Ion-atom Interaction Research Articles

On the relation of dispersion to induction energies, and to their damping functions

A simple, yet reliable, expression is suggested to represent the radial-dependent charge-overlap damping functions for the coefficients (Cn ) of the second-order induction and dispersion perturbation series expansions for arbitrary even n, and of the third-order charge-overlap induction damping functions for arbitrary odd n. By using a reduced coordinate of the form x = R/[λ + 2γ(<r 2 A >1/2 + <r 2 B>1/2)], where λ = 5·5 a 0 and γ = 1.25, it is shown that the charge-overlap damping functions for the H(1s) … H+ second-order induction coefficients can be made consistent with those for the H(1s) … H(1s) second-order dispersion coefficients over a wide range of internuclear distances. Hence, in view of previous work, they are suggested to study other ion-atom interactions. Based on the damped induction energy, reliable analytic fits to the exact numerical Born-Oppenheimer curves for the X 2Σ g + and 1 2Σ u + states of H+ 2 are presented.

A modified Tang–Toennies model for ion–atom potential curves

A modification of the Tang–Toennies potential model suitable for representing heteronuclear ion–atom interactions is proposed. It has the form V(r) =A exp(−br) −B exp(−br/2) −∑3n=2 f2n(r) C2n r−2n with f2n(r) =1−[∑2nk=0 (br)k/k!] exp(−br) and displays realistic behavior over a wide range of r. It provides an accurate representation of both spectroscopic data and potentials derived from mobility. Parameters are tabulated for HeNe+, HeAr+, HeLi+, and XeCl−.

The emission spectrum of ArXe + from a supersonic jet discharge

Ultraviolet and visible emission spectra from a d.c. discharge in supersonically expanding mixtures of Ar and Xe have been photographed under medium and high resolution. The low rotational temperature of the spectra facilitates the vibrational analyses of five band systems previously observed by Y. Tanaka, K. Yoshino, and D. E. Freeman [ J. Chem. Phys. 62, 4484–4496 (1975)] and interpreted by them as charge-transfer transitions of ArXe +. Four of the systems share two upper and two lower states. The fifth system has no state in common, but is linked to the other four band groups by fragments of a weak sixth system. The energies of all six ω components arising from Ar( 1S) + Xe +( 2P) and Ar +( 2P) + Xe( 1S) have been established. The ground state dissociation energy D 0 0 = 1445 ± 30cm −1 (0.179 ± 0.004 eV) was derived by extrapolation from the observed vibrational levels, taking into account the long-range behavior of the ion-atom interaction.

Long-Range Ion-Atom Interactions Studied by Field-Dissociation Spectroscopy of Molecular Ions

A long-range ion-atom potential is for the first time measured directly by electric-field-induced dissociation of selectively laser-excited $\mathrm{O}_{2}^{+}$ ions in a fast beam. The laser excitation spectrum reflects the molecular properties at normal internuclear distances of 1-2 \AA{} while the threshold field required to dissociate the excited molecule determines the ${\mathrm{O}}^{+}_{\ensuremath{-}}\mathrm{O}$ potential in the range 10-20 \AA{}.

Increased energy losses in off-beam directions for MeV protons traversing thin films

This article reports on a negative attempt to understand the experimental data of Ishiwari et al. (Phys. Rev. A25 (1982) 2524) on the angle-dependent energy-loss of MeV protons traversing thin films. They observed an increasing energy-loss with increasing exit angle, and an apparent saturation at large angles. The explanation proposed by Ishiwari et al., as a small angle multiple scattering process which reduces the effective impact parameters in the ion-atom interaction, is investigated. The predictions of multiple scattering theory, with a reasonable impact-parameter dependent inelastic energy loss function, are shown to be at least a factor of three below the reported experimental values.

Ion-atom interaction potential effects on the shape of energy spectra of ions backscattered by a thick target

Abstract Based on the general Gaudsmit-Saunderson-Lewis model for multiple scattering in a semi-infinite medium, an expression accounting for the effects of multiple scattering on the shape of the backscattering energy spectrum has been obtained. This expression is represented by a series of ion angular distribution moments. The limits of the applicability of the small-angle approximation to the description of multiple scattering are defined. It is shown that the sensitivity of the angular distribution moments of multiple scattered ions to the type of ion-atom potential increases with an increase in the moment order. The possibility of investigation of the ion-atom interaction potential experimentally over a wide range of impact parameters by the backscattering method is discussed.

Information on surface thermal vibrations from kev ion scattering

The possibilities of obtaining information on surface phonons from (quasi-)triple collisions in keV ion scattering (LEIS) are reinvestigated. Sensitivities to the ion-atom interaction potential can be elucidated with a semi-analytical three-atom model without the need for expensive computer simulations. The most reliable information is obtained from measurements along the low index (〈100〉 and 〈110〉) azimuthal directions. A simple recipe is proposed for acquiring potential-independent information on surface thermal vibrations.

Information on surface thermal vibrations from keV Ion scattering

Abstract The possibilities of obtaining information on surface phonons from (quasi-)triple collisions in keV ion scattering (LEIS) are reinvestigated. Sensitivities to the ion-atom interaction potential can be elucidated with a semi-analytical three-atom model without the need for expensive computer simulations. The most reliable information is obtained from measurements along the low index (〈100〉 and 〈110〉) azimuthal directions. A simple recipe is proposed for acquiring potential-independent information on surface thermal vibrations.

Neutralized ion-beam spectroscopy

Research on the neutralization of ion beams by electron-capture collisions with atomic particles has largely been within the domain of atomic and high-energy physics with the emphasis on understanding the fundamentals of ion-atom interactions. This account presented a chemist's perception of the possibilities to obtain new information about molecules formed by neutralization of beams of molecular ions. Only reactions in which a molecule is formed in an electron-capture process are discussed. In this article neutralized ion beam spectroscopy was defined as a technique for investigating modes of energy disposal in species formed when a fast beam of ions is neutralized in the electron-transfer reaction AB/sup +/ + M(g) ..-->.. AB* + M*/sup +/ where AB/sup +/ is any diatomic or polyatomic ion and M is a thermalized target atom. The target atom series M = Cs, K, Na, Ca, Mg, and Zn that covers an ionization potential range of 3.9 to 9.4 eV. The most significant technical advance from neutralized ion beam studies is the capability of producing beams of stable or metastable molecules that would be difficult, if not impossible, to obtain by other methods. 5 figures, 2 tables. (DP)

Charge transfer to helium ions from metastable helium

The cross section for charge transfer in He +He(2 3S) collisions is calculated using the asymptotic theory of ion-atom interactions, including the effects of polarization and momentum transfer. The results agree well with recent measurements by the merged beam technique.

Gaseous ion transport coefficients

Numerical techniques are discussed that allow both the mobility and diffusion coefficients of gaseous ions to be calculated efficiently and to a predetermined level of accuracy from knowledge of the atomic ion-atom interaction potential at a discrete set of separations. These techniques are applied to the case of Li + ions in He gas.

Velocity Dependence of the Ionization Probability of Sputtered Atoms

The velocity dependence of the ionization probability of sputtered ${\mathrm{O}}^{\ensuremath{-}}$ from chemisorbed oxygen layers on V and Nb is studied. The ionization probability is found to depend on the normal component of the emission velocity, which suggests that the ionization process is an ion-surface interaction and not an ion-atom binary interaction. For ${v}_{\ensuremath{\perp}}g1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cm/sec, the ionization probability shows an exponential dependence on ${v}_{\ensuremath{\perp}}$. However, this velocity dependence fails at lower velocities.

Dynamics of ion channeling at low energies: Preliminary trajectory studies

In order to understand better recent experiments which have examined the channeling of deuterium ions in tungsten single crystals at low incident energies, we have computed classical trajectories using various sets of ion–atom interaction parameters. Both the penetration depth profiles and the backscattered ion energy spectra display strong dependences upon the particular input parameters but to a much lesser extent upon the level of accuracy to which the individual trajectories are calculated. On the basis of our results we are able to suggest experiments which may resolve present uncertainties about the appropriate description of the microscopic interaction forces.

MCSCF potential energy surface for the high barrier adiabatic 1 4Σ− pathway of the O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction

The collinear 4Σ− pathway for the state-specific O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction has been surveyed with ab initio calculations. A ninety-nine configuration, fifteen orbital multiconfiguration self-consistent field (MCSCF) wave function, involving the use of a double-zeta plus polarization one-electron basis, was developed for the long range 4Σ− state. This long range 4Σ− state has the character of O++N2 for long RNO, or of N+NO+ for long RNN, and is for most geometries, the lowest, or 1 4Σ−, state. The ab initio exothermicity computed with the present wave function is 0.93 eV, compared to an accurate experimental value of 1.10 eV. The saddle-point in the energy surface is 8.0 eV above O++N2, with critical values of R*NN=1.48±0.02 Å and R*NO =1.38±0.02 Å. These values are 0.38 and 0.32 Å greater than the equilibrium bond lengths of N2(X 1Σg+) and NO+(X 1Σ+). The present wave function reproduces the experimental bond lengths of these two diatomics to within 0.01 Å when the third atom is removed to 500 a.u. (264.6 Å). Preceding the barrier on the O++N2 side is an appreciable polarization well. With the present wave function the attributes of this well are D° (N2–O+) =0.38 eV, RNNE=1.108 Å, RNOe =2.350 Å, ϑeNNO=180°, keNN=24.679 md/Å, keNO=0.295 md/Å, kϑ=0.0366 md/Å. There is also a smaller polarization well on the N+NO+ side. Although the ion–atom interaction is more stable at Cs the latter well is bound by 0.02 eV at 180° with ReNN=3.34 Å, and ReNO=1.07 Å.

Laboratory studies of some ion-atom reactions related to interstellar molecular synthesis

view Abstract Citations (66) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Laboratory studies of some ion-atom reactions related to interstellar molecular synthesis. Viggiano, A. A. ; Albritton, D. L. ; Fehsenfeld, F. C. ; Adams, N. G. ; Smith, D. ; Howorka, F. Abstract The rate coefficients and product ions have been determined for reactions involving C(+), CH(+), CH2(+), CH3(+), CH5(+), C2(+), C2H(+), C2H2(+), D(+), OH(+), and H2O(+) ions with N and O atoms and NO molecules at 300 K. The values obtained for the rate coefficients are generally substantial fractions of the collision-limiting values, indicating that ion-atom reactions can be important steps in the synthesis of some of the molecules observed in interstellar clouds, in accordance with previous models. Publication: The Astrophysical Journal Pub Date: March 1980 DOI: 10.1086/157766 Bibcode: 1980ApJ...236..492V Keywords: Interstellar Chemistry; Interstellar Matter; Ion Atom Interactions; Molecular Physics; Reaction Kinetics; Electron-Ion Recombination; Molecular Ions; Neutral Atoms; Positive Ions; Astrophysics full text sources ADS |

Mobilities of C+, S+, and F− ions in helium

The ion mibilities of atomic ions C+, S+ and F− in helium have been measured. The mobilities may be used to deduce the ion-atom interaction potential at collision energies than previously possible (AIP)

Potential and stopping-power information from planar-channeling oscillations

A method is developed for analyzing measurements of the backscattering of ions from a crystal under planar-channeling conditions in which the intensity of scattered ions is observed as a function of emergent energy. Attention is concentrated on two features of such observations, the energy separation of the strong series of peaks seen when the ion beam is exactly parallel to the planes and the angle ${\ensuremath{\psi}}_{M}$ of inclination of the beam to the planes that produces the maximum yield of backscattered ions. Computer simulation of ion trajectories using various assumed ion-atom interaction potentials is used to show that ${\ensuremath{\psi}}_{M}$ is a measure of the strength of the potential. The simulation results are also used to show how the half wavelength of the oscillations can be calculated from ${\ensuremath{\psi}}_{M}$. From the half wavelength and the separation of the peaks in the measured energy spectrum of the backscattered ions, the average stopping power the ions experience in their oscillating trajectories can be calculated. Applications of the method to experiments are discussed.

TRANSPORT PROPERTIES OF PARTIALLY IONIZED GASES

The main problems arising in the calculation of transport coefficients of partially ionized gases are discussed. Particular attention is given to the convergence of the Chapman-Enskog method, to the influence of electron-atom, ion-atom and ion-ion interactions and of the electronically excited states in affecting the transport properties of a plasma. The bulk of results refer to LTE plasmas, even though some non equilibrium effects are taken into account. The accuracy of available tabulations is then discussed and an application of present results to a problem of plasmachemistry interest is finally presented.

Monte Carlo simulation of ion motion in drift tubes

The motion of a swarm of ions in a uniform electric field is studied by simulating the motion of a single ion through many collisions with neutral atoms in order to obtain the drift velocity, average energy, and velocity distribution for the ions. For K+ ions in He at low field strengths, the results agree well with the solutions of the Boltzmann equation by Kumar and Robson; and for K+ in Ar at all field strengths, the computed mobilities demonstrate that the Viehland–Mason moment method can give useful results, especially if carried through to third order. The velocity distributions computed for O+ ions in He and Ar are used in the accompanying paper by Albritton et al. to analyze drift tube measurements of O+ reaction rates. Significant deviations from the Maxwell–Boltzmann form have been found and are seen to have important effects in that application. Velocity distributions have also been obtained for Li+ in He. The sensitivity of ionic mobilities to changes in the ion–atom interaction potential is examined with particular reference to K+ ions in Ar.

Isotope effects in ion-scattering double spectroscopy curves for He+→Zn, In, Sn, and Pb

An ion-scattering double spectroscopy (ISDS) method which traces the dependence of the height of an ISS mass peak on the ion-incident energy has been described. The resulting curves show marked differences for different ion–atom collision pairs and thus can be used for surface atom identification once standard ISDS curves are available. Here ISDS curves for He+→Zn, In, Sn, and Pd are presented for a larger energy range than formerly available. Furthermore, these data are presented for both isotopes of He. It is shown that there are significant differences in the magnitudes and positions of the extrema of ISDS curves when data from 3He and 4He→X are plotted and compared. These differences further distinguish the ISDS signatures of various surface atoms. An explanation for these differences is offered, and a method, exploiting the isotope effects, is described by which information regarding ion–atom interaction potentials might be obtained.