Table of contents

A fourth-order Wannier description of electron-pair escape has been extended to include nonzero total angular momentum. A key element is the electron-pair centre-of-mass momentum, which is fully represented by the momentum of the recoiling ion. General and Wannier-theory predictions are presented for recoil-ion angular distributions following double-photoionization of helium.

The band system in the and molecules is investigated by the polarization labelling spectroscopy technique. Accurate molecular constants are derived for the observed levels v = 0 - 30, J = 18 - 95 in the state.

We report on high-resolution vibrationally-resolved photoabsorption spectra of gas-phase silane ( and ) in the region of the Si-2p absorption threshold. Isotope shifts were observed for the energies of the core-to-Rydberg excitations between the vibrational ground states: in , these excitations were found to be shifted by meV as compared to . These isotope shifts are attributed to differences in the zero-point vibrational energies, , of and , both in the electronic ground and in the excited states. The results confirm isotope effects in the zero-point vibrational energies of core-excited molecules.

Absolute total cross section for electron collisions with molecules has been measured in a linear electron transmission experiment for impact energies between 0.75 eV and 250 eV. Experimental cross section function versus energy reveals a resonant-like broad enhancement with maximum between 3 and 4 eV, monotonically decreasing at higher energies.

We present results from R-matrix with pseudo-states (RMPS) and convergent close-coupling (CCC) calculations for electron impact total ionization of the 1S and 2S states of atomic hydrogen in the energy region from threshold to 100 eV. Particular attention is given to the near-threshold region. We find the results for energies more than 2 eV above threshold to be in excellent agreement with the available experimental data.

The triple differential cross section in coplanar asymmetric relativistic (e,2e) processes is not symmetric about the direction of momentum transfer. It is demonstrated that this symmetry, so familiar from the nonrelativistic first Born approximation, is in the relativistic case already broken at the first Born level due to the effect of retardation and magnetic contributions to the electron - electron interaction. A further shift away from the direction of momentum transfer is due to distortion effects in the incoming and outgoing channels. Similar effects are found in coplanar symmetric geometry.

The traditional theory of many-electron atoms and ions is based on the coefficients of fractional parentage and matrix elements of tensorial operators, composed of unit tensors. The calculation of spin-angular coefficients of radial integrals appearing in the expressions of matrix elements of arbitrary physical operators of atomic quantities has two main disadvantages: (i) the numerical codes for the calculation of spin-angular coefficients are usually very time consuming; (ii) f-shells are often omitted from programs for matrix element calculations since the tables for their coefficients of fractional parentage are very extensive.

The authors assume that a series of difficulties persisting in the traditional approach to the calculation of spin-angular parts of matrix elements can be avoided by using this secondly quantized methodology, based on angular momentum theory, on the concept of the irreducible tensorial sets, on a generalized graphical method, on quasispin and on the reduced coefficients of fractional parentage.

Cs vapour in , Ar, Kr, Ne, He and Xe buffer gases was optically pumped with a diode laser in a magnetic field of 1.516 T. The relaxation of the population imbalance in the hyperfine Zeeman sublevels of the ground state was detected with another diode laser. A phenomenological theory is presented which explains the optical transients and the time variation of the populations in the hyperfine Zeeman sublevels. Both experimental and theoretical results showed that there were relaxation transients of populations in some hyperfine Zeeman sublevels of optically pumped Cs with `anomalous' shapes. The experimental results showed that the `anomalous' relaxation depended strongly on the buffer-gas species and the buffer-gas pressure. Theoretically, it was found that the combined electron-spin - nuclear-spin transition, which is mainly induced by collisional modification , caused the `anomalous' shapes of the transients. The collisional transfer cross section for the binary collisional interactions was estimated to be . `Anomalous' relaxations of populations in the hyperfine Zeeman sublevels are discussed. The results of the calculation are in good agreement with the experimental results.

We report on one-photon laser spectroscopy experiments in which we have obtained the even z-parity diamagnetic spectrum of Sr by actively cancelling the motional Stark effect. This spectrum differs from previous experimental studies in that it contains no electric-field-induced structure. New calculations are reported at a resolution comparable with the experimental data, and an excellent agreement between theory and experiment is obtained.

We calculate the total direct double photoionization rates in the strong field approximation for the helium atom in an intense field of a circularly polarized plane wave. We study the role of the electron - electron interaction both in the initial and the final states. We find that for the frequencies and intensities considered here, the initial electron - electron correlation can be neglected, while the repulsion in the final state changes the total rate only quantitatively, and becomes irrelevant as frequency grows. The electron energy spectra are also obtained.

A recently proposed K-matrix technique to solve the random phase approximation equations projected on basis sets of functions has been applied to a theoretical study of the outer-shell ionization processes in . Cross sections, photoelectron angular distribution parameters and branching ratios for the ionization of the valence , 1 e and inner-valence shells of ammonia have been calculated as a function of the photon energy in a range of more than 100 eV above threshold.

An iterative method for calculating total cross sections for molecular photodissociation and atomic photoionization is presented. It is based on the recursive expansion of Green's operator with absorbing boundary conditions, recently developed by Mandelshtam and Taylor. The method is tested by calculating cross sections for photodissociation of the molecule and another model problem, as well as for the photoionization of a hydrogen atom in the absence and presence of a strong magnetic field. In all applications the discrete variable representation is employed.

Auger decay of resonantly excited states (n = 5, 6, 7, 8 and 9) in Kr has been studied by means of synchrotron-radiation-excited electron spectroscopy utilizing the Auger resonant Raman effect. Many transitions have been identified for the first time. Shake probabilities as well as branching ratios of final state parent multiplets have been studied for a wide variety of principal quantum numbers. Valence photoelectron satellite intensities have been found to increase clearly at resonance photon energies.

We have measured the 17.3 GHz circularly polarized microwave ionization (CPMI) threshold fields for Na Rydberg atoms and compared them to the previous 8.5 GHz measurements. In both cases the ionization fields are near for low n and fall below this value at high n. A small ellipticity in the polarization and a small static field in the plane of the rotating microwave field both lead to ionization at substantially lower fields, approaching . In addition, elliptically polarized microwave ionization (EPMI) of Rydberg atoms and CPMI in the presence of a small static field have much lower threshold fields for ionization and unexpected resonant peaks at microwave fields below the CPMI threshold fields are observed in both cases at 8, but not at 17, GHz. Sequences of resonant microwave transitions are proposed to explain these resonant structures.

Two-dimensional photoelectron spectroscopy has been used to investigate vibrational selectivity accompanying electronic autoionizing decays of Rydberg states between the and ionization thresholds in . Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron radiation. This study, which is the most comprehensive to date, has enabled the vast majority of the accessible vibrational levels of the ion to be investigated. The experimental resolution was good enough to observe individual vibrational levels and complex vibrational selectivity was evident throughout the two-dimensional spectrum. A significant number of the features in the spectrum could be ascribed to vibrational progressions in the symmetric stretch mode.

Using very stable intense fs Ti:sapphire laser pulses, new structures in the ion versus intensity plot of are observed which can be attributed to electron trapping in excited electronic vibrational levels. This is further studied by a new and more sensitive technique based on high-order harmonic generation.

We have calculated collision strengths for electron impact excitation of inelastic transitions in Ar II using the R-matrix method in two independent nine- and 19-state close-coupling approximations. In the nine-state calculation the , , , , , , , and states are included, while in the 19-state calculation these states plus an additional ten states , , , , , , , , and are considered. These target states are represented by fairly extensive configuration-interaction wavefunctions which yield excitation energies and oscillator strengths that are generally in good agreement with the available most accurate calculations and the experimental values. Rydberg series of resonances converging to the excited state thresholds are included in the calculation. The effective collision strengths are obtained assuming a Maxwellian distribution of electron energies which are tabulated over the temperature range K.

Experimental and theoretical differential cross sections (DCSs) for electron impact excitation of the five lowest levels of xenon are reported. The experimental data were obtained at impact energies of 10, 15, 20 and 30 eV for scattering angles in the range of to . The scattering intensities for excitation of various levels with respect to that of the combined two lowest levels (representing the corresponding DCS ratios) were measured with conventional electrostatic electron energy-loss spectrometers. Absolute DCSs for excitation of the combined two lowest levels were derived from DCS ratios for excitation of the combined two lowest levels and elastic scattering and utilizing the elastic DCSs of Register et al. These DCS ratios were obtained from scattering intensity ratios measured with conventional electrostatic electron energy-loss spectrometers after correction for instrumental effects by utilizing electron time-of-flight measurements for determining the proper value of this ratio at scattering angle for each impact energy. The unitarized first-order many-body theory and the unitarized distorted-wave approximation were applied to obtain the theoretical differential cross sections for impact energies of 10, 15, 20, 30, 50 and 80 eV. The present experimental and theoretical results are compared with each other and with other available experimental and theoretical DCSs.

Experimental and theoretical differential cross sections for electron impact excitation of Xe are reported for the lowest 6th to 20th levels. These data are an extension of the work discussed in the preceding paper. The theoretical work is based on the unitarized first-order many-body theory and unitarized distorted-wave approximation. The experimental data span the same respective range of electron impact energies and scattering angles as the preceding paper. The present theoretical and experimental data are compared with each other and with other existing experimental data.

Attachment of electrons with energies of 1.2 to 2 eV to ozone was found to lead to the production of O and vibrationally excited , the latter subsequently lose electrons by vibrational autodetachment. This type of electron scattering is intermediate between inelastic electron scattering and dissociative attachment. Spectra of the detached electrons have resolved vibrational structure, which can be assigned to the individual vibronic transitions, providing detailed information on the product state distribution. The absolute integral cross section for the production of in vibrational states was found to be substantial, for example at 1.7 eV it is (with an error bar of about and ). Measurements of the signal onsets permit an independent determination of the dissociation energy of ozone into O and with the result eV. Comparison of the peak width of the electrons detached by in this work with the width of peaks of resonances observed in electron- scattering indicates a moderate degree of rotational excitation in the present process.

Detailed information is given on an experiment which was carried out to investigate the spin-dependent attenuation of electron beams transmitted through a vapour of chiral molecules. The experiment is considered to be the first verification of electron dichroism. A transmission asymmetry for longitudinally polarized electron beams of opposite helicity was observed for , a camphor-like molecule containing an ytterbium atom, and also recently for bromocamphor. Asymmetries of about , which is clearly above the detection limit, were measured for both targets at electron energies between 0.5 and 10 eV. No asymmetry was found when enantiomeric camphor vapour was used as the target, thus contradicting a result which has been previously published by another group. The asymmetry measurements are compared with electron transmission spectra, which show that the energies where resonant temporary-ion formation takes place and those of the extrema of the asymmetry agree to a great extent.

The excitation of the and states of Hg by polarized electrons is discussed in close relation to recent experiments. A full set of state multipoles, necessary for a complete description of the experiments, has been calculated by a relativistic distorted-wave method for 15 and 30 eV incident electrons. Results for the Stokes parameters and the spin-asymmetry parameters are also presented. A comparison between our theoretical results and the available experimental data shows satisfactory overall agreement.

We consider radiative processes in collisions of electrons with metallic clusters and demonstrate that the cluster giant resonance structure manifests itself in the photon emission spectrum. This effect occurs because the cross section of the process is mainly determined by the dynamical polarizability of the cluster. We also demonstrate that the diffraction of electrons at the cluster edge adds some peculiar oscillatory structure to the cross section as a function of the photon frequency and as a function of the transferred momentum of the scattered electron. The results are generalized for ion - cluster and cluster - cluster collisions.

Plasma-screening effects are investigated on semiclassical ionization probabilities for electron-impact ionization of hydrogenic ions in dense plasmas. The projectile - target interaction Hamiltonian is obtained by an appropriate form of the non-spherical Debye - Hückel model. Semiclassical straight-line trajectory method is applied to the projectile path in order to visualize the ionization probability in dense plasmas as a function of the impact parameter and Debye length. The final state of the ejected electron is represented by a plane wave solution normalized in the energy scale. The plasma-screening effect on the ionization probability is appreciably increased with increasing projectile energy. The maximum position of the ionization probability recedes from the nucleus as the projectile energy increases.

We draw attention to the fact that part of a recent article by Chakraborty and Dewangan has already appeared in the literature.

A part of the above article published by Chakraborty and Dewangan [1] is an alternative proof of the addition theorem of regular spherical harmonics. We note, however, that this alternative proof follows a procedure also used by ourselves in proving the theorem [2]. The addition theorem of regular solid harmonics has been applied in numerous calculations over the years. The work of Deb and Sil [3] may be mentioned as one of its earlier applications.

References

[1] Chakraborty S and Dewangan D P 1995 J. Phys. B: At. Mol. Opt. Phys. 28 L769 - 74

[2] Deb N C, Sil N C and Mandal P 1994 Ind. J. Phys. 68B 267 - 70

[3] Deb N C and Sil N C 1983 Phys. Rev. A 28 2806 - 10