| 1. |
- Brage, Tomas, et al.
(författare)
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MCHF calculations of autoionization widths in two- and three-electron systems
- 1993
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 26:4, s. 621-640
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Tidskriftsartikel (refereegranskat)abstract
- We present calculations of autoionization widths using a fully numerical multiconfigurational Hartree-Fock method (MCHF). Energy positions and autoionization rates are given for several doubly-excited states lying below the n = 2 threshold in He I and H-. Particular attention is given to orthogonality considerations in the computation of the autoionization widths. The method is extended to the 1s2p2 2D, 1s2s(3S)3d 2D and 1s2s(3S)4d 2D states of Li I. To the authors' knowledge, our MCHF value, 12.5 meV, is the first result obtained for the autoionization width of the 1s2s(3S)4d 2D state. Comparison is made with other theoretical results and with experimental values and we point out several inconsistencies between the two sets of results.
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| 2. |
- Frasinski, L. J, et al.
(författare)
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Multielectron Dissociative Ionization of Co2 In Intense Laser Fields
- 1994
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 27:6, s. 97-109
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Tidskriftsartikel (refereegranskat)abstract
- The multiphoton ionization of CO2 has been studied using a laser of 794 nm WaVelength and focused intensities in the region of 10(16) W cm-2. The kinetic energy distributions and ion yields obtained using time-of-flight spectroscopy coupled with covariance mapping can be interpreted most simply in terms of charge-symmetric fragmentation, the strongest channel being O2+ + C2+ + O2+. These results are compared with two recent experiments on CO2, one using an intense laser at 248 nm, the other using Si8+ ion projectiles of 3 MeV u-1 energy.
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| 3. |
- Muffett, J. E, et al.
(författare)
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Numerical Modeling of the Spatial Profiles of High-order Harmonics
- 1994
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 27:23, s. 5693-5706
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Tidskriftsartikel (refereegranskat)abstract
- The far-field spatial distributions of high-order harmonics in an ionized medium have been investigated using a computer model that includes both the single atom effects and the collective effects of the medium. The results show that depletion of the non-linear medium due to ionization and radial phase variations across the gas jet cause broadening and complicated structures in the far-field spatial distributions. These phase variations are due to three factors: focusing, ionization, i.e. depletion of the non-linear medium and electron dispersion, and intensity-dependent phase variations of the atomic dipoles.
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| 4. |
- Starczewski, Tomas, et al.
(författare)
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Time-resolved Harmonic-generation In An Ionizing Gas
- 1994
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 27:14, s. 3291-3301
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Tidskriftsartikel (refereegranskat)abstract
- We report on a time-resolved study of the fifth harmonic generated in xenon by 140 ps pulses from a Nd:YAG laser in the 10(13) W cm-2 intensity regime. Absolute timing between the driving laser pulse and the harmonic pulse could be determined by means of reference harmonics generated in non-linear crystals and a single common streak camera. Above a certain laser intensity the centre of the observed harmonic pulse was shifted earlier in time relative to the laser pulse. We found the intensity dependence of this shift to be approximately linear within the intensity range used which is consistent with the results of numerical simulations taking the ionization and dispersion of the xenon gas into account.
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| 5. |
- Hägg, Lotten, et al.
(författare)
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Gaussian Approximations to Equilibrium Charge State Distributions with the Maximum Entropy Method
- 1994
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075. ; 27:1, s. 81-95
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Tidskriftsartikel (refereegranskat)abstract
- Charge-state distributions for ions in beam-foil experiments are discussed in detail and analysed using the maximum entropy method (MEM). The symmetric distributions can be well described by one Gaussian in the whole charge-state interval. It is found that asymmetric distributions, exhibiting the shell effect, can be treated using two disjoint Gaussians, in two separate intervals. The relative weights of these Gaussians are interpreted by using Fermi-Dirac statistics. Physically this corresponds the varying occupancy of some of the shells in the ions. The fact that no more than two moments, in one interval or two, is sufficient is of significant importance in the analysis of charge-state distributions in general. An interpolation procedure to obtain the charge-state distribution for an energy is developed and tested. The procedure, which is made possible by the Gaussian character, uses MEM and experimental information. It is only possible to predict distributions in energy regions where experimentally measured values for other energies exist. The method is tested on asymmetric distributions of copper ions in the energy range 0.902-1.505 MeV u-1 and symmetric distributions for calcium, titanium, yttrium, rhodium, holmium and bismuth ions for energies from 0.191 to 3.184 MeV u-1.
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| 6. |
- Karlsson, Hans O, et al.
(författare)
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A direct recursive residue generation method : application to photoionization of hydrogen in static electric fields
- 1994
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 27:6, s. 1061-1072
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Tidskriftsartikel (refereegranskat)abstract
- In studies of hydrogenic systems via the recursive residue generation method (RRGM) the major bottleneck is the matrix vector product HC, between the Hamiltonian matrix H and a Lanczos vector C. For highly excited states and/or strong perturbations the size of H grows fast leading to storage problems. By making-use of direct methods, i.e. avoidance of explicit construction on of large Hamiltonian matrices, such problems can be overcome. Utilizing the underlying analytical properties of the Laguerre basis e(-lambdar)L(k)2I+2(2lambdar) a direct RRGM (D-RRGM) for the unperturbed hydrogenic Hamiltonian is derived, changing the storage needs from scaling as N2 to 4N where N is the number of radial functions for each factorized H-0(l, m) block with the possibility of parallel processing. A further computational simplification is introduced by putting the expression for the photoionization (PI) cross section in the rational form conventionally used in the representation of density or states (DOS). This allows the construction of the PI cross section directly from the tridiagonal Lanczos matrix avoiding the explicit calculation of individual eigenvalues and eigenvectors. To illustrate and verify the method the PI cross section for a hydrogen atom in a static electric field, for both pi and cr polarization, was calculated for an electric field strength of F = 5714 V cm-1. Sufficiently large basis sets could be employed so that good comparison with experiment and other theoretical work was obtained, including the field-induced modulations near the zero-field ionization limit.
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| 7. |
- Karlsson, Hans O, et al.
(författare)
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Perturbed hydrogenic manifolds studied by the recursive residue generation method
- 1992
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 25:23, s. 5015-5028
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Tidskriftsartikel (refereegranskat)abstract
- A method for calculating the perturbation of hydrogenic manifolds, the emerging bound states and resonances, for arbitrary combinations of external fields, is presented. It requires the combined use of complex dilation, an orthonormal Laguerre basis e(-lambdar) L(k)2l+2 (lambdar) rather than the non-orthogonal Sturmians e(-lambdar) L(k)2l+1 (lambdar), and the recursive residue generation method (RRGM) version of the Lanczos algorithm. Generalized eigenvalue problems are avoided. Furthermore, direct computation of the residues of resolvents, transition amplitudes and sum rules is achieved, Comparison with other methods and with previous calculations, suitable for one perturbation at a time, indicates that high accuracy is achieved separately both for the 1s Stark resonance and for the 1s Zeeman effect. Accurate results for the 1s Stark-Zeeman resonance, for various combinations of fields, are given.
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| 8. |
- Glans, Peter, et al.
(författare)
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Experimental and theoretical investigation of the soft x-ray emission spectrum of molecular oxygen
- 1993
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - 0953-4075. ; 26:4, s. 663-673
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Tidskriftsartikel (refereegranskat)abstract
- A high-resolution x-ray emission spectrum of molecular oxygen is presented. Comparisons with ab initio calculations are made regarding relative intensities, energies, and band profiles for the outer valence, 2PI(g), 4PI(u), 2PI(u), 4SIGMA(g)-, 2SIGMA(g)-, levels. The importance of lifetime-vibrational interference is discussed.
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| 9. |
- Glans, Peter, et al.
(författare)
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X-ray emission spectroscopy measurements of fluorine substituted methanes
- 1994
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - 0953-4075. ; 27:15, s. 3399-3414
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Tidskriftsartikel (refereegranskat)abstract
- We present the carbon and fluorine x-ray emission spectra of methane and the fluoromethanes (CH4-xFx, x = 0, 1, 2, 3 and 4), measured in high resolution. Relative x-ray emission intensities in dipole velocity and length gauges have been derived from the independent particle approximation using Hartree Fock ground state molecular orbitals. The results of the calculations are compared with the experimental spectra. In the experimental carbon (K-V) emission spectrum of CF4 We observe dipole-forbidden electronic transitions. We suggest that these transitions arise due to vibronic coupling, i.e. intensity is borrowed from the dipole-allowed transitions.
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| 10. |
- Hägg, Lotten, et al.
(författare)
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Maximum Entropy and Equilibrium Charge State Distributions
- 1993
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Ingår i: Journal of Physics B. - 0953-4075. ; 26:15, s. 2345-2358
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Tidskriftsartikel (refereegranskat)abstract
- The maximum entropy method is implemented in order to describe equilibrium distributions arising in beam-foil spectroscopy. Since there are very few charge states involved, the usual moment conditions, based on simple powers xi, give rise to severe numerical difficulties already for three moments and cannot be applied in these systems. Earlier devised methods, based on Lagrange interpolation polynomials Qi(x) with abscissae chosen as zeros of Chebyshev polynomials in the interval being studied, are adapted and implemented for the present problems of charge state distributions. A reduced variable x= (q-)/, where q is the charge and the mean charge, is chosen. Using the method described above calculations of equilibrium charge state distributions for copper ions (exit energy range 0.599-2.304 MeV u-1) colliding with carbon foils are carried out in order to exhibit the usability of the method. The new moment conditions associated to the Qi(x) provide a framework for a systematic analysis of equilibrium distributions. In future work the algorithm will be applied for systematic studies of charge state distributions, of approach to equilibrium, of deviations from Gaussian behaviour, of the shell effect etc. Applications to related problems like fragmentation will also be possible.
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