Ar Collisions Research Articles

Target dependence of multi-electron processes in Iq+ (q=10, 15)+rare gas (Ne, Ar, Kr and Xe) collisions

We have determined experimentally the absolute cross sections for i-electron capture (to projectile) and j-electron removal (from target) ( sigma q,q-ij), total electron capture ( sigma q= Sigma i sigma q,q-i), j-electron removal ( sigma qj= Sigma i sigma q,q-ij) and i-electron capture ( sigma q,q-i= Sigma j sigma q,q-ij) for the processes: Iq+(q=10 or 15)+B to I(q-i)++Bj++(j-i)e (B identical to Ne, Ar, Kr and Xe) at collision energy of 1.5q keV. Charge-state distributions of the scattered ions were measured in coincidence with the recoil ions. The absolute electron-capture cross sections were measured by the initial growth-rate method. The experimental results for sigma q and sigma qj were compared with the predictions of the extended classical over-barrier model (ECBM). The branching ratios of the multiply-excited ions produced by multi-electron transfer have also been determined.

Space-time ambiguity of two- and three-fragment reduced velocity correlation functions.

Reduced-velocity correlation functions between two and three intermediate mass fragments are compared for central {sup 36}Ar+{sup 197}Ar collisions at {ital E}/{ital A}=50 MeV. Previously published {ital N}-body Coulomb-trajectory calculations, capable of reproducing the measured two-fragment reduced velocity-correlation function, describe the measured three-fragment correlation function equally well. Moreover, ambiguities between source size and lifetime observed in the analysis of two-fragment correlations remain unresolved in the three-fragment correlation function.

Velocity dependence of stabilized double electron capture in N 7+Ar collisions

We have studied the two-electron processes in the collision of N 7+ with argon at various energies. We have measured the radiative stabilization P rad, defined as the ratio of the true double capture σ TDC to the total double capture which is the sun of σ TDC and the autoionizing double capture σ ADC. A large enhancement of P rad (17–44%) with decreasing projectile velocity (0.6–0.14 a.u.) has been found. On the other hand, by using the photon-ion coincidence technique to select the double capture processes, we have observed an increase of the emission cross sections of optical transitions from (3, n) Rydberg states with decreasing projectile energy. We give a tentative explanation which is based on a combination of autotransfer to Rydberg states and the postcollisional “satellisation” of the Rydberg electron described by Roncin and Kazansky.

Cusp-electron production in intermediate-energy O 7+ + Ar collisions

Cusp-electron production has been investigated in 0.5–1.5 MeV/u O 7+ + Ar collisions. For this system, cusp electrons can result from electron capture or loss to the continuum (ECC or ELC), transfer ionization (TI), or double transfer ionization (2TI). Of particular interest are the two- and three-electron processes TI and 2TI. The ratios TI ECC and 2TI ECC each exhibit a maximum near 0.75 MeV/u, while the ELC ECC ratio increases monotonically with beam energy. The reasons for these behaviors, including electron correlation effects, are considered.

Study of multiple capture processes by recoil ion momentum spectroscopy for 150 keV Ne 10+Ar collisions

We report preliminary investigations of multiple electron capture in the collision of 150 keV Ne 10+ ions with Ar targets. These processes can be described by the following reaction: Ne 10+ + Ar → Ne (10− s)+ + Ar r+ + ( r- s)e −. The Ne 10+ projectile removes r electrons from the Ar target among which s electrons are stabilized on the projectile and ( r- s) are ejected into the continuum. Distributions corresponding to the production of up to septuple-charged recoils were obtained in coincidence with the charge state (10 − s) of the scattered projectiles. Stabilization rates and average scattering angles were also derived from this experiment and given here as preliminary results.

Absolute cross sections for the multielectron processes in 15 keV I 10+ + rare gas collisions

We have experimentally determined the absolute cross sections for total charge transfer ( σ q ), j electron transfer ( σ q j ), i electron capture ( σ q, q- i ) and each reaction process ( σ q, q- i j ) in 15 keV I 10+Ne, Ar, Kr and Xe collisions. The branching ratios were determined by the coincidence measurements between charge changing projectile and recoil ions. The electron capture cross sections were measured by the initial growth rate method. The experimental results for total and j electron transfer cross sections were compared with the predictions of the extended classical over-barrier model (ECBM).

Semiclassical study of polarization-dependent optical collisions: application to Mg () and Ar(1S0) collision

Abstract A semiclassical approach in the study of polarization-dependent optical collisions has been developed. General expressions for the cross-sections describing optical collisions of the atoms with arbitrary angular momentum and rare gas atoms in the ground state are obtained. The formation of different atomic multipoles in direct optical collision and the influence of the multipoles on light absorption in this process are considered from general positions of the semiclassical approach including the nonadiabatic dynamics of the collision. Light absorption in the far wings of the 1 P → 1 S atomic transition is considered in more detail. For such type of optical collision the resonance scattering is also included in the semiclassical analysis. It is obtained that there is no orientation-dependent part in the resonance cross-section describing circularly polarized light absorption. However, the alignment-dependent part describing the linearly polarized or unpolarized light absorption is significant for resonance collisions as well as for direct collisions. The calculation for the Mg (3 p 1 P 1 → 5 s 1 S 0 )− Ar ( 1 S 0 ) optical collision and comparison with experimental results are presented.

State resolved rotational relaxation of CS2 in argon free jet

Rotational relaxation of CS2 in argon free jet was studied using laser-induced fluorescence (LIF). The rotational populations were obtained from LIF detection at 19 different points along the jet axis from 3 to 15 nozzle diameters. On the basis of the energy gap power law, the master equation was solved to fit the experiment results, and the two temperature-independent parameters in the law were determined. The results show that the state-to-state relaxation cross sections vary as the inverse 1.7 power of the energy gap between the initial and the final states. The expressions for the state-to-state relaxation cross sections and the rate constants were written. The relaxation cross sections decrease with increasing temperature over the range of 2–60 K for the CS2+Ar collision.

Curvature in the scattered-beam growth curves in H-+(He,Ne,Ar) collisions.

In this paper, we report measurements of the curvature present in the ${\mathrm{H}}^{0}$ and ${\mathrm{H}}^{+}$ scattered-beam growth curves for the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ + (He, Ne, Ar) collision systems. The main motivation for this study is to provide an experimental test of the scattered-beam growth-curve rate-equation solutions for tenuous target thicknesses at one level beyond that used to determine cross sections. In doing so, this study provides an indirect verification of the magnitudes of the dominant charge-changing cross sections for ${\mathrm{H}}^{\mathrm{\ensuremath{-}},0,+}$ projectiles incident on noble-gas targets. The measured quadratic coefficients of the growth curves are compared with the expected quadratic coefficients derived from the previously measured, charge-changing cross sections. These curvature measurements indicate that additional measurements of cross sections for charge-changing processes between hydrogen projectiles incident on neon and argon targets may be warranted.

Coherent excitation of H(n=2) induced in H+-Ne and H+-Ar collisions

The collision-induced excitation of atomic hydrogen to the H(n=2) states in H+-Ne and H+-Ar collisions was measured in an energy range of 1 to 25 keV. From a polarization analysis of the emitted Lyman- alpha radiation as a function of an external electric field the partial cross sections sigma lm, for excitation to the H(2s) and the H(2pm) magnetic substates (m=0,+or-1) and the real part of the s0-p0 coherence were extracted. For both collision systems an energy dependent forward-backward asymmetry corresponding to a shift of the centre-of-charge relative to the centre-of-mass (electric dipole moment) was observed. The collision-induced electric dipole moment (Dz) is generally small or negative at incident energies below 10 keV and rises steeply towards higher energies. The present results are in reasonable agreement with close-coupling calculations employing a molecular representation.

Comments on the shape of the “cusp” in the spectrum of electrons emitted in H+, H0→Ar collisions

The symmetry-asymmetry as well as the width of the cusp in the spectrum of electrons emitted in H+, H0 (75 keV)→Ar collisions was studied in singles and in coincidence (with the outgoing projectile ions) arrangement.

Observation of collisionally induced (1s2p2p')4Pe shape resonance of He-

Autodetachment via the collisionally populated (1s2p2p'${)}^{4}$${\mathit{P}}^{\mathit{e}}$ shape resonance of ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ has been observed by means of 0\ifmmode^\circ\else\textdegree\fi{} electron spectroscopy in a measurement of the double-differential cross section for single and double electron loss to the continuum in ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ on Ar collisions at 75-keV/u projectile energy. An alternative parametrization has been developed for the autodetachment lines emitted with low energy in the projectile reference frame. The determined position and width of the shape resonance are in agreement with the results of theoretical calculations and photodetachment measurements.

A coincidence study of multiple-electron capture in 15N7+-Ar collisions

We present energy spectra of autoionization electrons resulting from collisions between fully stripped 15N7+ ions aid Ar in the energy range of 1-10 keV amu-1. These spectra have been measured in coincidence with charge-state analysed Ar ions. Observed populations of the various configurations are discussed in the framework of the classical overbarrier model. Our measurements suggest that target-ion excitation together with two-electron capture is an important process in collisions between bare N7+ ions and Ar. We find evidence that radiative stabilization of doubly excited N5+ (4l, 4l') states is even stronger than was thought up to now.

Doubly differential cross sections for collisions of 100 eV positrons and electrons with argon atoms

Energy distributions of scattered positrons (e+) and ejected electrons (e-) resulting from 100 eV e+-Ar collisions have been determined at 30 degrees and 45degrees in coincidence with the remnant Ar+ ions. A calibration method has been employed to convert the measured intensities into absolute cross sections. The experimental data are compared with values calculated by a classical-trajectory Monte Carlo method. The agreement is good for the secondary e- but poor for the scattered e+. The ratios of the measured secondary e- intensities at 15 eV have been determined for e- and e+ impact and compared with existing experimental and theoretical values.

The collision-induced electric dipole moment of H(n=2) in H-He, Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.

Alignment and orientation of Ar+ in He+-Ar collisions

We have measured the alignment and orientation parameters of the2F 7 2/0 and2F 5 2/0 states of Ar+ formed in the two-electron process; He++Ar→He(1s2)+Ar+(3p44p′). These have been measured at a collision energy of 0.25 keV/amu and for scattering angles ranging from 0.94° to 3.75°. First, by comparing the orientation parameter for the Ar+[(3p4[1D]4p′2F 7 2/0 ] and the Ar+[(3p4[1D]4p′2F 5 2/0 ] states, we have experimentally determined the importance of the spin-dependent interactions for the present collision system, by testing the Percival-Seaton hypothesis of spin independence. If the Percival-Seaton hypothesis holds for this system, the orientation parameter should beJ-independent. Secondly, the magnitude of the orientation parameter can be interpreted as resulting from the collective circulation of the unexcited 3p4 electrons and the excited 4p electron. The direction of this collective circulation is compared to the propensity rule for colliding di-atom systems.

Differential cross sections for fine structure transitions in O(3P2)+Ar collisions

We present experimental and theoretical differential cross sections for the fine structure changing process Ar + O(3P2)→Ar + O(3Pjmj) with (j,mj)=(1,1), (1,0), and (0,0). The measured cross sections refer to a collision energy of 3.2 kcal/mol, and were obtained from Doppler line shapes associated with (2+1) resonance-enhanced multiphoton ionization of O(3Pjmj) after scattering in a crossed-beam apparatus. The theoretical results are based on 3Σ− and 3Π potential curves obtained from high quality ab initio calculations, and on quantum coupled-channel calculations. The calculated differential cross sections show strong Stuckelberg oscillations, with similar magnitudes and phases for all three final states. With slight adjustment (0.03 Å) of the hard wall on the 3Σ− curve, the calculated angular distributions match up well with the corresponding experimental results, much better than is found using previously derived empirical potentials. At the same time, the integral total cross sections obtained from the ab initio measurements are in satisfactory agreement with previous measurements.

Measurement of the V–T energy transfer rates of highly excited 2A1 NO2

Production of electronic ground state NO2 (2A1) from 248 nm photolysis of HNO3 was detected by laser induced fluorescence (LIF). A growth in the LIF signal was observed following the photolysis and has been interpreted as the relaxation of NO2 through the higher vibrational levels of the X(2A1) state; an energy region where the probe laser photodissociates the NO2 instead of inducing fluorescence. The rate coefficients for NO2 relaxation through these high vibrational levels were determined by fits of time resolved LIF signal to a stepladder kinetic model. The results of the kinetic analysis suggest that the observed relaxation begins at the 2B2 threshold near 9500 cm−1 and extends downward through approximately 5 vibrational levels of the ground electronic surface. The derived quenching rate coefficients (in units of 10−12 cm3 molecule−1 s−1) are 0.51±0.05, 1.0±0.1, 1.4±0.2, 2.6±0.6, and 8.7±1.1 for Ar, He, N2, O2, and CO2 collision partners, respectively. The discrepancies between these coefficients and previous literature values are rationalized in terms of a dependence of the vibrational relaxation rate on total internal energy.

Analysis of charge-state-correlated multiple-capture and -ionization cross sections in fast Arq+-Ar collisions.

We consider the problem of charge-state-correlated cross sections in the 1.05-MeV/nucleon Ar[sup [ital q]+-]Ar collision system with [ital q]=10--16. Theoretical results from a semiclassical ([h bar]=0) shell model that treats the Ar([ital L]) and Ar([ital M]) shells in an uncoupled approximation are compared with recent experimental measurements. For the dominant channels agreement can be obtained within a factor of 2--3, but larger disagreements remain in the direct multiple-ionization channel for recoil charges above 8 and generally for the multiple-capture (MC) channels. The experimental data are analyzed to understand whether in principle they are compatible with an independent-particle model evaluation. Single-particle capture and ionization probability functions are found that describe the experimental MC cross sections.

Multielectron processes in 10-keV/u Arq+ (5

We have used time-of-flight coincidence techniques to study multielectron reactions in 10-keV/u (v=0.632 a.u.) ${\mathrm{Ar}}^{\mathit{q}+}$ (5\ensuremath{\le}q\ensuremath{\le}17) on Ar collisions. Absolute cross sections for total charge-transfer (${\mathrm{\ensuremath{\sigma}}}_{\mathit{q}}$), projectile charge-change (${\mathrm{\ensuremath{\sigma}}}_{\mathit{q},}$${\mathit{q}}_{\mathrm{\ensuremath{-}}\mathit{k}}$), recoil production (${\mathrm{\ensuremath{\sigma}}}_{\mathit{q}}^{\mathit{i}}$), and phenomenological cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{q},}$${\mathit{q}}_{\mathrm{\ensuremath{-}}\mathit{k}}^{\mathit{i}}$) have been obtained by normalizing to cross sections reported in the literature [H. Klinger, A. M\"uller, and E. Salzborn, J. Phys. B 8, 230 (1975)]. The data have been used to critically test the predictions of the molecular classical overbarrier model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)] and rather impressive agreements have been obtained. In particular, the predictions of target outer-shell excitation seem to have supporting evidence in this set of data. A stabilization scheme for the multiply excited projectile following charge transfer is proposed to complement the MCBM predictions and the gross features of the final reaction products are fairly accounted for. In addition, enhanced electron loss from the projectile-target system is observed in hard collisions for low-charged projectiles (q\ensuremath{\le}8) and is attributed to inner-shell excitation via molecular-orbital promotion.