Table of contents

The author presents a classical analysis of the energy and field strength dependence of the stability coefficients (Liapunov exponents) for certain periodic orbits of a hydrogen atom in a uniform magnetic field.

The authors report wavelength spectra of the photoelectron yield resulting from three-photon ionisation and four-photon excess-photon ionisation (EPI/ATI) of xenon in a magnetic field. The spectra are scanned over a wavelength region of 1 nm to 307 nm (three-photon threshold for ionisation of Xe). The EPI behaviour is studied while they scan through the threshold. They conclude from the measurements that the phenomena which are typical for EPI do not only take place above the threshold, but also below it as long as the ionisation pulse is short compared with the time period that elapses before the wavefunction of the intermediate state is relocalised around the core. This time period is at least one classical revolution, but is increased in this experiment by the application of a magnetic field.

There has been discussion recently as to whether the dominant mode of multiphoton ionisation in Xe is sequential (single-electron) or collective (many-electron) in character. In order to address this question, the authors have performed an experiment with the isoelectronic molecule HI using a laser of a peak power density 3*10¹⁵ W cm^-2. The results show categorically that the process cannot be collective. In addition, they indicate how the dynamics of the multielectron dissociative ionisation of a molecule such as HI can be investigated on a time scale of femtoseconds using a picosecond laser.

The cross section for mutual neutralisation in collisions of protons with negative hydrogen ions has been calculated for impact energies of 8.0 and 50.0 keV. The multi-state impact parameter method with rectilinear trajectories and an expansion in travelling atomic orbitals on both centres is employed. The sensitivity of the results to the wavefunction for H^- is investigated.

Absolute elastic differential cross sections for electrons scattered from cadmium vapour at incident electron energies of 60, 75, 85, 100 and 150 eV and scattering angles of 10-70' have been measured. The generalised oscillator strength of the transition 5¹P to 5¹S was used to place the elastic cross sections on an absolute scale. No calculations of elastic differential cross sections are available for cadmium.

Hyperfine interaction constants A and B in the 10²D_5/2 state of ⁸⁷Rb were obtained by the analysis of quantum beats in the fluorescence 10²D_5/2-5²P_3/2 following two-quantum excitation. The resulting values are mod A mod =0.510 (10) MHz, mod B mod =0.098 (11) MHz and A/B<0.

The probability density functions P(u) for the interelectronic distance u in the 2³P and 2¹P states of the two-electron ions from He to Mg¹⁰⁺ are calculated from highly accurate explicitly correlated wavefunctions and also from self-consistent field wavefunctions of near Hartree-Fock quality. Effects of electron correlation on the P(u) densities ae examined with the help of their moments as well as Coulomb holes. Counter-intuitive effects such as an increase in interelectronic repulsion upon inclusion of electron correlation occur for helium and smoothly disappear as the atomic number increases.

The spin-orbit interaction is considered as a sequence of magnetic percussions in which the particle Hamiltonian jumps stochastically. It is possible to connect the depolarisation rate with the problem of random walk in the plane. A quantitative study is made for polarised and thermal neutrons passing through a lead slab; it is found that spin-orbit interaction does not modify the neutron polarisation.

A new absorption spectrum of PN has been obtained and photographed at high resolution in the spectral region 1300-1500 AA. The rotational analysis of the bands reveals four new electronic states of PN which are named alpha ¹ Pi , beta ¹ Pi , gamma ¹ Sigma ⁺ and delta ¹ Sigma ⁺. Two Pi states as well as two Sigma states are found to interact with each other homogeneously. A discussion on electron configuration is also given.

The degree of alignment induced in excited levels by two-step excitation with short broadline optical pulses having perpendicular linear polarisations is considered for different sequences of angular momenta J₀ to J₁ to J₂. The results are compared with those recently reported for the simpler case of two-step excitation with pulses having parallel linear polarisations. It is found that for certain sequences, such as J to J to J+1 and J to J+1 to J+1, for which the degree of alignment in the case of parallel polarisations is small or even zero, the degree of alignment in the perpendicular case is large. These results can be applied to determine the optimum excitation polarisations in Zeeman quantum beats experiments for a given two-step excitation sequence.

The autoionising energy levels of Cu atoms in the vapour phase have been investigated by ejected-electron spectroscopy. Following excitation by electron impact at 9-500 eV incident electron kinetic energy, autoionised electrons were detected with energies in the range 0-3.5 eV corresponding to 41 non-radiative transitions. The autoionising levels have been assigned with the aid of new ab initio pseudorelativistic Hartree-Fock calculations with configuration interaction, and by comparison with earlier photoabsorption data. For high incident electron energies, the observed autoionisation features are predominantly associated with decay of the Cu I 3d⁹4snp (n=4, 5, 6 . . .) J=1/2, 3/2 levels to continua based on the Cu II ground state, although the absence of autoionising features whose energies exceed that of the Cu II 3d⁹4s ³D₃ final state indicates that when energetically permitted this is the preferred channel. At low incident electron energies, both parity-forbidden and J-forbidden levels are observed to autoionise.

The relative intensities and energies of the KLL, KLM, and KMM Auger electrons of iron (Z=26) were measured using an electrostatic spectrometer. Discrepancies exceeding five standard deviations were observed between the experimental KL₁L₁(¹S₀), KL₁L₂(¹P₁) and KL₂L₃(¹D₂) intensities and those calculated relativistically in the intermediate-coupling scheme with configuration interaction. The measured KL₃M_2,3/KL₂M_2,3 intensity ratio disagrees completely with the prediction based on the jj-coupling scheme while it is close to the value calculated in the approximation of intermediate coupling. The intensity ratios KLL : KLM : KM_1-3M_1-3 : K-14.4 keV=(4.0+or-0.6) : (1.0+or-0.1) : (0.05+or-0.02) : (3.0+or-0.3) were determined.

A theory of resonant multiphoton ionisation with perturbed Rydberg series as intermediate states is developed which parametrises the atomic density matrix equations with the help of multichannel quantum defect theory. The authors apply the theory to stepwise three-photon ionisation of Ba near the 5d7d ¹D₂ state.

The work reported has been carried out using an intrinsic germanium detector in order to extend to a lower energy region previous investigation on Rayleigh scattering between 0.1 and 1 MeV. Differential cross sections for Rayleigh scattering of 59.5 keV photons from Sn, Mo, Ta, Er, Cd, Nb and Yb, measured for angles ranging from 60 to 165', are presented. The results are discussed in comparison with theoretical calculations.

Spectroscopic and kinetic studies have been made on the mercurous halides HgX (X=Cl, Br, I) photodissociated from HgX₂ by an ArF excimer laser excitation at 193 nm. The lifetimes and quenching rates of the HgX B(² Sigma ⁺₁2/) state have been determined from the properties of the B to X (² Sigma ⁺₁2/) emission. Comparison is made of the spectroscopic behaviour of the three halides and also between photon and electron dissociation. The HgX (B to X) spectra consist of numerous overlapping bands extending from the visible to the UV. The vibrational spectral distribution narrows from Cl, through Br to I and the peak emission wavelength becomes shorter. Characteristics of gain and laser action in HgBr have been investigated.

The multiphoton dissociation of the mercury halides HgX₂ (X=Cl, Br, I) to yield highly excited Hg* atoms using ArF and KrF excimer laser excitation is described. The dominant excitation mechanism for the production of Hg* atoms is shown to be via a three-photon sequential excitation path from HgX₂(x) through HgX₂(b), HgX(B), HgX(virtual) to Hg (7³S and 6³D).

A three-body problem is considered in toroidal coordinates. It is shown that the distance between two particles placed at the foci of the toroidal coordinate system can be expressed in terms of the hyperradius of a three-body system; the variables in the kinetic energy operator are separable for zero total angular momentum. An equivalent treatment of the original problem is also possible in four-dimensional space in the cylindrical coordinate system. In this approach the free motion of three particles is characterised by three quantum numbers: hypermoment, poloidal and azimuthal.

Using thermodynamic Green's functions, effective potentials and related virial coefficients for electron-atom interactions are derived within the second Born approximation. In addition to the exact results for hydrogen, improved values for the virial coefficients of electron-alkali-atom interactions are obtained.

The fluorescence intensities for the D lines in non-resonant excitation of the alkali-helium pairs under single-collision conditions were obtained from calculations of the optical collision spectra by the quantum coupled-channels method. The close relation of the non-adiabatic transition probability from a molecular level to an atomic state during the final stage of an optical collision and the spectral dependence of the relative fluorescence intensities is investigated. An analysis of the impact region in terms of line-broadening and fine-structure cross sections calculated separately for all systems confirms the factorisation approximation.

N⁺ excited states were observed in a N₂/He plasma. The plasma, which was confined in a capillary tube, was created by current pulses. Concentrations of the excited species He*, N*₂, N⁺*₂, N*, N⁺* were measured spectroscopically during the discharge pulse. N⁺* and N* lines were recorded over the entire visible spectrum at a time corresponding to the maximum emission. From these studies the following remarks can be drawn. Only five line series were detected in N⁺*. They belong to the triplet system and their upper levels are nearly resonant with the singlet n=2, 3 He levels. This strongly selective emission suggests a He* (singlet)+N⁺ (triplet ground state) to He (singlet ground state)+N⁺* (triplet state) excitation transfer process in which energy and spin are conserved. From the kinetic study, N⁺ (ground state) appears to be mainly produced by the resonant He⁺-N₂ charge transfer process, and excited in the triplet states by excitation transfer from He* (singlet). This last point is directly evidenced in a laser-induced fluorescence experiment. When pumping the 3 ¹P He level at 5016 AA, induced fluorescence is observed on the 5001 and 5005 AA transitions starting from the 3d ³F N⁺ levels. A kinetic model limited to the observed species is compared with the experiment. From this comparison, the rare coefficient for He*-N⁺ excitation transfer is estimated to be 2*10^-9 cm³ s^-1.

The relativistic Born series for the transition amplitude for electron capture is summed using approximations which permit the closure relation to be used, thus producing the relativistic symmetric eikonal approximation. Two different forms of the prior eikonal phase factor are obtained. One form includes the operator which transforms the interaction potential from the projectile frame to the target frame while the other form does not. It is demonstrated that both forms provide legitimate and closely equivalent expressions for the transition amplitude for electron capture assuming that 'soft' elastic collisions occur before and after electron capture takes place.

The target continuum distorted wave (TCDW) theory is derived and developed. It describes charge exchange in asymmetric collisions, in particular the collision of a projectile ion of low charge with a target ion of high charge. The final state is represented by a TCDW function. The initial state is taken as a travelling atomic orbital, while the intermediate states are represented by a Coulomb-Green function. The theory is applied to capture by protons from the inner shells of heavy atoms. For s-s transitions, it is found that within a standard on-shell full-peaking approximation the second-order amplitude does not contribute. The total cross section for K-K electron capture in proton-carbon, neon and argon atom collisions is calculated in the energy range corresponding to values of Z_T/v from 0.5 to 3.0, where Z_T is the nuclear charge of the target and v the velocity of impact in atomic units. Agreement with experiment is excellent in all three cases and the cross section values compare extremely favourably with those of the well known strong-potential Born theory.

Translational-energy spectra for state-selective single-electron capture by Ar²⁺ from atomic (He, Ne, Ar, Kr, Xe) and molecular (N₂, O₂, NO, N₂O, NH₃, CO₂, CH₄, C₂H₆, 1-C₄H₈, C₆H₆) target gases are recorded at 6 keV impact energy using a reversed-geometry double-focusing mass spectrometer. Spectra indicate the presence of the ³P, ¹D and higher excited states of Ar²⁺, the populations of which are controlled by varying the ionising electron energy E_e. For Ar²⁺-He at E_e=70 eV, capture from the first excited ¹D state of Ar²⁺ into the ground state Ar⁺ is most intense, whereas at E_e=43 eV, ground state to ground state capture dominates. For Ar²⁺ -rare-gas systems, relative cross sections are discussed in terms of calculated reaction windows. For molecular targets, dissociation is evident in many cases. Broad peaks and long tails in the translational energy spectra indicate reaction channels involving closely spaced energy levels. An additional peak due to the capture of delocalised electrons is observed for unsaturated hydrocarbons which has not been previously reported.

The convergence of a finite L² expansion of the exchange interaction is studied in the single-centre close-coupling formulation of electron-molecule scattering at the static exchange level. It satisfies no variational principle; some confusion about the convergence process may have resulted in the past from the emphasis given to a diagonal representation of the exchange operator. A number of expansion sets are tested, which include short-range multicentre molecular orbitals derived from Slater-type atomic basis sets and long-range one-centre numerical orbitals. An expansion set optimising the rate of convergence is derived. Illustrative results are presented for e-H₂ and e-N₂ scattering and compared with previous calculations in order to assess the accuracy of the method and its range of applicability.

The use of the (e, 2e) technique as a tool to investigate electron momentum densities in atoms and molecules has until now been essentially confined to symmetric kinematics. Its application has often been hampered by the long time needed to perform the experiments. This work shows that asymmetric (e, 2e) events can also be described in the framework of the simple plane-wave impulse approximation even for energies of the ejected electron as low as 100 eV and intermediate incident energies (about 1500 eV), provided the kinematics belong to the Bethe ridge. The larger cross section associated with the asymmetric kinematics allows the integration time to be reduced by one order of magnitude. Electron momentum distributions of the 1s orbital of He and the 3 sigma g and 2 sigma u orbitals of N₂, as determined in asymmetric kinematics, are presented, together with an estimate of the achieved reduction in integration time compared with the symmetric conditions at equal incident energy.

Second-order effects for the problem of electron excitation of the 2p state of hydrogen are examined. Some different types of second-order amplitudes which result from the distorted-wave series and the plane-wave series are discussed and fundamental amplitudes of each type are evaluated exactly. The effect of using different Green's functions as well as the effect of distortion in both the first- and second-order terms is analysed. The validity of the closure approximation for both the second-order Born approximation and the second-order distorted-wave approximation is examined through comparison with equivalent exact results. The accuracy of the local-potential approximation is discussed.

The accuracy of several pseudostate basis sets for calculating second-order amplitudes is determined by comparing second-order distorted-wave amplitudes calculated using pseudostates with recent exact results. The accuracy of both the total basis sets and individual s-, p- and d-state components of the basis sets is determined and improved basis sets are formed by combining the best s-, p- and d-state basis sets. Both the improved basis sets and the original basis sets are used to calculate physically measurable quantities and the errors for the various pseudostates are determined. The primary objective for the study is to find an accurate pseudostate basis set which may be used in a close-coupling calculation since this method shows the most promise for obtaining accurate cross sections. The question of whether the pseudostate basis sets found in the paper which give accurate DWB2 results for excitation of the 2p state of hydrogen may also be used in a close-coupling calculation with comparable accuracy is addressed.

The presence of an axially symmetric angular distribution of energetic electrons in hot plasmas can be detected by measuring the linear polarisation of the X-ray lines emitted from collisionally excited levels of multiply charged ions. The authors have evaluated the degree of linear polarisation of Fe²⁴⁺ resonance lines and Fe²³⁺ inner-shell excitation satellite lines observed in soft X-ray emission of solar flares and tokamaks. Collision strengths for exciting upper line magnetic sublevels were calculated using a distorted-wave method for the electron impact energy range up to 20 times the excitation threshold. It is shown how a plasma diagnostics of these non-Maxwellian electrons can be derived in a practical case.

The ejected-electron spectra of europium arising from autoionising and Auger transitions in Eu I and Eu II following excitation by electron impact have been measured in the energy region from 0 to 27 eV; incident-electron energies were in the range from 20 to 500 eV. 151 spectral features are tabulated and excitation functions are reported for a number of autoionising and Auger transitions. The chemi-ionisation electron spectrum of europium atoms reacting with oxygen and water vapour is reported in the electron energy region from 0 to 1.5 eV.

The threshold electron spectrum of molecular nitrogen has been studied using a high-energy-resolution electron impact spectrometer incorporating a penetrating field analyser for low-energy scattered electrons. Spectra are presented for the energy region from 6 to 19.2 eV. The assignments of the observed features are discussed.