using a pseudo-shock (with space homogeneity). In a second step, the structure of normal shock waves is calculated by using both models (HS; LJ for helium and argon). Comparison of density profiles with existing shock tube experiments show fair agreement for the LJ-model but too small shock thickness for the HS-model. Results for the stress term σxx and the heat flux qx are in good agreement with simple estimates for large Mach numbers, M1≫1, where we find, for the extrema of these moments, that |σxx|∼M12 and |qx|∼M13. The complex interaction of loss and gain terms and their dependence on the energy of the interacting particles has some influence on the shock thickness. We find that, with decreasing upstream temperature T1, also the shock wave thickness decreases, while the minima of σxx and qx practically remain unchanged. When testing the collisional terms used in the BGK-model, with a constant loss term ν and a gain function fg=fM (M=Maxwellian), we find the typical upstream tail in the density and temperature profiles, as it was calculated before.

Evidence of collisional coherences in the transport of hydrogenic krypton through amorphous carbon foils

We study theoretically and experimentally the population dynamics of the internal state of 60 MeV/u Kr 35+ ions traversing amorphous carbon foils. A quantum transport theory is developed that incorporates the state mixing induced by the wake field of the ion as well as all the coherences generated by the collisional and radiative redistribution of states. We show that the internal state of the ion is sensitive to collisional coherences and the wake field. The results of the full simulations are found to be in good agreement with experimental data.

Quantum transport of the internal state ofKr35+ions through amorphous carbon foils

The population dynamics of the internal state of 60 MeV/u ${\mathrm{Kr}}^{35+}$ ions traversing amorphous carbon foils is studied theoretically and experimentally. This system is of particular interest as the times scales for collisional distribution, mixing due to the wake field, and radiative processes are comparable to each other. A transport theory based on a quantum-trajectory Monte Carlo method is developed, which treats the collisional and radiative redistribution of states on the same footing. The simulations exhibit clear signatures for the interplay between radiative decay and collisional mixing. Good agreement with experimental data is found.

Radiative Decay of the 4d9 and 4d94f1 Configurations in Ba2+ and Neutral Barium

An novel interpretation of Ba 4d spectrum observed in BaF2 excited by a discrete photon radiation is proposed, based on a model of collisional redistribution following a first order excitation process. In the case of Ba2+ and neutral barium, we performed MCDF calculations of the transitions related to the 4d94f1 excited and 4d9 ionized configurations, by considering the entire population of the J-levels of the 4d94f1 configuration statistically redistributed. Good agreement is obtained between the Ba2+ simulated spectrum and the experimental one observed following excitation by photons of 4d94f1 3D1 energy. This underlines the applicability of our model for cases involving an interaction of an excited level with a continuum, or with other excited levels.

Infrared, Far‐Infrared, and Maser Emission from the Nascent Water Formed in the OH − H2Reaction

It is known that most water in the interstellar medium is formed in the OH + H2 reaction. It is also known that the newly formed water molecules from this reaction are born in highly excited vibrational and rotational states. Even at densities of 109 cm-3 in the interstellar medium these states relax in a fast IR radiation cascade to the ground state(s) of water emitting IR, far-infrared (FIR), and microwave light. Because this effect has not been included in the astrophysical literature before, we analyze here the intensity of this spontaneous emission at 6 μm, in the FIR and the microwave region. We calculate stationary rotational state distributions using a quantitative model that includes the nascent formation, the subsequent spontaneous emission, photolysis, and collisional redistribution. The radiation cascade yields FIR radiation and an alternative pump mechanism for water masers. For regions of massive water formation it is shown that the stationary rotational population is much less affected by collisions than by the nascent water formation. It is speculated that nascent reactive water might be responsible for the near-IR spectra at 6 μm, for the pure rotational transitions in the FIR radiation, and for the H2O maser radiation.

The profile of the hydrogen Hβ emission line in proton aurora

Models of hydrogen‐proton transport in proton aurora predict the line profile of the hydrogen emissions from specified incident proton precipitation. We are using a model that includes collisional angular redistribution which leads to upward moving proton and hydrogen fluxes. For ground‐based observation in the magnetic zenith this causes a small Doppler broadening toward the red in the line profile. The precipitating energetic hydrogen atoms are responsible for the prominent Doppler shift toward the blue. The resulting line profile has thus both a widened red and a widened blue wing. Using a spectrometer with sufficient spectral resolution to distinguish the red‐shifted wing of the line from the instrumental line broadening, we obtain Hβ line profiles (486.1 nm). Comparing the predicted line shapes to our observations, we find the red‐shifted wing is due to upward moving hydrogen as predicted by the angular redistribution in the model calculations. The shape of the blue‐shifted wing, rather than the location of the peak of the blue‐shifted line profile, is a suitable indicator of the mean energy of the precipitating proton flux.

Magnetic mirroring in an incident proton beam

We point out that the influence of magnetic‐field nonuniformity on redirecting the pitch angle of a particle is independent of the particle's charge and thus is identical for protons and neutral hydrogen atoms. Under certain circumstances one can then speak of “magnetic mirroring” of hydrogen atoms as well as of protons. In the case of an energetic proton beam incident on the upper atmosphere, the study of the influence of magnetic field on both protons and H atoms can be relevant to inferring information about proton aurora from measurements of upgoing energetic particles observed from space. In a model that here neglects collisional angular redistribution of the particles, the total particle and energy albedos are approximately independent of the energy of the incident particles and of the atmospheric temperature. However, the separate proton and H atom albedos have a strong dependence on the incident energy. We also reinvestigate how to handle energy conservation properly in the presence of a nonuniform magnetic field, to provide a good validation for proton transport models.

ICRF heating scenarios for the IGNITOR machine

A large frequency range ICRF antenna (70< f<140 MHz) is proposed for the IGNITOR machine, in order to allow either hydrogen or helium-3 minority heating for all the planned operating scenarios. Results from a feasibility study for such a heating system are presented. The power absorbed by the different plasma species as a function of plasma parameters for different heating scenarios was calculated using both 1-dimensional and 2-dimensional kinetic wave codes. Different alternatives are proposed in order to improve heating performances while accounting for the effects of collisional redistribution of the absorbed power. Wave absorption is predicted to be very strong for a number of different RF scenarios. Single pass absorption by alpha particles, as given by a 1-dimensional integral kinetic wave code, is also discussed.

Collisionally promotednear-infrared excitation of green triplet emission in NaK molecules

A collisionally promoted green emission band appearing in mixed sodium-potassium vapour is observed when the vapour is irradiated with a broadband continuous-wave laser at wavelengths around 780 nm. Previously not reported bound-bound transitions from the 3 and 5 + states down to a + in NaK molecules are found to be the origin of the emission. By fitting contributions from the individual vibrational states to the shape of the fluorescence band the Te for 5 + and 3 are estimated as 24 770 and 24 630 cm-1, respectively. Calculations of two-photon excitation spectra, resonantly enhanced by the A + state, show that the triplet manifold can be accessed from the 3 state after collisional vibrational redistribution to reach gateway levels perturbed by 5 +. Additional collisional redistribution transfers a fraction of this population to the 3 state. This seems to be the first time a collisionally promoted all-molecular one-colour scheme to excite triplet emission in heteronuclear alkali diatomic molecules with wavelengths shorter than that of the excitation light is reported and extensively analysed, a process of potential interest in the construction of new laser types.

Superfluorescence from optically trapped calcium atoms

We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 \ensuremath{\mu}m were generated on the Ca $4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}$ transition from a column of calcium vapor buffered with Ar by optically pumping the ${4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}$ transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.

Quantum close-coupling calculations of collisional redistribution of light for Ba(1P 1S) perturbed by helium

Quantum close-coupling calculations of collisional redistribution of light for Ba(1P 1S) perturbed by helium

Quantum close-coupling calculations of collisional redistribution of light for Ba(1P ← 1S) perturbed by helium

Cross-sections for optical collisions of the barium atom with He are calculated for a range of detunings around the atomic resonance. Results are obtained from fully quantum mechanical coupled-channels calculations including the ground and two excited molecular 1σ and 1II states. Close-coupling calculations are carried out using the theoretical potential curves for Ba—He, obtained by means of a pseudopotential SCF-CI technique (Czuchaj, E., Rebentrost, F., Stoll, H., and Preuss, H., 1995, Chem. Phys., 196, 37). For accurate comparison with the experiment (Ni, S. Y., Goetz, W., Meijer, H. A. J., and Andersen, N., 1996, Z. Phys. D, 38, 303), the calculated absorption profile and polarizations have been thermally averaged over the collision energy. The theoretical absorption coefficient and linear polarization ratio agree quite well with the experimental data.

Theoretical study of collisional redistribution of light near the resonance of the Ba, Sr and Mg atoms perturbed by He and Ne

Absorption coefficient and polarization of collisionally redistributed fluorescence light in a range of detunings around the atomic resonance have been calculated for Ba, Sr and Mg perturbed by He and Ne. Results are obtained from fully-quantum mechanical coupled-channels calculations including the relevant ground and two excited and molecular states for each diatomic. Close-coupling calculations are carried out based on the theoretical potential curves obtained by means of a pseudopotential + valence configuration-interaction (CI) technique. For accurate comparison with experiment the calculated absorption coefficients and polarizations have been thermally averaged over the collision energy. The theoretical absorption profiles and linear polarization ratios agree, in general, quite well with the available experimental data.

Bounce-averaged Fokker-Planck code for the description of ECRH in a periodic magnetic field

A bounce-averaged Fokker-Planck code has been developed for studying electron cyclotron resonance heating of fusion plasmas in a periodic magnetic field, where several classes of trapped particles exist. Such a problem is of relevance, e.g., for the analysis of heating close to the plasma axis in modular Stellarators. The main characteristic of the code is the capability of describing the collisional redistribution between the different classes of electrons of the particle, momentum and energy fluxes driven by the external heating. The solution for the distribution function is obtained by means of a conservative, finite difference, two-step operator splitting scheme.

Detection of titanium in electrothermal atomizers by laser-induced fluorescence. Part 1. Determination of optimum excitation and detection wavelengths

A detailed investigation has been performed of suitable excitation and detection wavelengths of Ti for the technique of laser-induced fluorescence in electrothermal atomizers. Fluorescence spectra (most often in the 250–350 nm region) from the 39 excitation wavelengths conjectured to be the most important (in the 220–270 nm region) have been investigated in more detail by the use of an intensified CCD detector. The fluorescence spectra were found to have a rich occurrence of peaks (about 50 each), of which many cannot be found in existing atomic wavelength data in the literature. All of the peaks found (with one exception) could be identified thanks to a home-made program that calculates atomic wavelengths from existing energy level data. Most of the spectra are dominated by “indirect” transitions (i.e. fluorescence from an upper level that is different from the one accessed by the laser) despite the prediction of existing “direct” transitions. This indicates that the collisional redistribution processes the excited levels, in general, are faster than typical fluorescence rates for Ti in graphite furnaces. Suitable excitation and detection wavelength combinations are given. One such suitable choice is excitation at 264.662 nm since the fluorescence following this excitation consisted of peaks of almost equal magnitude in three different wavelength regions (around 295, 319 and 338 nm).

Detection of titanium in electrothermal atomizers by laser-induced fluorescence. Part 1. Determination of optimum excitation and detection wavelengths

A detailed investigation has been performed of suitable excitation and detection wavelengths of Ti for the technique of laser-induced fluorescence in electrothermal atomizers. Fluorescence spectra (most often in the 250–350 nm region) from the 39 excitation wavelengths conjectured to be the most important (in the 220–270 nm region) have been investigated in more detail by the use of an intensified CCD detector. The fluorescence spectra were found to have a rich occurrence of peaks (about 50 each), of which many cannot be found in existing atomic wavelength data in the literature. All of the peaks found (with one exception) could be identified thanks to a home-made program that calculates atomic wavelengths from existing energy level data. Most of the spectra are dominated by “indirect” transitions (i.e. fluorescence from an upper level that is different from the one accessed by the laser) despite the prediction of existing “direct” transitions. This indicates that the collisional redistribution processes the excited levels, in general, are faster than typical fluorescence rates for Ti in graphite furnaces. Suitable excitation and detection wavelength combinations are given. One such suitable choice is excitation at 264.662 nm since the fluorescence following this excitation consisted of peaks of almost equal magnitude in three different wavelength regions (around 295, 319 and 338 nm).

Far-wing collisional redistribution of light in the barium-rare gas systems

We have measured intensity and polarization of far-wing collisionally redistributed light from Ba perturbed by He, Ne, and Kr in a heated cell (T∼800 K). Collisional alignment decay rates were also determined. The light was detuned ±3-900 cm-1 from the 5535 As BaI 61S-61P resonance line. The rare gas pressure was about 5 mbar. Together with the earlier data of Alford et al. [24], the results form a complete set of data for comparison with forthcoming predictions based on stateof- the-art theory for potential curves of Ba-rare gas systems. With emphasis on systematical trends, the results are discussed in terms of earlier theoretical and experimental findings for similar two-electron systems.

Selective rovibrational energy transfer: A classical trajectory study of collisional energy redistribution in methyl radical

Abstract A classical trajectory study was performed on planar CH 3 to investigate the cause of anomalous experimentally observed state selectivity in the collisional deactivation of CH 4 . Despite complicated and possibly chaotic dynamics in some of the vibrational states, one vibration was found stable enough to simulate the conditions of the experiment. The results agree with experiment and confirm that the selectivity is due to simple time-scale differences rather than the more commonly used energy-gap arguments. A detailed description of the nonstandard method for assigning the initial and final states is given.

Line shape study of Ba ions in laser produced plasmas

Detailed measurements of spatio-temporal variations in the line strengths and line shapes for Ba ion absorption lines, observed in laser-ablated YBCO plasma plumes, show that the presence of ambient oxygen leads to enhanced local number densities and temperatures at the expanding front of the plasma plume. Enhanced local temperatures (and local excitation rates) are attributed to the increased collisional redistribution of high initial z-directed plume velocities in the presence of ambient oxygen.

Collisional redistribution of light: a comparative study of the density operator and optical collisional approaches

We present an outline of both the redistribution theory and the optical collision approaches. Starting from the general theory of collisional redistribution we show how it reduces to the weak-field binary-collision limit. The optical collision method is discussed here for the case of a degenerate atom perturbed by e.g. a rare gas. This approach also provides a natural basis for a nonadiabatic theory of collisionally broadened lineshapes. The connection between both approaches is demonstrated by the equivalence of the collision operator of redistribution theory and the scattering matrices for optical collisions. As applications the quasistatic limit of the theory and the depolarization of the fluorescence for the model adiabatic reorientation are discussed.