Table of contents

Molecular autoionization (MAI) is a Penning-like process involving high Rydberg state electrons from two neighbouring excited atoms and can be the dominant mode of free electron production in a cold excited gas. The density of a gas may be determined employing the sensitivity of MAI to the average inter-ionic separation R_N. We show by explicit calculation that a sharp correlation exists between the gas density and Rydberg state quantum number n, manifested through an abrupt increase in the number of ejected electrons as n is increased to a critical valuen_th = R_N^1/2/2. The MAI electrons have discrete energies allowed by energy conservation, at 6/n⁴ Ry, 2/n³ Ry, 4/n³ Ry, and so on.

R-matrix calculations of the photoionization of the core-excited 1s2s2p ⁴P^o state of atomic lithium have been performed. The cross section is shown to be dominated by hollow atom resonances, in contrast to valence-excited Li where these hollow resonances are small modulations of the background continuum cross section in the higher energy region.

Strong correlations between the cross sections and threshold energies for positronium formation in positron-impact single ionization of noble gases are shown to exist over a wide energy range. These correlations also extend to higher degrees of ionization, and they can be used as an empirical tool for predicting such cross sections within the relevant Ore gaps. Similar correlations are shown to exist for the exothermic positronium formation process in positron-impact single ionization of the alkali atoms. All the single ionization data are consistent with the hypothesis that the magnitude of the positronium formation cross section is determined by the modulus of the difference between the kinetic energies of the incident positron and the emerging positronium. An explanation is given for the qualitative difference between the alkali atoms and the noble gases in the magnitude of the ratio of the cross sections for positronium formation into the ground and first excited states.

The R-matrix method is used to calculate elastic and the excitation cross sections of the six lowest lying electronically excited states of the ClO molecule. These states, of symmetry 1 ⁴Σ^-, A ²Π, 1 ²Σ^-, 1 ²Δ, 2 ⁴Σ^- and 1 ²Σ⁺, have vertical excitation energies in the range 3.48 to 6.99 eV. Except for the state A ²Π, all other excited states are dissociative. We find a bound state of ClO^- with ¹Σ⁺ symmetry with an adiabatic electron affinity of 1.128 eV at an equilibrium bond length of 3.25 a₀. There are shape resonances of ¹Π and ³Π symmetries at 1.6 and 2.9 eV respectively. Rotationally summed cross sections are obtained for elastic and electronically inelastic scattering for electron-impact energies up to 10 eV.

We present measurements of the resonant Auger electron emission at an electron kinetic energy of ~810 eV in the Ne atom following 1s → 3p excitation at an excitation photon energy of 867.12 eV, with a photon band pass of 60-68 meV, an electron energy resolution of 13 meV, and the Doppler width due to thermal motion of the sample Ne atoms of 79 meV. Under these conditions, the overall resolution of 100-105 meV (FWHM) is much smaller than the natural width, 250 meV, of the Ne 1s hole state, and the resonant Auger final states 2p^-2(¹D₂)3p (or 4p) ²P, ²D, ²F are completely (or partially) resolved. The obtained β values and branching ratios are in good agreement with first principles calculations by means of the multi-configuration Dirac-Fock methods within the framework of the two-step model.

We present calculations of single- and double-ionization rates of helium at 390 nm, accurate to within 10%, obtained from a full-dimensional integration of the time-dependent Schrödinger equation. The theoretical results are compared with experimental data at the same wavelength. Excellent agreement is obtained, allowing for likely uncertainties in the experimental determination of laser intensity.

The recollision model has been applied to separate the probability for double ionization into contributions from electron-impact ionization and electron-impact excitation for intensities at which the dielectronic interaction is important for generating double ionization. For a wavelength of 780 nm, electron-impact excitation dominates just above the threshold intensity for double ionization, ≈1.2×10¹⁴ W cm^-2, with electron-impact ionization becoming more important for higher intensities. For a wavelength of 390 nm, the ratio between electron-impact ionization and electron-impact excitation remains fairly constant for all intensities above the threshold intensity for double ionization, ≈6×10¹⁴ W cm^-2. The results point to an explanation of the experimental results, but more detailed calculations on the behaviour of excited He⁺ ions are required.

We have evaluated charge transfer cross sections for Na₉⁺ + Cs collisions using a molecular close-coupling formalism in the framework of the independent electron model. Our results are in good agreement with recent experimental measurements below v = 0.04 au and confirm existing discrepancies with previous experimental data. At low velocity, the calculated cross section exhibits the typical behaviour found in ion-atom collisions: a rapid increase with v followed by a plateau. More interestingly, our theory predicts that the cross section increases again at higher velocities, which calls for experimental confirmation.

We present new results for electron transfer cross sections in C⁶⁺-H collisions at low and intermediate impact energies (0.1-500 keV amu^-1). The impact parameter semiclassical method is used within the atomic-state close-coupling approach. Special attention is concentrated on the C⁵⁺(n = 5) capture channels, for which severe discrepancies have been reported at low impact energies. We identify and discuss the mechanisms giving rise to the discrepancies in terms of avoided crossings in the molecular energy diagram. Trajectory effects are also introduced with the use of channel-selective deflection patterns obtained by classical trajectory Monte Carlo calculations.

A new exact solution of the time-dependent quantal equation is obtained for the full array of angular momentum mixing transitions nℓ→nℓ' in atomic hydrogen induced by collisions with charged particles at ultralow energies. Based on this new solution, efficient numerical procedures are devised. It is proven that the present (fixed-frame) solution is equivalent to the rotating-frame approach described by Kazansky and Ostrovsky (Kazansky A K and Ostrovsky V N 1996 Phys. Rev. Lett.77 3094) and that it overcomes the difficulties therein. Analytic expressions for low quantum numbers n are presented. Numerical results for the transition array with n = 28 are reported.

Strong variation in the branching ratio of partial decay channels across the resonance is observed for the 1s^-13p resonance in Ne photoemission. It is attributed to the interference between the resonant and direct amplitudes of photoionization. The observed phenomenon is used to obtain the experimental ratio of the direct and resonant contributions to the resonance.

The first results for electron emission associated with dissociative and non-dissociative capture processes in low velocity ¹⁸O⁸⁺ + CO₂ collisions are discussed. The electron spectrum is very similar to those obtained with heavy atomic targets; it is typical of multiple capture processes followed by autoionizing cascades, with strong lines superimposed on a weak continuum. No indication of a molecule target effect is found. It is shown that detailed information can be extracted from the dissociation patterns which concern the excitation of the target. As an example, the population of O⁶⁺(4l4l') states is analysed thoroughly. It is shown to correspond mainly to a double capture process accompanied by a target excitation. When the autoionization has occurred, the target dissociates into various channels for which kinetic energy release distributions are obtained. The similarity between the present findings and photoionization data is emphasized.

An analysis of photon emission by He⁺ interacting with intense, short laser pulses is presented. Employing one- and two-dimensional model atomic systems, the time-dependent Schrödinger equation is solved numerically for few-cycle laser pulses with wavelength 800 nm and a peak intensity of 5.6×10¹⁵ W cm^-2. The low ionization probability of positive ions leads to photon emission spectra that display prominent plateaus whose height and cut-off energy depend strongly on the phase and duration of the laser pulse. These features can be explained using the semiclassical three-step model of harmonic generation. Ions also allow the investigation of effects due to the magnetic field component of the laser pulse. For the laser parameters used, we find that photon emission polarized along the propagation direction is three to four orders of magnitude weaker than photon emission polarized parallel to the polarization direction of the incident wave.

A second Born calculation of the (e, 2e) triple differential cross section for excitation-ionization of helium to He⁺(n = 2) in coplanar asymmetric geometry is found to be in excellent agreement with recent (relative) measurements of Rouvellou et al (Rouvellou B, Rioual S, Pochat A, Tweed R J, Langlois J, Nguyen Vien G and Robaux O 2000 J. Phys. B: At. Mol. Opt. Phys.33 L599). The second Born cross section is very much larger, by about a factor of four, than the corresponding first Born cross section, a trend that is consistent with absolute total cross section measurements for ionization to He⁺(2p) at the same impact energy.

The 3p photoelectron spectrum of atomic scandium, the first 3d transition metal, has been investigated experimentally using monochromatized synchrotron radiation. The threshold energies for direct 3p photoionization have been extracted from the photoelectron spectrum. The 3p^-1 multiplet structure is discussed based on the predictions of Hartree-Fock (HF) calculations and recent results from multi-configuration Hartree-Fock (MCHF) calculations by Altun and Manson (1999 Phys. Rev. A 59 3576). Due to the characteristics of the 3p^-13d configuration, scandium reveals a peculiar multiplet splitting, which differs significantly from those of other 3d transition metal atoms.

We study the elastic scattering of positronium atoms by hydrogen atoms at medium energies using partial-wave Born-Oppenheimer (BO) exchange amplitudes and report accurate BO cross sections in the energy range 0 to 60 eV. The present BO results agree with a 22-state R-matrix and a five-state coupled-channel model potential calculation, but disagree strongly with a conventional close-coupling calculation as well as its input BO amplitudes at medium energies.

Measurements of the triply differential cross section (TDCS) for double photoionization of Ca are presented, in a photon energy region away from any resonant structure. The data differ markedly from similar data for He. An attempt has been made to measure the ratio of the TDCS at resonant and non-resonant photon energies of 31.41 and 43.03 eV respectively and a ratio of (3800±2100) : 1 was obtained.

We present results for the ionization of oriented HCl by low energy longitudinally polarized electrons. We use a distorted wave method introduced by Dorn et al (1997 J. Phys. B: At. Mol. Opt. Phys.30 4097), and solve equations for the continuum functions with static, local exchange and spin-orbit interactions. We calculate the ionization asymmetry between parallel and anti-parallel polarized electrons in a coplanar scattering geometry and with HCl oriented perpendicular to the scattering plane. The asymmetry depends strongly on the directions of the two ejected electrons, and generally has a magnitude of the order of 10^-3.

We compute the shift of the critical temperature T_c with respect to the ideal case for a weakly interacting uniform Bose gas. We work in the framework of the canonical ensemble, extending the criterion of condensation provided by the canonical particle counting statistics for the zero-momentum state of the uniform ideal gas. The perturbative solution of the crossover equation to lowest order in powers of the scattering length yields (T_c-T_c⁰)/T_c⁰ = -0.93aρ^1/3, where T_c⁰ is the transition temperature of the corresponding ideal Bose gas, a is the scattering length, and ρ is the particle number density. This result is at variance with the standard grand canonical prediction of a null shift of the critical temperature in the lowest perturbative order. The non-equivalence of statistical ensembles for the ideal Bose gas is thus confirmed (at the lowest perturbative level) also in the presence of interactions.

The stability of trapped dilute Fermi gases against collapse towards large densities is studied. A Hermitian effective contact interaction for all partial waves is derived, which is particularly suited for a mean-field description of these systems. Including the s- and p-wave parts, explicit stability conditions and critical particle numbers are given as a function of the scattering lengths. The p-wave contribution determines the stability of a single-component gas and can substantially modify the stability of a two-component gas. Moreover it may give rise to a novel p-wave stabilized high-density phase.

Angular distributions of photoelectrons from both C and O K-shells of the fixed-in-space CO molecule have been measured using the angle-resolved photoelectron-photoion coincidence technique. The measurements have been performed at several photon energies from the ionization thresholds up to about 30 eV above them, where the σ* shape resonances occur. Experimental results are compared with the multiple-scattering calculations of Dill et al (1976 J. Chem. Phys. 65 3158) and with our new calculations in the relaxed-core Hartree-Fock approximation. Our calculations are in a better agreement with the experimental data though numerical discrepancies remain. The experimental angular distributions are fitted by the expansion in Legendre polynomials containing up to ten terms and the extracted parameters are compared with the corresponding theoretical values.

Using the experimental angular distributions of photoelectrons from the K-shells of an oriented CO molecule reported in a companion paper, we have performed a so-called complete experiment and determined 18 dynamical parameters (ten moduli of transition moments and eight phase differences) for the O K-shell, and 16 dynamical parameters (nine moduli of transition moments and seven phase differences) for the C K-shell, and compared them with the results of our calculations in the relaxed-core Hartree-Fock (RCHF) approximation. The agreement between theory and experiment is only qualitative, therefore the model has to be improved by including electron correlations. From the analysis of experimental data we proved that the σ* shape resonance is due to not only the f-wave, as was widely believed earlier, but is due to approximately equal contributions of three partial waves with 1⩽l⩽3 for the C K-shell, and four partial waves with 0⩽l⩽3 for the O K-shell, with a rather substantial contribution of other partial waves with l⩽5. From the analysis of the transition moments determined from the experiment it follows that several Cooper minima are likely to exist in partial photoionization cross sections, in particular, in the C 1sσ→εsσ and in the O 1sσ→εdσ transitions.

Time-of-flight (TOF) techniques have been used to measure singly and multiply ionized Ar recoils in coincidence with energy-analysed, forward-ejected electrons in 1 MeV H⁺-Ar collisions. Electron energies up to 2800 eV and charge states 1-4 are covered. It is shown experimentally that the peak of binary-encounter (BE) electrons in the electron energy spectrum is associated almost entirely with single ionization of Ar, whereas the peak of electron-capture-to-the continuum (ECC) electrons is associated with multiple ionization. This shows that the BE electrons originate exclusively from the valence shell of Ar (the M shell) and the ECC electrons predominantly from an inner shell (the L shell).

Experimental investigations of resonant structure in doubly charged ion (A²⁺) formation were carried out during Sr and Ba atom ionization by infrared laser radiation (ω = 8400-9100 cm^-1). It has been shown that in this case the mechanism of ion formation is a two-electron process - the ions are produced immediately from the neutral atoms. It has been found for the first time that the resonant structure in A²⁺ ion formation in such conditions is due to the excitation and ionization of neutral-atom mixed states. The atomic-state mixing in this case results from the Stark effect provided that the polarizability of these states is very high.

The nonlinear optical response of interacting charge-transfer excitons is investigated theoretically by means of pseudospin operators for the model Hamiltonian. The generalized optical Bloch equations including the static dipolar interaction of charge-transfer excitons are derived in terms of the Heisenberg equation of motion and the mean-field approximation. It is shown that the intrinsic optical bistability due to the strong interaction of charge-transfer excitons may exist in the interacting charge-transfer exciton system. The absorption spectrum of interacting charge-transfer excitons associated with a weak probe field in the presence of a strong pump field is also calculated using the linear-response theory. The results show that a gain without population inversion can be observed in the interacting charge-transfer exciton system.

We have studied the ionization behaviour close to threshold for the fragment ion production from CF₄, CHF₃, CH₄ and C₃H₈. In particular, we use a reliable method to extrapolate weak ion signals towards the appearance energy. Our evaluation is based on an extension of Wannier's law for the ionization of atoms towards the case of polyatomic molecules. In the experiment we use a hemispherical electron monochromator with an energy resolution of ≈135 meV. The vibrationally cold target molecules are prepared in a supersonic expansion and the resulting ions are mass analysed using a quadrupole mass spectrometer. We observe several disagreements with recommended ionization energy values, reaching up to 5 eV in the CHF₃ case. This is in contrast to a number of recent reliability tests that we have performed on Ne, Ar, Kr, Xe, H₂, D₂, N₂, O₂ and N₂O and that have reproduced the tabulated literature data within the resolution of the monochromator.

The scattering of positronium (Ps) by lithium (Li) is investigated using the static-exchange approximation and a two-state close-coupling approximation (CCA) including exchange. The major effect of very high polarizability of Li, at least 30 times higher than hydrogen and more than four times higher than Ps, has been considered in the present CCA scheme in a dynamic way by using the Li(2p) state of the target Li atom in the basis set. Different scattering parameters, e.g. s-, p- and d-wave phase shifts below the excitation threshold, the partial wave elastic and excitation cross sections for s-, p- and d-waves, the integrated elastic and excitation cross sections, the quenching cross sections, and the ortho-para conversion ratios are presented in the incident energy range from 0.068⩽E⩽100 eV. The present study reveals that the long-range dipole correlation potential has an appreciable effect in the cases of Ps and Li scattering in the lower energy region 0.068⩽E⩽30 eV. In the incident energy region above 30 eV, both the results almost coincide with the first Born results. A sharp maximum in the elastic cross sections near 3 eV and three sharp peaks in excitation cross sections near the incident energies of 0.425, 13.0 and 19.0 eV appear in the present results, which are consistent with the minima in the conversion ratios. The p-wave singlet elastic phase shifts calculated by the CCA method are higher by a factor of nearly π/2 than the corresponding static exchange results below 0.439 eV.

We present an atomic physics package called AVERROÈS/TRANSPEC for studying plasma spectroscopy of complex L-, M-shell emitters or even core-excited multielectron K-shell emitters. The model, which is also intended to give some insights on ionization properties of M-shell ionized plasmas, is divided into two parts. The first part (AVERROÈS) is based on the superconfiguration concept and on the supertransition array method. It generates superconfiguration average-energies, collisional and radiative rates needed for a calculation of population kinetics. It also calculates the statistical shift and width associated with each possible radiative electron jump between selected superconfigurations. All the previously mentioned quantities are stored on files readable by a multicell time-dependent collisional-radiative model (TRANSPEC) that calculates population kinetics and synthetic emission spectra. This last code can be employed with a hydrodynamics code to provide simulated x-ray ouputs of non-steady-state inhomogeneous plasmas.

Using a wavefunction with the radial correlation only for the bound electrons and a correlated double-continuum wavefunction for the ejected electrons, an analytical expression is obtained in the Born approximation for the fully differential cross section of double ionization of helium-like ions by electrons. The result is found within the framework of the shake-off model considering only the radial correlation of electrons in the target.

An analytical expression is obtained for the fourfold differential cross section following integration of the fully differential cross section over the solid angles of the ejected electrons. For the first time the total cross sections (TCS) are calculated for direct double ionization of two-electron atomic systems from H^-(Z = 1) to N⁵⁺(Z = 7). The calculations are carried out for energies extending up to a maximum 200 times the double-ionization threshold. The calculated values of TCS are found to be in fair agreement with the available experimental data for He and Li⁺. Disagreement between theory and experiment is discussed. TCS are found to be very sensitive to the inclusion of repulsion between the ejected electrons in the final state.

Valence shell relativistic configuration interaction calculations indicate three bound states for Lu^- by 6p attachment to the 5d6s²J = 3/2 ground state. These include two J = 2 levels with electron affinities of 329 and 124 meV, as well as one J = 3 level at 63 meV. The first value is 139 meV larger than the previously published value, while the latter two are predicted here for the first time. We find second-order effects moderately important in the binding of these states, while certain core excitations (particularly involving the 4f subshell) were found to have a negligible impact.

The spectra of Auger electrons are measured for the cascade decay of the photoexcited 1s^-13p resonance in Ne. Angular distributions of the Auger electrons for the prominent peaks of the spectra are determined. It is shown that in the first step of the cascade which corresponds to the spectator (shake-modified spectator) transitions 1s^-13p→2s^-12p^-13p(4p) and 1s^-13p→2s^-23p(4p) the electrons are emitted almost isotropically. In contrast, the majority of the second-step transitions 2s^-12p^-13p(4p)→2p^-2 and 2s^-23p(4p)→2s^-12p^-1 shows strong anisotropy. A qualitative explanation of the observed results is given.

The angular dependence of the neon K-L_2,3L_2,3 (¹D₂) Auger line intensity is measured and calculated for proton-neon collisions at two projectile impact energies. At one of the energies the projectile moves faster than the Auger electron and at the other energy the projectile moves slower. The Auger line intensity measured along the beam direction shows a significant energy dependence compared with other directions.

The shape and the intensity of the neon K-L_2,3L_2,3 (¹D₂) Auger line is calculated for proton and antiproton impact, respectively. It is used to compare the Coulomb focusing and defocusing effect as a special case of the post-collision interaction along the beam direction. It is shown that these two types of projectile have opposite effects on Auger electrons. The analysis confirms that the post-collision interaction of a structureless heavy particle and light Auger electron emitted along the beam direction, as studied here, is a particular case of the so-called glory and rainbow effects, known from optics.

The influence of the vibrational degree of freedom on the strong-field ionization of molecules is investigated. On the basis of numerical simulations it is found that vibrational motion leads to a reduced ionization rate for H₂ and O₂ compared with the rate of an atom with identical ionization potential. In the case of N₂ the vibrational motion has, however, a negligible influence on the ionization rate. While these results are in qualitative agreement with recent experimental findings, the predicted reduction of the ionization rate for H₂ and O₂ is too small to fully explain the experimental results.

We present a convergent frozen-correlation approximation (CFCA) to evaluate multiple-ionization cross sections in ion-atom collisions. The theory allows the evaluation of total cross sections for double ionization without dynamic correlation. We have studied double ionization of He by impact of high-energy protons and antiprotons. Our results for antiproton impact are in excellent agreement with experiment, thus proving the negligible role of dynamic correlation. However, a discrepancy is obtained for proton impact, which suggests that dynamic correlation may play a significant role in this case.

We present the results of extensive close-coupling calculations of electron-impact excitation of the C-like ion, Ne⁴⁺. We first compare effective collision strengths determined from a 20-level Breit-Pauli R-matrix calculation with those obtained from a 20-level intermediate-coupling frame transformation (ICFT) R-matrix calculation. The ICFT method was also employed to perform two much larger calculations; we compare the effective collision strengths determined from these calculations with each other and with those obtained from the 20-level calculations in order to assess the effects of increasing both the size of the configuration-interaction expansion of the target and the size of the close-coupling expansion. Our final calculation, with 130 terms and 261 levels in the configuration-interaction expansion of the target and 66 terms and 138 levels in the close-coupling expansion, provides improved data for excitation between the levels of the 2s²2p², 2s2p³ and 2p⁴ configurations and the first close-coupling results for excitation to the levels of the 2s²2p3ℓ configurations in Ne⁴⁺.

The single differential cross section (SDCS) for electron-impact single ionization should be symmetrical about half the total energy of the system. However, numerical results for the convergent close-coupling approximation do not exhibit this symmetry. Consequently, a controversy has arisen as to whether this is characteristic for all fully converged time-independent close-coupling calculations or whether this indicates a lack of convergence. In this paper, the behaviour of the SDCS is examined for time-dependent close-coupling calculations and it is found that a fully converged calculation would be symmetrical about half the total energy.

The quantum-electrodynamic theory of photon-polarization effects in two-photon bound-bound atomic transitions is developed, including relativistic and retardation effects. The differential cross section for photon scattering from a randomly oriented target is expressed in terms of eight polarization correlation parameters a_i which are associated with vector products involving the photon polarizations. We restrict our detailed discussions to the case of photon scattering from atoms, though the results are easily generalized to other situations. Symmetry arguments are used to establish the necessary conditions for these polarization effects to occur. One of the eight parameters a_i describes circular dichroism in the case that the scattered photon has a suitable linear polarization, and another describes the appearance of elliptically polarized scattered photons for the case of a suitable linearly polarized incident beam. The particular cases of atomic scattering with J_i = J_f = 0 and 1/2 (where J_i and J_f are the total angular momenta of the initial and final states of the target, respectively) are analysed in detail. Numerical estimates of the parameters are given for ground-state atoms in the J_i = J_f = 0 approach, using the independent-particle approximation, and for 1s_1/2→1s_1/2 and 2p_3/2 transitions in hydrogen-like ions, demonstrating the feasibility of experimental observation of the above-mentioned circular dichroic effect in high-Z atoms and ions.

We examine an extension to the theory of Gaussian wavepacket dynamics in a one-dimensional potential by means of a sequence of time-dependent displacement and squeezing transformations. Exact expressions for the quantum dynamics are found, and relationships are explored between the squeezed system, Gaussian wavepacket dynamics, the time-dependent harmonic oscillator, and wavepacket dynamics in a Gauss-Hermite basis. Expressions are given for the matrix elements of the potential in some simple cases. Several examples are given, including the propagation of a non-Gaussian initial state in a Morse potential.

Dissociative and non-dissociative electron capture cross sections for medium-velocity collisions of carbon ionic cluster projectiles with helium atoms have been measured (v = 2,2.6 and 4 au C_n⁺ n⩽5). This has been done using a 4π multifragment detector operating either in the full-efficiency mode for the extraction of total electron capture cross sections, or in a partial-efficiency mode (anticoincidence method) for the extraction of the dissociative and non-dissociative parts. The dissociative part has been found to be the largest one in all cases, increasing with n and v. The role of the cluster temperature, not precisely known, may be important but cannot explain all the observed experimental results, in particular the appearance of quite large branching ratios for dissociation into three fragments (n⩾3). In order to explain this multifragmentation, the multi-electron process of capture-projectile excitation is invoked. Another multi-electron process is directly extracted from these collisions, namely, the transfer-ionization two-electron process whose relative contribution with respect to single capture is found to increase with the velocity.

The formation and recombination of a protonated acetone dimer with electrons was studied in the flowing afterglow. H₃O⁺ ions were formed in the early post-discharge region. The subsequent addition of acetone leads to the formation of the protonated ions CH₃COCH₃H⁺. These ions further associate with acetone forming the cluster ions H⁺·(CH₃COCH₃)₂ that react further with acetone, but very slowly. This facilitates the creation of an afterglow plasma with a dominant population of H⁺·(CH₃COCH₃)₂. The evolution of electron number density (n_e) and electron temperature (T_e) is measured using the Langmuir probe. The recombination rate coefficients that are obtained indicate a negative temperature dependence: α = (3.4±1)×10^-6 cm³ s^-1 at T_e = (580±150) K and α = (7±2.5)×10^-6 cm³ s^-1 at T_e = (450±100) K.

A two-parameter representation is suggested for sonoluminescence (SL) spectra. The suggested spectrum representation fits the experimental data in various SL preparations very well. The spectrum representation can be interpreted by the Fermi-Thomas model describing atoms of a noble gas remaining in a bubble. With constriction of the bubble, the boundary of the sphere representing an atom is constricted, causing the electronic system in the atom to deviate from an equilibrium state. The diffusion current of electrons restoring the system to equilibrium radiates electromagnetic waves. The characteristic frequency of this radiation spectrum is determined by the atomic weight of the noble gas and the radius of the sphere representing the atom, which is approximately dependent on the pressure and temperature of the gas system. The theoretical predictions of the characteristic frequency fit experimental data very well.

A variational method is described that is of value in either single- or many-channel scattering problems. Its use is exhibited in a number of test cases.

We describe a method for the determination of rate coefficients for collisional excitation out of the 1s states of a noble gas using continuous-wave laser collisionally induced fluorescence (CW LCIF, i.e. fluorescence from an upper level which is not that of the laser transition). The intensity of CW LCIF lines from an upper 2p level that is strongly coupled to an influential 1s level is dominated by the electron collisional rate out of that level. By an appropriate choice of LCIF line we fit our recently published theory to experimental observations to determine a set of electron collisional direct 1s mixing and 1s-2p excitation rate coefficients. The values of coefficients determined in this way show very good agreement with those which have been calculated using cross section data from the literature with the measured electron temperature. For cases where accurate cross section data are available for the 1s-2p transitions CW LCIF diagnostics should, via these rate coefficients, provide reliable values of the bulk electron temperature.

Angle-resolved electron energy-loss spectra for the nitrous oxide (N₂O) molecule have been measured at an impact energy of 1 keV. The spectra cover an excitation energy range from 0 to 50 eV and an angular range from 1.5° to 10.0°. The absolute inelastic differential cross section and the generalized oscillator strength (GOS) for the D  ¹Σ⁺←X ¹Σ⁺ transition (9.6 eV) were determined in the 0.05-2.22 au K² range. The absolute elastic differential cross section was also determined spanning an angular range from 1.5° to 15.5°.

The Kapitza-Dirac effect (KDE), or the scattering of electrons by a standing light wave, is considered quantum mechanically with electron states before scattering taken in the form of wavepackets with homogeneously distributed `centres of mass'. The angular distribution functions of electrons after scattering and the average angle of scattering are found. This approach is used to establish the applicability conditions of purely classical, purely quantum-mechanical and intermediate results in the theory of KDE, as well as to describe some new regimes of scattering, characteristics of which depend explicitly on the size of the wavepackets before scattering.