Ar Collisions Research Articles

Experimental study on rotational energy transfer in LiH (X1∑+ v) + Ar collisions

Abstract Using high-resolution transient laser spectroscopy, the rotational energy transfer between LiH (12, 8) and Ar through collisions was studied. LiH (12, 8) was generated via degenerate stimulated hyper-Raman scattering. The population of LiH (12, J″ ≠ 8) generated during collisions were obtained using transient laser-induced fluorescence. According to the rate equation, the rate coefficients for the transfer from (12, 8) to (12, J″) states are between 7.1 × 10−12 and 3.5 × 10−13 cm3 molecule−1 s−1 within 2 μs of the collision occurring. Above 2 μs, the rate coefficient is no longer constant. Meanwhile, no vibrational relaxation occurs before 10 μs. The rotational energy E rot is the sum of the rotational energies of rotational states (12, J″). Within the period of 0–10 μs, the value of E rot decreases from 541 cm−1 to 390 cm−1. The distribution of translational energy E trans at different delay time of LiH (12, 8) is obtained by measuring the Doppler broadened line widths. It increases from 707 cm−1 at 0 μs to 852 cm−1 at 10 μs. Therefore, when rotational relaxation occurs, the decrease in rotational energy is approximately equal to the increase in translational energy.

High-precision cavity-enhanced spectroscopy for studying the H2-Ar collisions and interactions.

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2-Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2-Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.

Influence of collisional broadening on resonance photoplasma parameters in a sodium-argon mixture

A study of photoplasma parameters in a Na–Ar mixture was carried out in a wide range of Ar pressures, considering the Voigt profile of the D1 and D2 resonance lines of Na in the Biberman-Holstein approximation. The effect of collisional broadening on the full width at half maximum (FWHM) of these lines is analyzed in detail, using the constants of the resonance (Na–Na collision) and van der Waals (Na–Ar collision) line shape broadening. 2D simulations of the main parameters (density of resonance (3p) levels of Na; electron density and temperature; photo-EMF) of the resonance photoplasma in a two-chamber cell at the Na pressure 0.02 Torr and Ar pressure 0.1 − 100 Torr have been carried out. It has been established that the difference between these parameters and previously obtained data for the case of the Doppler contour of the resonant line for the exact geometry of the gas cell and gas pressures is significant. Numerically, it reaches one-two orders of magnitude for the electron density and two times for the electron temperature. Also, a noticeable difference in the electron flux spatial picture in the cell for these two cases has been found. Therefore, it is shown that it is necessary to consider the collisional broadening of the resonance line of Na to determine plasma parameters even for relatively small pressures of Na and Ar. The developed approach and obtained data open up an opportunity to make a quantitative design of a photoelectric converter in a mixture of Na–Ar.

Charge-Overlap Effects on Electronic Transitions in Moderate-Energy K+–Ar and Cs+–Ar Collisions

Excitation mechanisms in K+–Ar and Cs+–Ar collisions at moderate laboratory energies Elab = 0.05–2 keV are investigated by calculating repulsive potentials V(R) = 0.01–1 keV at distances R = 0.8–10...

Evidence for target outer-shell excitation mediated by electron correlation in single-electron-capture collisions of slow He2+ ions with Ar atoms

We have performed kinematically complete experiments on single-electron capture in slow ${\mathrm{He}}^{2+}$-Ar collisions. Besides the pure capture to the ${\mathrm{He}}^{+}(n=2)$ level, capture into the deep ${\mathrm{He}}^{+}(n=1)$ state with simultaneous excitation of another target electron is also observed. In contrast to the pure capture, the total cross section for this two-electron transition decreases with increasing collision energy, and its angular-differential cross section exhibits a much slighter slope. We take these observations as evidence for electron capture mediated by electron-electron interactions.

Target K-shell X-ray emission associated with single and double electron capture for [formula omitted] + Ar collisions

Cross sections for target Ar K-shell ionization associated with single and double electron capture, as well as the same cross sections corresponding to projectile F K X-rays and the total cross sections for single and double capture, were determined for 1.8–2.2 MeV/u F9+,8+,7+ projectiles. This work was performed at Western Michigan University using the tandem Van de Graaff accelerator. Coincidences between emitted X-rays and their corresponding charge-changed particles were detected. The Ar K X-ray coincidence cross section ratios for double to single capture are found to depend strongly on the incident charge state, with the ratios for F9+ well exceeding unity. Possible explanations for this anomalous behavior are discussed but a conclusive explanation is not found. The results are compared to previous experiments using fully-stripped fluorine where coincidence techniques were not employed.

Differential Study of Projectile Coherence Effects on Double Capture Processes in p + Ar Collisions

We have measured differential yields for double capture and double capture accompanied by ionization in 75 keV p + Ar collisions. Data were taken for two different transverse projectile coherence lengths. A small effect of the projectile coherence properties on the yields were found for double capture, but not for double capture plus ionization. The results suggest that multiple projectile–target interactions can lead to a significant weakening of projectile coherence effects.

Target dependence of post-collision effects in ionization by proton impact

SynopsisDouble differential cross sections (DDCS) for ionization in p + Ne, Ar collisions were measured and calculated for a broad range of projectile energy losses as a function of scattering angle. PCI effects in the DDCS were found to be become increasingly prominent with increasing ionization potential I probably due to a greater importance of the nucleus-nucleus interaction when I becomes larger.

Electron transfer plus double ionization in slow He2+ + Ar collisions

We performed a kinematically complete experiment on double ionization accompanied by single capture in 30 keV/amu ${\mathrm{He}}^{2+}$ + Ar collisions. The data were systematically analyzed using three different techniques, namely, fully differential cross sections as a function of the electron emission angle of each electron, the correlation function, and four-particle Dalitz plots. The data can to a large extent be explained within the independent electron model. However, the correlation function reveals small but significant effects from correlations in the initial target state. These correlations strongly depend on the interaction between the nuclei of the collision partners. Furthermore, a surprisingly strong correlation between the electron momenta with the momentum transfer, reported earlier, was confirmed.

Electron emission from transfer ionization reaction in 30 keV amu−1 He2+ on Ar collision

A model is presented that describes the transfer ionization process in collision at a projectile energy of 30 keV amu−1. It is based on a semiclassical independent-particle close-coupling method that yields a reasonable agreement between calculated and experimental values of the total single-ionization and single-capture cross sections. It is found that the transfer ionization reaction is predominantly carried out through simultaneous capture and ionization, rather than by sequential processes. The transfer-ionization differential cross section in energy that is obtained satisfactorily reproduces the global behavior of the experimental data. Additionally, the probabilities of capture and ionization as function of the impact parameter for and collisions are calculated, as far as we know, for the first time. The results suggest that the model captures essential elements that describe the two-electron transfer ionization process and could be applied to systems and processes of two electrons.

Ionization and Single and Double Electron Capture in Proton-Ar Collisions.

Total cross sections for formation of H and H-, and electron production, in H+ + Ar collisions have been calculated at energies between 100 eV and 200 keV by employing two methods: for E < 10 keV, a semiclassical treatment with an expansion in a basis of electronic wave functions of the ArH+ quasimolecule and, for E > 10 keV, the switching-classical-trajectory-Monte Carlo method (s-CTMC). The semiclassical calculation involves transitions to molecular autoionizing states, calculated by applying a block-diagonalization technique. The s-CTMC method is adept to treat two-electron processes and yields total cross sections for H- formation in reasonably good agreement with the experimental data. Cross sections for electron- and H-production processes, which are dominated by one-electron transitions, are in good agreement with the experimental data.

Study of inelastic processes in Li+–Ar, K+–Ar, and Na+–He collisions in the energy range 0.5–10 keV

Absolute cross sections are measured for charge-exchange, ionization, and excitation processes within the same experimental setup for the LiAr, KAr, and NaHe collisions in the ion energy range of 0.5–10 keV. The results of the measurements and schematic correlation diagrams are used to analyze and determine the mechanisms for these processes. The experimental results show that the charge-exchange processes occur with high probabilities and electrons are predominantly captured in ground states. The contributions of various partial inelastic channels to the total ionization cross section are estimated, and a primary mechanism for the process is identified. In addition, the energy-loss spectrum is applied in order to estimate the relative contribution of different inelastic channels, and to determine the mechanisms for the ionization and for some excitation processes of Ar resonance lines for the Ar collision system. The excitation cross sections for the helium and for the sodium doublet lines for the NaHe collision system both reveal some unexpected features. A mechanism to explain this observation is suggested.

Study of multiply charged Argon-ions formed by decay of 2p-hole state under e-- Ar collision employing energy selected ion coincidence technique

SynopsisIn this presented work we have performed the energy selected electron-ion coincidence technique to investigate the formation of multiply charged argon ions and determined correlation probabilities for individual charge state of formed ions and several other features of the employed technique is presented and discussed.

Fully differential study of few-body dynamics in multi-electron atomic fragmentation processes

We have measured fully momentum analyzed Ar3+ recoil ions and two ejected electrons as well as He+ projectiles in coincidence with each other for 30 keV amu−1 He2+ + Ar collisions. Fully differential cross sections for electron transfer from the target to the projectile accompanied by the ejection of two additional target electrons were extracted. To a large extent the data can be reproduced by an independent electron model. However, we also observed a surprisingly strong correlation between the electron momenta and the projectile momentum transfer.

Signature of Single Binary Encounter in Intermediate Energy He2+—Ar Collisions**Supported by the National Basic Research Program of China under Grant No 2010CB832902, the National Natural Science Foundation of China under Grants Nos U1332128 and 11274317, 10979007 and 11004202, and the Program of One Hundred Talents of the Chinese Academy of Sciences.

We experimentally observe the signature of electron emission resulting from a single binary encounter mechanism in the intermediate collision energy regime of 30 keV/u He2+ on argon. Electron emission spectra in the transfer ionization are obtained and compared with classical calculations from a two-step model considering the initial electron velocity and re-scattering of the binary encounter electron in the recoil potential. Although the present reaction is actually a four-body problem, the model starting from a binary encounter gives out surprisingly good agreement with the experimental data. Our studies show that orbital velocities of the electron affect the emission patterns of ionized electrons significantly.

Stereodynamics in NO(X) + Ar inelastic collisions

The effect of orientation of the NO(X) bond axis prior to rotationally inelastic collisions with Ar has been investigated experimentally and theoretically. A modification to conventional velocity-map imaging ion optics is described, which allows the orientation of hexapole state-selected NO(X) using a static electric field, followed by velocity map imaging of the resonantly ionized scattered products. Bond orientation resolved differential cross sections are measured experimentally for a series of spin-orbit conserving transitions and compared with quantum mechanical calculations. The agreement between experimental results and those from quantum mechanical calculations is generally good. Parity pairs, which have previously been observed in collisions of unpolarized NO with various rare gases, are not observed due to the coherent superposition of the two j = 1/2, Ω = 1/2 Λ-doublet levels in the orienting field. The normalized difference differential cross sections are found to depend predominantly on the final rotational state, and are not very sensitive to the final Λ-doublet level. The differential steric effect has also been investigated theoretically, by means of quantum mechanical and classical calculations. Classically, the differential steric effect can be understood by considering the steric requirement for different types of trajectories that contribute to different regions of the differential cross section. However, classical effects cannot account quantitatively for the differential steric asymmetry observed in NO(X) + Ar collisions, which reflects quantum interference from scattering at either end of the molecule. This quantum interference effect is dominated by the repulsive region of the potential.

Electron capture and single ionization in H+ + Ar collisions: classical calculations

A classical model is used to study electron capture and single ionization (SI) following H+ + Ar collisions at projectile energies varying from 400 to 40 keV. In the present model, the Ar electrons are treated independently from each other, and only the 3s and 3p electrons are supposed to be captured by the projectile. In addition, a Coulombic potential with an effective charge Zeff = 6.75, derived from Slater rules, is used in the calculations to simulate the screening of the Ar nucleus due to the presence of the core and 2l electrons. Total cross sections for single electron capture and SI are calculated and compared with previous experiments and earlier calculations based on a semiclassical approach. The reasonable agreement we observed allows a preliminary study of double electron capture (DC). The total cross section for DC is found to be much larger than the experimental one. Possible reasons for this disagreement are discussed.

Collision Cross Sections for O + Ar(+) Collisions in the Energy Range 0.03-500 eV.

The interatomic potentials of the a(2)Π and b(2)Π states of the OAr(+) molecule are calculated using the relativistic complete-active space Hartree-Fock method followed by a multireference configuration interaction calculation with an aug-cc-pwCVNZ-DK basis sets where N is 4 and 5. The calculations were followed by an extrapolation to the complete basis set limit. An avoided crossing between the two potential energy curves is found at an internuclear separation of 5.75 bohr (3.04 Å). As the transition probability between the curves is negligible in the relative collision energy range 0.03-500 eV of interest here, collisions on the lower adiabatic a(2)Π potential may be treated without reference to the upper state. For low energies and orbital angular momentum quantum numbers, the one-dimensional radial Schrödinger equation is solved numerically using a Numerov algorithm method to determine the phase shift. The semiclassical JWKB approximation was employed for relative energies greater than 5 eV and orbital angular quantum numbers higher than 500. Differential, integral, transport (diffusion), and viscosity cross sections for elastic collisions of oxygen atoms with argon ions are calculated for the first time for the range of relative collision energies studied. The calculated cross sections are expected to be of utility in the fields of nanotechnology and arc welding. The combination of an Ar(+)((2)P) ion and a O((3)P) atom gives rise to a total of 12 different molecular electronic states that are all coupled by spin-orbit interactions. Potential energy curves for all 12 states are computed at the complete active space self-consistent field (CASSCF) level and scattering calculations performed. The results are compared with those obtained using just the lowest potential energy curve.

Inelastic processes inNa+-Ne,Na+-Ar,Ne+-Na, andAr+-Na collisions in the energy range 0.5–14 keV

Absolute cross sections for charge-exchange, ionization and excitation in Na$% ^{+}-$Ne and Na$^{+}-$Ar collisions were measured in the ion energy range $% 0.5-10$ keV using a refined version of a capacitor method, and collision and optical spectroscopy methods simultaneously in the same experimental set-up. Ionization cross sections for Ne$^{+}-$Na and Ar$^{+}-$Na collisions are measured at the energies of $2-14$ keV using a crossed-beam spectroscopy method. The experimental data and the schematic correlation diagrams are used to analyze and determine the mechanisms for these processes. For the charge-exchange process in Na$^{+}$ $-$Ar collisions two nonadiabatic regions are revealed and mechanisms responsible for these regions are explained. Structural peculiarity on the excitation function for the resonance lines of argon atoms in Na$^{+}$ $-$Ar collisions are observed and the possible mechanisms of this phenomenon are explored. The measured ionization cross sections for Na$^{+}-$Ne and Ne$^{+}-$Na collisions in conjunction with the Landau-Zener formula are used to determine the coupling matrix element and transition probability in a region of pseudo-crossing of the potential curves.

Rotationally inelastic scattering of NO(A(2)Σ(+)) + Ar: Differential cross sections and rotational angular momentum polarization.

We present the implementation of a new crossed-molecular beam, velocity-map ion-imaging apparatus, optimized for collisions of electronically excited molecules. We have applied this apparatus to rotational energy transfer in NO(A(2)Σ(+), v = 0, N = 0, j = 0.5) + Ar collisions, at an average energy of 525 cm(-1). We report differential cross sections for scattering into NO(A(2)Σ(+), v = 0, N' = 3, 5, 6, 7, 8, and 9), together with quantum scattering calculations of the differential cross sections and angle dependent rotational alignment. The differential cross sections show dramatic forward scattered peaks, together with oscillatory behavior at larger scattering angles, while the rotational alignment moments are also found to oscillate as a function of scattering angle. In general, the quantum scattering calculations are found to agree well with experiment, reproducing the forward scattering and oscillatory behavior at larger scattering angles. Analysis of the quantum scattering calculations as a function of total rotational angular momentum indicates that the forward scattering peak originates from the attractive minimum in the potential energy surface at the N-end of the NO. Deviations in the quantum scattering predictions from the experimental results, for scattering at angles greater than 10°, are observed to be more significant for scattering to odd final N'. We suggest that this represents inaccuracies in the potential energy surface, and in particular in its representation of the difference between the N- and O-ends of the molecule, as given by the odd-order Legendre moments of the surface.