Showing papers in "Journal of Physics B in 1983"

Ionisation of atoms by ion impact

Abstract: Total cross sections are calculated for the ionisation of a hydrogen atom by multicharged fully-stripped ions in the 20-1000 keV amu-1 impact energy range. Distortion is accounted for in the entrance channel (via the eikonal approximation) and in the exit channel (via the continuum distorted-wave approximation). The transition amplitude is calculated in the post form so that the electronic nonorthogonal kinetic energy is treated as the perturbation. It is concluded that of the currently available models this theory is the most successful and versatile over a considerable range of energies and charges. Specifically for ionisation of a hydrogen atom by 50 keV protons the authors present doubly differential cross sections for electrons ejected in the forward direction and singly differential cross sections as a function of emission energy. The question of cusps and peaks in the differential cross sections is considered as is the question of charged scaling of the total cross section.

On the reliability of semi-empirical pseudopotentials: simulation of Hartree-Fock and Dirac-Fock results

Abstract: Semi-empirical pseudopotentials can be modified in a simple way so as to reproduce frozen-core Hartree-Fock or Dirac-Fock results, respectively. The influence of relativistic effects is quantitatively discussed (i) for (rn) expectation values and dipole polarisabilities of group IA and IB atoms, and (ii) for the bond lengths of K2+ and Cu2.

Measurements of the dissociative recombination coefficients of O2+, NO+ and NH4+ in the temperature range 200-600K

Abstract: Measurements are presented for alpha t, the dissociative recombination coefficients with electrons of O2+, NH4+ and NO+ under truly thermalised conditions within the temperature range 200-600K using a flowing afterglow/Langmuir probe apparatus, alpha t(O2+) is found to vary as approximately T-0.7 in close accord with previous pulsed afterglow data for alpha t(O2+) and alpha e(O2+) and with values for alpha e(O2+) inferred from ion trap data. alpha t(NH4+) is found to vary as approximately T-.06. alpha t(NO+) is found to vary as approximately T-0.9 which is reasonably consistent with previous pulsed afterglow data for alpha t(NO+). These data are also compared with values of alpha 3(NO+) measured in a pulsed afterglow experiment and those derived from ion trap and merged beam cross section data and from atmospheric observations.

Ponderomotive force and AC Stark shift in multiphoton ionisation

Abstract: A non-perturbative approach to the photoionisation of a model atom in arbitrarily strong fields is described. An explanation is given for the disappearance of the lowest energy electrons in a multiphoton ionisation experiment at very high intensities, in terms of the AC Stark shift of the ionisation potential of the atom, followed by ponderomotive acceleration of the photoelectrons.

Multiply charged ions induced by multiphoton absorption processes in rare-gas atoms at 1.064μm

Abstract: Multiply charged ions are easily formed in rare gases by multiphoton absorption processes. For Kr and Xe up to quadruply charged ions are formed. They are induced by a bandwidth-limited 50 ps laser pulse at 1.064 W cm-2 intensity range. Doubly charged ions are formed through the absorption of a very large number of photons (29 for Xe) in a direct transition from the ground state of the atom to the second ionisation limit. The percentage of Xe2+ ions relative to Xe+ ions is 1% at 1013 W cm-2, and reaches 20% at 1014 W cm-2. At 1.06 mu m, the removal of two electrons from the ground state of rare-gas atoms in a single step is responsible for the creation of doubly charged ions. This could be a general rule for atoms which have two or more electrons in the outer shell.

Electron scattering in intense laser fields

Abstract: The authors report quantitative measurements of multiphoton free-free (FF) transitions in a controlled three-beam experiment: electrons elastically scattered from argon in the presence of a CO2 laser pulse having an average flux density between 106 and 108 W cm-2. They present FF spectra for four incident electron energies, two scattering angles, and two directions of the laser polarisation vector. Electrons that have changed their kinetic energy by as many as eleven photons have been detected. From the spectra the absolute average flux density of the laser pulse was determined and the validity of the sum rule examined. Both results are indirect verifications of the theory.

Non-empirical model for the imaginary part of the optical potential for electron scattering

Abstract: The authors propose a new kind of model for absorption of flux from the elastic scattering channel in electron-atom or electron-molecule collisions. It is a non-empirical model so it predicts the absorption cross section as well as the elastic scattering wavefunction and elastic differential cross sections. They have tested the model for electron scattering by He, Ne and Ar at impact energies 30-400 eV. Agreement with experiment is very good for both the absorption cross sections and elastic differential cross sections.

Highly-excited double Rydberg states of He-

Abstract: By measuring the yield of metastable atoms resulting from electron-impact excitation of helium, in the range of incident electron energy from 19.8 to 24.6 eV, the authors have obtained evidence for He- resonances in which both the excited electrons have high values of the principal quantum number. The resonances occur in distinct groups, the lower members of which form Rydberg series characterised by effective core charges that are nonintegral. Classification schemes for these resonances are discussed.

Elastic scattering of electrons from neon and argon

Abstract: The authors continue their earlier study on the effects of polarisation and exchange in low-energy elastic scattering of electrons from noble gases. They present detailed results for argon and conclude that including only the dipole part of the polarisation potential and treating exchange exactly produces the most reliable results. The behaviour for neon is also very similar to that for helium. Values for phaseshifts, differential, total elastic and momentum transfer cross sections are given for neon and argon and compared with recent experimental and theoretical results.

Theoretical study on low-lying electronic states of Na2

Abstract: Nineteen ground and excited states have been calculated using a HF pseudopotential, a valence CI, and a core-valence perturbation. This work provides extremely precise adiabatic potential curves that will facilitate the interpretation of the experimental data. Three currently employed calculational methods (model potential, frozen-core all-electron, HF pseudopotential) are also discussed.

Effect of symmetry on two-electron escape at threshold

Abstract: The threshold law for the escape of two electrons from a positive ion, first derived by Wannier (1953) for 1Se states, is examined for more general values of the quantum numbers S, L and pi of the final state. In all cases except 3Se and 1Pe, the threshold behaviour of the cross section has the same Wannier exponent as for 1S3, namely sigma varies as E1.127 for a singly charged ion. Only the 3Se and 1Pe symmetries exhibit a much larger exponent. In all other symmetries there is an analogue of the +/- quantum numbers familiar from the study of doubly excited states, with the + component always exhibiting the Wannier exponent. The relevance of these findings to recent measurements of spin-selected cross sections in e-atom ionisation is pointed out. The authors also discuss the dependence of the angular distribution of the ejected electrons on L and S.

Charge transfer and ionisation processes involving multiply charged ions in collision with atomic hydrogen

Abstract: Total cross sections are presented for the charge transfer reaction Xq++H(1s) to X(q-1)+H+ and ionisation process Xq++H(1s) to Xq++H+-e-. Here Xq+ is a fully or partially stripped ion. The collision energy range lies between 25-200 keV amu-1. Cross sections are also given for charge transfer into particular ionic n states and n, l states for energies of 25 and 50 keV amu-1, where Xq+=H+, He2+, C6+, O8+. The calculations were performed by the classical trajectory Monte Carlo method of Olson and Salop (1977), modified to allow for a better representation of the H(1s) electronic radial distribution. Good agreement is seen with previous classical calculations and with quantum calculations performed by Ryufuku and Watanabe (1979) and others. The authors' results exhibit excellent accord (within 20%) with the measurements of Shah and Gilbody (1981).

Total scattering cross sections for low-energy positrons in the molecular gases H2, N2, CO2, O2 and CH4

Abstract: The method described by Charlton et al. (1980) to measure total cross sections for the scattering of electrons from helium and argon has been used for similar measurements with positrons in the molecular gases H2, N2, CO2, O2 and CH4 over an energy range from 2 to 20 eV. The apparatus used is, with minor modifications, the same as that of Charlton et al. and incorporates a localised scattering cell close to the slow positron source which requires the measured attenuations to be normalised to previously known absolute cross sections. The apparatus used in this study, together with the normalisation procedure and the method of data analysis, is described in detail.

High-resolution electron impact excitation functions of metastable states of neon, argon, krypton and xenon

Abstract: Electron impact excitation of the lowest-lying metastable states of neon, argon, krypton and xenon have been studied as a function of incident electron energy over the range from threshold to the 2P1/2 ionisation potential and at an energy resolution of 20 meV or better. A wealth of structure is observed. Several of the features are very narrow and are coincident in energy with excited states having the classification np5n'p(J=0), while several other features correspond to pairs of resonances that have almost the same energy splitting as that of the corresponding grandparent ion doublet. The interpretation of the structures and classification of the resonances are discussed.

Inclusion of core-valence correlation effects in pseudopotential calculations. II. K2 and KH lowest Σ+ potential curves from valence-correlated wavefunctions

Abstract: For pt.I see J. Chem. Phys., vol.77, p.3571 (1982). The recently proposed perturbative treatment of core-valence correlation effects is extended to valence correlated wavefunctions. As expected, the valence correlation slightly diminishes the core-valence correlation for the ground state. This effect is illustrated for the atomic electron affinity of K. The core-valence correlation plays a very important role in the spectroscopy of K2 and KH, where it increases the lowest X1 Sigma + to A1 Sigma + transition energies by 2300 and 1100 cm-1, respectively.

Merged electron-ion beam experiments. V. Dissociative recombination of OH+, H2O+, H3O+ and D3O+

Abstract: For pt.IV see ibid., vol.14, no.8, p.1353-671 (1981). Absolute cross sections for dissociative recombination of OH+, H2O+, H3O+ and D3O+ have been measured using a merged electron-ion beam apparatus through an energy interval from 0.01 to 1 eV. The recombination cross section for the diatomic OH+ is small, one of the smallest measured by the authors for a diatomic ion. The cross sections for the polyatomics H2O+, H3O+ and D3O+ are large and nearly identical. Like most other polyatomic ions they exhibit nearly an Ecm-1 energy dependence below approximately 0.1 eV. At higher energies the slope of the cross section versus electron energy is much steeper with an energy dependence near Ecm-2. Cross sections for D3O+ are less than those for H3O+ indicating the presence of a small isotope effect.

Self-interaction correction to the local density Hartree-Fock atomic calculations of excited and ground states

Abstract: The application of the self-interaction correction to the local density Hartree-Fock calculations of atomic levels is discussed. For fixed-basis-set calculations a unified Hamiltonian is derived so that diagonalisation of the matrix of this operator gives both the occupied and excited levels. The formulation has advantages over the earlier ones where an orbital-dependent Hamiltonian must be solved separately for each state. Excited-state energies for eight light elements have been calculated by using this unified Hamiltonian approach and the results are in excellent agreement with experiment.

Ab initio approach to the multichannel quantum defect calculation of the electronic autoionisation in the Hopfield series of N2

Abstract: The two-step formulation of the multichannel quantum defect theory, applied to molecular electronic autoionistion by Giusti-Suzor (1980) and Lefebvre-Brion (1973) is used to calculate the total and partial photoionisation cross sections in the region of the Hopfield series on N2, the 700-730 AA wavelength range. The electronic quantities necessary for this treatment are obtained from ab initio calculations. The cross sections and the photoelectron angular distribution parameters are in satisfying agreement with experiment. The results clearly show that the absorption series correspond to the (B2 Sigma u+)nd sigma g series and the apparent emission series to the (B2 Sigma u+)nd pi g series.

Effect of crossed electric and magnetic fields on sodium Rydberg states

Abstract: The excitation spectrum for Rydberg states near n=11 of atomic sodium in crossed electric and magnetic fields has been recorded. The theoretical analysis of the spectroscopic data reveals degeneracies among some sublevels even though different sublevels are mixed by the external fields. The degeneracies are traced to a dynamical symmetry that is characteristic of the Coulomb potential.

Rotational excitation of CH4 and H2O by slow electron impact

Abstract: Cross sections for rotationally elastic and inelastic scattering of slow electrons by spherical (CH4) and asymmetric top (H2O) molecules are calculated in the adiabatic-nuclei-rotation (ANR) approximation. The elastic scattering amplitude is derived in the molecule-fixed frame and the corresponding K matrix is obtained by solving a set of coupled differential equations; in which the total interaction potential includes an ab initio molecular core potential from the ground state near Hartree-Fock wavefunctions, an exchange term in the free-electron-gas-exchange (FEGE) model plus the orthogonalisation procedure and the distortion of the target through a recently proposed nonparametric polarisation potential of Jain and Thompson (1982). Symmetry properties and the selection rules for the asymmetric rotor in general are discussed. In order to avoid convergence problems for the molecule with a permanent dipole moment, an alternative two-stage hybrid S-matrix theory of Collins and Norcross (1978) along with the closure formula of Crawford and Dalgarno (1971) is adopted to obtain converged cross sections (in which proper use of the unitarised Born approximation is made). Results are compared with recent available measurement and calculations; there is a qualitative agreement for CH4 with recent calculations performed in a different model. In the case of H2O, some difficulties are discussed in comparing directly the authors' data with recent experimental cross sections.

Positron annihilation in the positronium negative ion

Abstract: A theoretical study of positronium negative ions is carried out. Hylleraas-type wavefunctions are employed to describe the system and an improved ground-state energy is reported. In addition, the two-photon annihilation rate and the lifetime against two-photon annihilation are also calculated, as well as the two-photon angular correlation function.

Positronium formation cross sections in the inert gases

Abstract: New measurements of positronium formation cross sections are reported for the inert gases at energies up to 150 eV. These data show pronounced structure in the cross sections of all gases in the form of secondary peaks and plateaus above the ionisation potentials.

Second-order effects in large-angle coplanar symmetric (e, 2e) processes

Abstract: Theoretical and experimental triple differential cross sections are compared for the ionisation process e-+He(11S) to He+(1s)+2e-, at an incident electron energy of 200 eV, in a coplanar symmetric geometry. At large angles, the theoretical curve obtained by including the second Born contribution due to the initial target state differs markedly from the first Born result and exhibits a characteristic behaviour which is confirmed by the experimental data.

Threshold structures in the cross sections of low-energy electron scattering of methane

Abstract: The energy and angular dependences of the elastic and inelastic scattering of electrons from methane have been measured in the range of collision energies from 0.1 to 1.8 eV. Of special interest are the shapes of the excitation functions of the four possible vibrational modes of the molecule, because the elastic cross section shows a pronounced Ramsauer-Townsend minimum and this is compatible with the existence of a short-lived compound state of the electron-molecule system being detectable very close to the thresholds of the inelastic channels. The measurements show structures right at the thresholds, which have half widths of 0.1-0.3 eV and seem to be predominantly in the s and f-waves of the scattered electrons. In addition, a cusp structure has been found in the nu 2, nu 4 channel. The transmission functions of both the electron gun and the electron optics of the detector have been carefully controlled and therefore the peak shapes and half widths of the threshold structures should be more accurate than the results of Rohr (1980) who first reported the existence of the threshold peaks in methane at a 60 degrees scattering angle.

State-selective electron capture by C2+, C3+, N2+ and Ar2+ ions in rare gases

Abstract: Energy loss/gain spectroscopy is used to study state-selective one-electron capture by 1.6-6 keV C2+ in He, Ne and Ar, by 0.8-8 keV N2+ in He and Ne, by 0.14-5 keV Ar2+ in He and Ne, and by 3-18 keV C3+ in He. The relative contributions of the various collision product channels are determined. Moderately exothermic processes involving curve crossings at moderate internuclear separations are shown to be dominant and a strong influence of metastable ions in the primary beam is observed for some reactions. Serious discrepancies with some previous measurements are noted. The main collision channels observed are in accord with the Wigner total electron spin conservation rule.

Excitation of vibrational and electronic states in a glow discharge column in flowing N2

Abstract: The excitation and ionisation number densities of different states of N2 have been measured in a long positive column of a glow discharge in flowing N2 when the residence time of excited species in the column was varied between 5*10-4 and 2.5*10-1 s. The vibrational population of the ground state N2 (X 1 Sigma g+, nu ) and the electron energy distribution function (EDF) have been calculated for different values of gas pressure, electric field and electron densities. For residence times greater than 10-2 s the N2(X, nu ) and the EDF tails are strongly overpopulated. The vibrational population of the excited states N2(C3 Pi u, nu ') and N2+(B2 Sigma u+, nu ') have been experimentally determined by measuring the second positive and first negative band intensities. The measured N2(C, nu ') distributions are in good agreement with the calculated populations assuming that the C3 Pi u state is created by electron collisions and destroyed by radiative emission. The ionic lines emitted from N2+(B, nu ') have large intensities for long residence times t>10-2 s suggesting that the heavy particle reaction: N2(X, nu >or=12)+N2+(X) to N2(X)+N2+(B) is the main process.

The importance of continuum d states for the interaction nsnp6↔ns4d in the rare gases

Abstract: Calculations are described of the interaction between the nsnp6 2S term and the ns2np4md/ epsilon d2S series in atoms isoelectronic with the halogens, particularly the rare gases For the halogens, it has been found earlier that the interaction with the continuum d series is crucial for a correct description of the term structure A similar, although weaker, effect of the continuum is established for the rare gases Detailed results for the admixture of nsnp6 2S in bound and continuum d states are reported for Cl I, Ar II, K III and Kr II Only in K III is the effect of the continuum found to be negligible The present results are important for the interpretation of satellite structures connected with the ejection of an ns electron from the rare gases It is shown that there is good agreement between the present calculations and the satellite intensities in photoelectron experiments The experimental uncertainties in photoelectron experiments are at present the limiting factors in the comparison and the need for better experimental values in order to test the calculations is stressed Disagreements are found between the calculations and the results of (e, 2e) experiments These disagreements are attributed to difficulties in the interpretation of the (e, 2e) experiments

Double excitation in the K absorption spectrum of neon

Abstract: The high resolution spectrum of gaseous neon in the 1s region was obtained by using synchrotron radiation. In addition to previously observed single-electron excitation lines, the authors observed two new sharp lines of comparable intensities in the continuum at 35.1 and 38.1 eV above the 1s to 3p resonance line. On the basis of the known Ne 1s XPS shake-up lines the authors attribute them to transitions to 1P0 levels of the 1s2s22P53p2 final-state configuration.

Theoretical study of the lowest states of CsH and Cs2

Abstract: The 19 lowest states of CsH and the 11 lowest states of Cs2 have been calculated using a non-empirical relativistic effective core potential considering the caesium atom as a one-electron system. The core-valence polarisation and correlation energy has been included employing a recently developed perturbation treatment. The experimental atomic spectrum of caesium is well reproduced. The calculated energy curves show the remarkable precision of this procedure for the ground-state characteristics. For CsH, Re=4.47 Bohr (experimental 4.71 Bohr), De=1.80 eV (experimental 1.83 eV), omega e=871 cm-1 (experimental 891 cm-1); for Cs2, Re=8.75 Bohr (experimental 8.78 Bohr), De=0.40 eV (experimental 0.45 eV), omega e=41 cm-1 (experimental 42 cm-). The well-to-well transition energies are in error by less than 400 cm-1 for experimentally known transitions.

Diffuse bands in the visible absorption spectra of dense alkali vapours

Abstract: The authors report the results of extensive absorption measurements in dense alkali vapours with the main emphasis on the diffuse bands, located close to the short wavelength end of the B1 Pi u-X1 Sigma g+ alkali-band system. At extremely high pressures they succeeded in observing Li2 and Na2 diffuse bands at about 422.5 and 436.5 nm, respectively. Detailed measurements of the absorption coefficient of potassium diffuse band at 575 nm show square dependence on the atom density and independence on the temperature. The relevant data for dense rubidium and caesium vapours have been remeasured. The authors believe that diffuse bands originate in a transition from the ground triplet state towards the 23 Pi g state, which suffer peculiar configuration interaction with the first excited ionic state of the same symmetry.