Collisions Of Helium Atoms Research Articles

Adiabatically corrected scattering lengths and effective ranges for collisions of helium atoms

Abstract The accurate relativistically retarded potential of Janzen and Aziz [J. Chem. Phys. 107 (1997) 914] and the adiabatic correction of Komasa et al. [Chem. Phys. Lett. 304 (1999) 293] are used in a calculation of scattering lengths and effective ranges for 3 He– 3 He, 3 He– 4 He and 4 He– 4 He collisions; the binding energy and the number of Efimov states of the 4 He– 4 He dimer are also presented.

A route to chaos in classical atom–diatom collisions

We study classical atom–diatom collisions in the `T-shaped' configuration. Considering collisions of helium atoms with a bound I 2 molecule at high energies, the scattering process becomes increasingly complicated as the projectile energy is lowered. The complexity manifests itself in phase space structures which change in a well defined way forming a `route to chaos'. An analysis of this route provides a deeper understanding of the appearance of chaos in three-body collisional systems. Our perspective is complementary to approaches dealing directly with the fractal properties of observables in the chaotic regime.

Cross section for disalignment of excited neon atoms due to neon or helium atom collisions

In a positive column plasma of glow discharge in neon the population of metastable atoms ( configuration) is substantial. Linearly polarized pulsed laser light excited these atoms to one of the configuration levels, and the direct fluorescence was observed with its polarized components resolved. From the temporal development of the intensities of these components, the disalignment (depolarization) rate due to atom (neon or helium) collisions was determined. From its atom-density dependence the rate coefficient was determined, and from the temperature (42-650 K) dependence of the rate coefficient, the disalignment collision cross section was estimated as a function of energy in the energy range of the order of 10 meV. The cross section of the (Paschen notation) atoms has positive dependence on energy, while that of the atoms has a negative dependence. The cross sections thus determined serve as a quantitative test of the molecular potentials to the order of . In order to confirm that the temperature of the metastable atoms in the plasma is equal to the temperature of the surrounding wall, an additional experiment was performed; the line profile of the CW laser-induced fluorescence was observed for a plasma at liquid nitrogen temperature, and the metastable atom temperature was found to be 78 K.

Collision dynamics for small helium cluster ions

Classical trajectories have been carried out for the collision of helium atoms with He2+ and He3+ using a potential energy which includes the effects of dispersion, polarisation and charge exchange. At a collision energy of 0.1 eV the cross-section for atom exchange in He + He2+ is calculated to be 21.5Å, exchange occurring through both simple collisions and long-lived complexes. However, even for very long-lived complexes exchage never exceeds 50% so that complete permutation of atoms in the complex does not occur. The cross-section for exchange in He + He3+ at the same collision energy is found to be much smaller at 0.7Å2 which is attributed to the much smaller dissociation energy of He4+. Rotational energy transfer for both collisions is similar, for impact parameters less than 3.5Åonly 50% of collisions transfer more than 10% of translational energy to rotation.

Excitation and ionization of excited helium atoms in collisions with bare ions

A comparative study has been made on the cross sections for excitation of He(n = 2 → 2 – 5) and He(n = 3 → 3 – 5) and for ionization of the He n 1,3S and n 1,3P states with n = 2–5 in collisions with H+, Be4+ and B5+ in the Glauber and the continuum distorted wave-eikonal initial state approximations, respectively, besides the Born approximation. A comparison is made between the present results and other theoretical predictions.

Two-centre effects in ejected-electron spectra from ionization of helium atoms in collisions with fast, fully stripped ions

A joint experimental and theoretical study of the double-differential cross sections (DDCS) for ejection of electrons from helium by 1.00 and 1.84 MeV amu-1 p+, He2+, C6+ and O8+ is presented. Electron-emission angles from 20' to 160' are surveyed, and the electron energies range from 4 to 2500 eV. Single-differential and total cross sections, obtained by integrating the DDCS over angle and electron energy, are also presented. Large departures from the q2 dependence (where q is the projectile charge) predicted by the first Born approximation are observed in the cross sections. The deviations, which are seen to increase with projectile charge are discussed and compared with calculations, using the first Born approximation and a continuum-distorted-wave eikonal-initial-state model.

Energy and angular distributions of electrons emitted and ions recoiled in proton-impact ionization of helium atoms

Single ionization of helium atoms in collisions with high-energy protons is theoretically treated in the eikonal distorted-wave approximation. The momentum and angular distributions of the ejected electrons and the recoil ions are calculated as functions of the proton scattering angle.

The formation and destruction of highly excited helium atoms in 4–25 keV collisions with the rare gases and simple molecules

The production of highly excited (10 < n < 20) helium atoms has been investigated in 4–25 keV He + collisions with the inert gases, hydrogen and nitrogen using the field ionization technique. Cross-sections are found to scale as n −3, and in the case of heavier rare gases there are signs of structure at low energies. The destruction of highly excited (10 < n < 20) helium atoms in collisions with N 2, He, Ar and Xe has been investigated within the same energy range. Cross-sections are in good accord with the independent particle model, in which the core and excited electron scatter quasi-freely.

Time-dependent quantum-fluid density-functional study of high-energy proton-helium collisions.

A quantum-fluid density-functional theory (QF DFT) is proposed, yielding a time-dependent (TD) generalized nonlinear Schr\"odinger equation (GNLSE) as the equation of motion (EOM) for dealing with proton--helium-atom collisions from ``start'' to ``finish.'' The EOM contains the Weizs\"acker term as the kinetic-energy functional, apart from local exchange and correlation functionals. The GNLSE is numerically solved in cylindrical polar coordinates by a leapfrog-type finite-difference algorithm. Various TD quantities such as difference density (DD), induced-dipole moment (IDM), dipole polarizability tensor component, reaction probability, etc., have been studied to obtain physical insights into the mechanism of the TD collision process. In particular, the DD and the oscillating IDM permit a natural partitioning of the p-He collision process into approach, encounter, and departure regimes. The TD DD profiles reveal that, as a result of the interaction, p\ensuremath{\sigma} densities mix substantially into the 1s density of the He atom. Critical comments are made on the usefulness of the QF DFT approach for understanding TD processes.

Electron capture in C6++He and O8++He collisions at intermediate energies in the atomic-orbital-expansion method.

A multistate two-center atomic-orbital-expansion method is applied to study the subshell capture cross sections from helium atoms in collisions with bare ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ ions. The energy dependence of subshell capture cross sections, i.e., the n, nl, and nlm distribution, is examined. In view of the lack of detailed subshell cross section measurements, we compare our calculations with experimental total capture cross sections and with polarization fraction for the 2p-1s transition in ${\mathrm{O}}^{8+}$-He collisions. For the latter process, the cascade effect must be included in the theoretical analysis. We show that our results are in good agreement with existing experimental data.

An experimental investigation of double ionisation of helium atoms in collisions with fast, fully stripped ions

Experimental cross sections for single and double ionisation of helium by 0.13-15 MeV AMU-1 ions of charge 1-8 are presented. Where necessary, coincidence techniques were applied to distinguish between pure ionisation and ionisation involving electron capture. For high ion velocities, double ionisation takes place either through a shake-off or through a two-step process. The perturbative shake-off mechanism dominates only for the lowest ion charges and projectile energies above 10 MeV AMU-1. A reliable semi-empirical expression which determines the ratio between the double and single ionisation cross sections over a wide region of ion charge and velocity is given. The difference between double ionisation by equi-velocity electrons and positive ions is discussed. It is concluded that no reliable theoretical description of double ionisation exists for combinations of ion charge and ion velocity which give cross sections close to the maximum values.

Impact ionization of helium atoms by multiply charged ions

Cross sections for the single-electron ionization of helium atoms in collisions with multiply charged ions are calculated using a three-state close-coupling method. The results are compared with the available experimental data and other theoretical calculations.

Quantum-dynamical study of the translational-vibrational energy transfer in the collinear collisions of atoms with triatomic molecules

A quantum-dynamical method is described for investigating the translational-vibrational energy transfer that occurs in the collinear collision of an atom with a linear triatomic molecule. The technique is applied to the computation of vibrational transition probabilities for the collinear collisions of helium atoms with CO2, OCS and HCN over a range of energies. Realistic potentials are used for the triatomic molecules while simple exponential or Morse potentials are used to describe the helium-molecule interaction. The effects of varying the atomic masses and the potential function parameters are examined. It is found, that the symmetric stretch vibrational modes of CO2, OCS and HCN are preferentially excited (or relaxed), by collision with the atom, over the asymmetric stretch modes.

Methods for Monte Carlo simulation of the exospheres of the Moon and Mercury

A general form of the integral equation of exospheric transport on moon‐like bodies is derived in a form that permits arbitrary specification of time varying physical processes affecting atom creation and annihilation, atom‐regolith collisions, adsorption and desorption, and nonplanetocentric acceleration. Because these processes usually defy analytic representation, the Monte Carlo method of solution of the transport equation, the only viable alternative, is described in detail, with separate discussions of the methods of specification of physical processes as probabalistic functions. Proof of the validity of the Monte Carlo exosphere simulation method is provided in the form of a comparison of analytic and Monte Carlo solutions to three classical, and analytically tractable, exosphere problems. One of the key phenomena in moonlike exosphere simulations, the distribution of velocities of the atoms leaving a regolith, depends mainly on the nature of collisions of free atoms with rocks. It is shown that on the moon and Mercury, elastic collisions of helium atoms with a Maxwellian distribution of vibrating, bound atoms produce a nearly Maxwellian distribution of helium velocities, despite the absence of speeds in excess of escape in the impinging helium velocity distribution.

Quantum-Mechanical Transition Probabilities for Atom–Atomic-Oscillator Collisions with Morse Potential Interaction

Exact quantum-mechanical transition probabilities for the collision of an atom with an atomic oscillator for the case in which the interaction has the form of a Morse potential are obtained by numerical integration of the relevant close-coupled scattering equations. These transition probabilities are compared with approximate ones calculated from distorted-wave theory over a range of parameters which have been of interest in examining the collision of helium atoms with a tungsten surface. Some information is obtained concerning the range of validity of the first-order theory, and certain suggestions that have appeared in the literature for correcting the distorted-wave results are found to be inadequate.

Production and Loss of Fast Metastable Helium Atoms in Collisions with Permanent Gases

Cross sections for production and destruction of fast ${\mathrm{He}}^{0}$ atoms in triplet states have been measured for 10- to 30-keV ${\mathrm{He}}^{+}$ ions incident on gas targets of Kr, ${\mathrm{N}}_{2}$, and Ne. Values of the fractions of neutral ${\mathrm{He}}^{0}$ atoms in the triplet metastable state emerging from thin targets of Kr, ${\mathrm{N}}_{2}$, and Ne are 0.12\ifmmode\pm\else\textpm\fi{}0.005, 0.06\ifmmode\pm\else\textpm\fi{}0.03, and 0.03\ifmmode\pm\else\textpm\fi{}0.02, respectively, for incident ${\mathrm{He}}^{+}$ energies of 30 keV.

Production and Loss of Fast Metastable Helium Atoms in Collisions with Xe,H2, Ar, and He

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A quantitative analysis of fast metastable helium atoms in collisions involving electron loss

The attenuation of a fast beam of helium atoms by electron-loss collisions during passage through hydrogen has been carefully investigated. The measurements allow the metastable atom population of the beam to be determined and, for the first time, permit separate measurements of the electron-loss cross sections for both ground-state and metastable helium atoms. At 120 kev these are 7.3 ± 0.8 and 48 ± 8 cm2 × 10-17 respectively.

Messungen der van der Waals-Wechselwirkung zwischen Edelgasatomen

Total cross sections for collisions of Helium atoms with other rare gases have been measured by modulated molecular beam technique. The results are used to determine the interaction constants of the van der Waals potential (V(r)∼ −C/r6). For this purpose, an indirect procedure is adopted, since the well known approximation ofMassey andMohr (Q∼C2/5) is not applicable for Helium scattering experiments at thermal energies.

Polarization of the He II line λ = 4686 Å in electron - helium atom collisions

Polarization of the He II line λ = 4686 Å in electron - helium atom collisions