Comment on ionisation of the first excited state of hydrogen by ion impact using the continuum distorted wave eikonal initial state approximation

The continuum distorted wave eikonal initial state (CDWEIS) approximation is used to study the ionisation of hydrogen in it's first excited state by proton and antiproton impact in the energy range 10–1000 keV. The total cross sections presented are found to be in disagreement with similar CDWEIS calculations by Fainstein, Ponce and Rivarola [6]. It is concluded that Fainstein et al. have erroneously included a constant multiplicative factor (of 2) in their calculations.     

A quasielastic scattering approximation for target ionisation by heavy structured projectiles

A modification of the impulse approximation for capture to continuum is used to describe the ejection of loosely bound target electrons by heavy ions. In contrast to the currently applied elastic scattering models based on the binary encounter approximation, the present model treats only the short-range part of the ionic field within the elastic scattering theory, but uses the more sophisticated impulse approximation for the Coulomb tail of the interaction potential. Calculations have been performed for B ^q+, Fe ^q+, Au ^q+ and U ^q+ colliding with helium at energies between 0.5 and 6 MeV/amu. The comparison with absolute experimental spectra shows that theory works well for light projectiles, being able to describe the intensity and its variations with ionic chargeq _ion in the binary encounter peak region. For very heavy low-energy projectile ions the intensity is, however, seriously under-predicted at small emission angles, indicating a breakdown of perturbative treatments.     

Effects of the cluster surface on the electronic shell structure: faceting,roughness and softness

Several simple models have been used to study the effects of the surface on the electronic shell structure in metal clusters. The main results are as follows: The icosahedral clusters have the same electronic shell structure as the sphere up to about 1000 atoms. The surface roughness causes the distribution of the level spacings to be a Wigner distribution. By varying the softness of the potential we can obtain potentials where the simplest classical orbits are the five-point star or even the three-point star.This paper was originally submitted in connection with the 2nd. Int. Conference on Atomic and Nuclear Clusters held in Santorini from 28. June–2. July 1993 and is published here as a regular article after an independent refereeing procedure according to the standards of Z. Phys. D     

A method for measuring cross sections for electron-impact excitation out of metastable levels of atoms

A method for measuring cross sections for electron-impact excitation out of the metastable levels of the He atom is described. A hollow cathode discharge is used to produce an atomic beam consisting of ground-level He atoms and the He(2¹S) and He(2³S) metastable atoms. An electron beam of energy below 20 eV crosses the atomic beam exciting the metastable atoms to higher levels, and the intensity of the radiation emitted by atoms in these higher levels is utilized to determine the cross sections. Because of the very low concentration of metastable atoms in the atomic beam, the emission signal is extremely weak. A number of special techniques have been developed to detect these very low-level signals. Absolute calibration of the cross section is accomplished by referencing the emission signal that resulted from electron excitation out of the metastable level to the emission signal that resulted from the 2³S3³P or 2¹S3¹P laser optical absorption.     

Photoionization of small silicon clusters: ionization potentials for Si2 to Si40

Photoionization thresholds of Si _n (n=2–40) were examined by vacuum UV radiation (5.7–8.5 eV) generated by stimulated Raman scattering of narrow-bandwidth 193-nm radiation in high pressure hydrogen and deuterium gases. A strong threshold energy dependence on cluster size is observed, featuring major maxima at 10 and 20. The magic behavior atn=10 is consistent with the results of the photofragmentation and CID experiments reported previously.     

Effects of an avoided crossing on linewidths of resonances in the H2 i3Π g state

We have investigated the influence of nonadiabatic effects on the linewidths of quasi-bound rovibrational levels in the i³ _g state of molecular hydrogen. This state has a potential barrier due to the interaction of then=3³ _g Rydberg series and the Q1³ _g valence state. The radial coupling with the higher lying³ _g states has been modelled into a diabatic problem. The widths of thev=4 and 5 levels, a measure for their lifetimes, have been determined both in the adiabatic i³ _g potential, and in the two-state system. The results show small shifts of the i³ _g (v=4,5;N=1) levels of a few wavenumbers, and a significant increase of 49% in the lifetimes of these levels. Both effects are large enough to be important in spectroscopic experiments. The magnitude of the effect is in accordance with observed linewidths obtained in recent experiments. Model calculations for differently shaped potential barriers have been carried out to study the generality of the above conclusions.     

Optimal Hylleraas wave functions

Hylleraas wave functions composed of the optimally combinedN terms (2 N 20) are presented for two-electron atoms with nuclear chargesZ = 1 (H^–), 2(He), 3(Li⁺), 5(B³⁺), and 10(Ne⁸⁺). The spherically-averaged electron density (r) and electron-pair densityh(r ₁₂) are constructed in a simple and analytical functional form from the 20-term functions. Comparison of several one- and two-electron moments r ^k and r ₁₂ ^k shows that the present density functions have near-exact accuracy.     

Millimeter wave rotational spectra of indium monofluoride

The millimeter wave rotational spectra of indium monofluoride (InF) have been studied in the 250–300 GHz frequency region. Three rotational transitions of¹¹⁵InF in thev=0, 1 and 2 vibrational states and two rotational transitions of the less abundant¹¹³InF in thev=0 state with resolved hyperfine structures have been measured. The analysis including previous measurements on the lower rotational transitions in the microwave region yielded improved and extended sets of Dunham parameters. The comparison of the results for the two isotopic species of InF indicates a similar deviation from the Born-Oppenheimer approximation as for InCl.     

Optimal Kinoshita wave functions for heliumlike atoms

An extensive optimization has been performed for the composition ofN terms, as well as the exponent and the mixing coefficients, of Kinoshita wave functions for heliumlike atoms with atomic numberZ. The optimalN-term Kinoshita functions have been constructed forN=1–10, 20, 30, 40, 50, 100 andZ=1(H^–)–10(Ne⁸⁺). The present results demonstrate that the optimal term selection dramatically improves the accuracy of the Kinoshita function: In the case of He, for example, the optimal 100-term Kinoshita function gives – 2.903 724 376 95 hartrees, which is only 8×10^–11 hartrees higher than the most accurate literature value.     

Time resolved EUV spectroscopy on foil-excited 5.9 MeV/u Xe and 13.2 MeV/u Au ions

N=3, n=0 transitions in Na-like ions have been observed and the resonance level lifetimes have been measured by time-resolved spectroscopy of foil-excited ion beams of Xe (Z=54) and Au (Z=79). Other lines in the same wavelength range are identified withn=3, n=0 transitions in Ne-, Mg-, and Al-like ions. Wavelengths and transition rates for these new lines are compared to the available theoretical data. A number of experimental problems are discussed.     

Calculation of the electronic properties of metal dimers

The low-energy excitation spectra of metal dimers are determined by solving exactly a realistic many-body Hamiltonian with inter- and intra-atomic Coulomb interactions. Our results for Cu₂ and Ag₂ are in very good agreement with the excitation energies derived from recent photodetachment experiments. The characteristics of the many-body excited states in these clusters are briefly discussed.     

Energy transfer in collisions of alkali-metal clusters and ions

The electronic energy loss of a positively charged projectile passing through a sodium cluster is calculated by the semiclassical Vlasov method. The electron density before impact is obtained within the localdensity approximation in the jellium model. For the dynamical response, the Vlasov equations are solved by test particle discretization. The model gives the correct breathing mode and dipole Mie frequencies. The energy deposit is calculated as a function of projectile velocity, charge and impact parameter and is compared with theoretical and tabulated values for bulk sodium.     

The collision cross sections for excitation energy transfer in Rb*(5P 3/2) + K(4S 1/2) → Rb(5S 1/2) + K*(4P J ) processes

The collisional excitation transfer for the processes Rb*(5P _3/2)+K(4S _1/2) Rb(5S _1/2)+K*(4P _J ),J=1/2, 3/2, was investigated using two-photon laser excitation technique with a thermionic heat-pipe diode as a detector. The population densities of the K 4P _J levels induced by collisions with excited Rb atoms as well as those produced by direct laser excitation of the potassium atoms were probed through the measurement of the thermionic signals generated due to the ionization of the potassium atoms emerging from the K(4P _J ) K(7S _1/2) excitation channel. The measurements of the thermionic signals in addition to the spectroscopic determination of the potassium number density yield the following values for the excitation transfer cross sections: ₁(Rb 5P _3/2 K 4P _1/2)=8 Ų and ₂(Rb 5P _3/2 K 4P _3/2)=11 Ų. The accuracy of the presented results is 15%. The obtained results are compared to those for the opposite processes K*(4P _J )+Rb(5S _1/2) K(4S _1/2)+Rb*(5P _3/2).     

Collisional excitation transfer between lithium isotopes

Excitation transfer between the 3S _1/2 states of the lithium isotopes⁶Li and⁷Li is measured in a thermionic diode. The 3S level is excited by off-resonant two-photon transitions with a single mode ew laser. The relative densities of the directly excited and collisionally populated levels are probed by further laser excitation to the 12P levels. An energy transfer cross section of 585 Ų±160 Ų is found at the experimental temperature of about 850 K. A simple semiclassical calculation yields a cross section of 450 Ų.     

Exchange effects on electronic states in jellium clusters

Hartree-Fock calculations are done for finite number of valence electrons (8, 20, 40, and 58) in a positively charged uniform background (jellium) with the density of bulk sodium. Differences and similarities with the results of Kohn-Sham local-density calculations are discussed. State-dependence and suppression of the wave functions are the two important effects coming from the nonlocality of the Hartree-Fock potential. These two effects, however, cancel in the density profile of electrons. The Kohn-Sham calculations consequently produce similar density profiles as in the Hartree-Fock calculations. The dipole polarizability is also calculated for 8-mers. The calculated value is still smaller than the measured one and is not an improvement of the insufficient Kohn-Sham result.deceased     

Coherence parameters for He+(2p) capture and H(2p) excitation in He2+-H(1s) collisions

Semiclassical coupled channel calculations have been carried out for the collision system He²⁺-H(1s) in the velocity range 0.15–3.0 a.u. (impact energies 0.5–225 keV/amu) in order to study capture probabilities and alignment and orientation parameters for the dominant He⁺(n=2) channels. A 14-state AO basis set calculation has been combined with an analytical treatment of the asymptotic collision region. For impact velocities about and abovev=0.6 a.u. a strong propensity for resonance capture into an oriented He⁺(2p) state with the same sense of rotation as the collisional rotation of the internuclear axis is predicted together with a very smooth behaviour of the alignment angle as function of impact parameter. Eikonal method calculations of differential capture cross sections predict that the left/right orientation asymmetry will prevail in differential scattering experiments. The resulting total cross sections for capture into specificnl-substates (n=2, 3) and the total light polarisation parameter for He⁺(2p) capture compare well with previous work. Finally we report H(2s,2p) excitation cross sections, probabilties and H(2p) alignment and orientation parameters, following the established propensity rule for orientation in H(2p) excitation.     

Classical theory of vibration energy transfer in collinear collisions

Classical theory of collisions is cast in a form which also includes the uncertainty principle. This theory is used for analyzing the vibration energy transfer in the collinear collision which approximates the He-H₂ system. The results are compared with the quantum calculations and several classical and semiclassical approaches. Very good agreement with quantum theory is found, for all the parameters investigated.     

Sodium doped binary clusters I: Ionization potentials of SinNam clusters

Sodium doped silicon clusters (Si_nNa_m, 3n11, 1m4) were produced by two independent laser vaporization methods and their ionization potentials were measured by scanning the wavelength of the UV dye laser. The IPs of most Si_nNa_m clusters decrease monotonously with the number of Na atoms, but IPs of Si₇Na_m and Si₁₀Na_m clusters show an apparent even-odd alternation; odd numbers of Na atoms efficiently decrease the IP but even numbers of Na atoms never significantly decrease the IPs. In addition, the reactivity of Si_nNa_m clusters for NO molecules was investigated with a fast flow reactor, and an anti-correlation between IP and the reactivity was clearly observed; clusters having high IP show low reactivity andvice versa.     

Inner-shell ionization in near-central collisions of 6.7 MeV/amu4He ions with mid-Z atoms

We report a self consistent tight binding calculation of the cohesion energy of small MgO clusters, up to 12 atoms. We discuss the size variation of the charges, the bond lengths, the electronic gaps. For the first time, we predict the existence of a strong dimerization of the bond length in the small clusters. We underline the consequences of the competition between covalent and electrostatic effects in these ionocovalent systems.     

Resonant double electron capture by fast He2+ from helium: the first-order Born approximation with correct boundary condition

The first-order Born approximation with correct boundary condition is applied in a study of the resonant double electron capture by fast He²⁺ from helium. A configuration-interaction wave function is employed to describe the ground state of helium. Total cross section as a function of the impact energy is calculated and compared with experimental and theoretical values.