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A modified effective potential for an valence-electron - -core interaction is shown to substantially improve the accuracy of calculated energies for Ca terms containing significant components, while maintaining high accuracy for other terms. In fact, we obtain near spectroscopic precision for those series which have little 3d character. Our calculations support the assignments of some 4s ng levels recently identified in the solar spectrum and, in particular, we confirm that lies below. The energy of the unidentified calcium term is predicted.

An interpretation is given of the threshold photoelectron spectrum of Ar between the and double-ionization thresholds. In this region the observed spectrum is less cluttered than below the thresholds and the interpretation is more straightforward. One of the characteristic features of the spectrum is the excitation to Rydberg orbitals with high angular momentum l in agreement with the Wannier picture as first discussed by Fano in 1974.

We report the observation of a two-photon ionization process in the XUV wavelength regime. In a near-resonant 1 + 1 ionization scheme, Ar atoms are ionized absorbing the 15 eV third harmonic photons produced in a gas jet by the 0.5 ps intense laser pulses of a KrF excimer laser emitting at 248.6 nm. The present demonstration of a non-linear process in the XUV regime reveals feasibility of high-intensity applications utilizing the uniquely high peak power of non-conventional short wavelength radiation sources based on harmonic generation.

Elaborate wavefunctions representing low-energy positron - helium elastic scattering, which were obtained in the course of calculating accurate values of the scattering phase shifts, are used to determine the electron - positron annihilation rate and the Doppler-broadened annihilation -ray spectrum. This spectrum is also measured using room-temperature positrons in a Penning trap and a non-Gaussian lineshape is observed for the first time. Excellent agreement is obtained between the theoretical spectrum and the present results.

The harmonic oscillator energy level spacing for atomic clusters as a function of the particle number N is expressed analytically in terms of the parameters of a Woods - Saxon (or Symmetrized Woods - Saxon) potential which approximates the effective spherical self-consistent jellium model potential. The expressions derived depend on the particular scheme adopted to approximate the potential by the harmonic oscillator one and on the assumed dependence of the potential radius R on N. It is also observed, considering the case of sodium clusters, that for large N the expressions of are in good agreement with the well known expression of in terms of the Wigner - Seitz radius.

A basis of Hermite splines is used in conjunction with the collocation method to obtain accurate solutions of the Schrödinger equation for . The spectral transform variation of the Lanczos method is found to be the most suitable method for solving the matrix eigenvalue problem, while the preconditioned conjugate gradient method is very effective for solving the systems of linear equations necessary to evaluate the matrix - vector products that occur for each Lanczos iteration. The lowest eigenvalue is obtained to more than six significant figures without taking advantage of the separability of the orbital equations.

Results from systematic multiconfiguration Hartree - Fock calculations of transition probabilities, isotope shifts and hyperfine structures of the - and - transitions in B I, the - transitions in C II and the - transition in C I are reported. Higher-order correlation effects beyond the pair-correlation approximation are shown to be important for the transition probabilities, and are included in large configuration interaction calculations. For all the transitions except one, the agreement between the length and velocity forms of the weighted oscillator strength is better than 0.4%. The present results are believed to be substantially more accurate than those of other theories.

The stability of the free dianion is investigated by using ab initio multireference-CI (MRD-CI) methods (including the Z-stabilization technique for resonances) and AO basis sets containing diffuse s, p and d functions. The valence-like resonant state of gaseous lies eV above the X ground state of , i.e. the isolated dianion is unstable upon electron attachment to . The dianion is also less stable, by about eV, then the X ground state of BN. With respect to dissociation, however, the resonance lies eV below the products . The ion embedded in the continuum autoionizes through the mixing of with autodetaching states. Arguments are presented in favour of the existence of a stable unit within a crystal environment. To prove this point, exploratory Møller - Plesset calculations were carried out for the ground states of linear LiBNLi, NaBNNa and BNCa. These polyatomic species are thermochemically stable in the gas phase, with dissociation energies of 5.9 eV for LiBNLi, 4.8 eV for NaBNNa and 3.44 eV for BNCa, and can therefore serve as precursors for generating and isolating in-crystal ions.

Many-body perturbation theory, formulated within the algebraic approximation, is used to determine the correlation energy of the ground state of the nitrogen molecule. Systematically constructed even-tempered basis sets of Gaussian-type functions, which have been shown to achieve an accuracy approaching Hartree in matrix Hartree - Fock calculations, have been developed to afford basis sets suitable for electron correlation studies using the second-order many-body perturbation expansion. Over 98% of an empirical estimate of the ground-state correlation energy of the molecule at its equilibrium geometry is recovered by using a basis set constructed from even-tempered sets centred on the atoms and on the bond midpoint and containing functions of s, p, d, f, g and h symmetry. The importance of functions with i symmetry is also assessed. It is estimated that the calculated correlation energy corresponds to 99.1% of the exact second-order energy. The correlation energy obtained in the present study is compared with second-order energies obtained by using previously reported basis sets, which are shown to recover, at best, about 10% less of the empirical correlation energy estimate.

We have recorded the - 50) bound Rydberg spectrum of cadmium. We have fully resolved the multiplet up to n = 19 and partially up to n = 24. Furthermore, we have observed the odd singlet and triplet 5sn f Rydberg series for n = 10 - 22 and n = 4 - 21, respectively. We have applied two- and three-step laser schemes to excite Cd vapour in a heat pipe-like oven which was operated as a thermionic diode for detection. Theoretical calculations were performed for the energies of the 5sn p and 5sn f series using perturbation theory with a zero-order model approximation. There is a good agreement between theoretical and experimental results for the 5sn p series, the deviation decreasing with n and being 1 - 2 for n = 50. The agreement for the 5sn f is satisfactory. From the quantum defect of the 5sn f series we approximate a value of the effective dipole polarizability of the ion of in units of the Bohr radius.

Using the isolated-core-excitation scheme we have measured the energies and widths, as well as resonance profiles of the Sr 5 (J = 1,3) autoionizing Rydberg states for . We have characterized the excitation spectra by the sum of two Lorentzians extracting the observed energies and widths. The manifestations involved in the spectra are interpreted in detail. Some of the J = 3 data are compared with the available ones, and good agreement between them is achieved.

We report on measurements of dissociative recombination of ions populated in a range of initial vibrational levels with free electrons in the energy range of 0 - 20 eV. A photodis-sociation technique was applied to modify the vibrational distribution, and the influence on the dissociative-recombination (DR) spectrum was studied. To estimate the vibrational distribution in the experiment, the results of a simple model calculation describing a part of the measured spectrum are presented. From the comparison between the experimental data and the model calculation, we conclude that when laser photodissociation is not applied, our result is consistent with a Franck - Condon vibrational distribution from the ion source. When laser photodissociation is applied, a dramatic change of the DR spectrum is observed, which is consistent with the expected effect, based on theoretical photodissociation cross sections.

Using the advanced adiabatic method we have calculated the cross sections for excitation, ionization and electron capture of H(1s) and H(2s) colliding with in the energy ranges 0.1 - 20 keV/u and 0.1 - 7 keV/u, respectively. All radial and rotational couplings among the adiabatic states with united atom principal quantum number have been included in the calculations, as well as the electron promotion to the continuum via the S- and Q-superseries of hidden crossings. The obtained results are compared, when possible, with the results of other cross section calculations or measurements.

Numerical results for the Nikitin exponential model at non-zero impact parameters are presented. It is confirmed that in order to derive the correct strong-coupling approximation of the transition probability, one need only include the leading terms of the asymptotic expansions of the confluent hypergeometric functions. Development beyond leading order is inappropriate to the particular model, because by construction the non-analytic wavefunction is discontinuous in its second-order derivative at the turning point, which follows from the cusp-like discontinuity in the first-order derivative of at least the off-diagonal Hamiltonian potential matrix elements.

It is therefore concluded that the Stueckelberg phase-integral derivations of Crothers are essential if transition probabilities and cross sections are not to be underestimated at large impact parameters, due to neglecting the bending of the double Stokes line.

We present a first Born approximate method for evaluating cross sections for collisions of one-electron projectile ions with one-electron target atoms. We consider those reactions in which the projectile electron is excited to a specific state but no measurement is made of the target electron in regard to its final disposition. Comparison with accurate first Born calculations is made. The approximation scheme involves evaluation of a one-dimensional integral. Explicit analytic forms are given for the asymptotic region. The method is applied to shed light on an interesting result obtained from the static mean field model.

Cross sections for K-vacancy and L-x-ray production by 1 GeV protons for elements ranging from Cu to U are measured and compared with calculations based on the plane-wave Born approximation. The analysis revealed the need to include the current - current interaction between the projectile and target electron and the spin-flip term.

The spin depolarization of transversely polarized electrons scattered by molecular oxygen in its ground state is reported. The R-matrix method has been used to calculate T-matrices for purely elastic scattering X and vibrational excitation X for energies in the range 4 - 13 eV. The present elastic results have been obtained by using the T-matrices of 10 scattering symmetries at the equilibrium geometry and the vibrationally averaged T-matrices for the resonant and symmetries. The depolarization fractions are in better agreement with the experimental measurements than our earlier work in which the fixed-nuclei T-matrices of all 12 scattering symmetries were used. For vibrational excitation, only the vibrationally averaged and symmetries were considered since the contributions to the cross sections from the other 10 symmetries are negligible. The results show that exchange effects are much more significant in vibrational excitation than in elastic scattering.

Two independent calculations have been carried out to obtain the differential cross sections (DCS) for the excitation of n = 2 and 3 states from the ground state of hydrogen at electron impact energies lying between the n = 3 and n = 4 thresholds. They are the 15-state R-matrix calculation of Fon et al and the algebraic variational method of Wang and Callaway. The cross sections show complicated and irregular structures associated with overlapping resonances in different partial waves. Resonance analysis has been carried out. In the absence of experiments and other theoretical calculations, the 15-state R-matrix calculation is used to describe the shape of the DCS while the absolute magnitude is estimated by the algebraic variational method.

A simple model of electron detachment from negative ions by negatively charged projectile impact is developed as a combination of quantum tunnelling and classical field ionization. While the tunnelling theory has been discussed previously by Smirnov and Chibisov, we have substantially amended Solov'ev's classical model for above-barrier transition. As the most important modification we introduce the concept of decay rate for the classical ensemble related to the quantum state and evaluate the rate in terms of the simple free-fall atomic model. The present theory compares very well with recent experiments while the isolated application of the tunnelling model and of the classical model yields results that are a factor of two too large and too small, respectively.

We present data on the optical constants of thin molecular films formed from phthalocyanines and fullerenes obtained by means of thin film spectrophotometry. On the basis of the optical constants, the oscillator strength values for the relevant electronic transitions could be calculated. For phthalocyanines, our data agree well with Henriksson and co-workers' theoretical predictions.

Two new classes of nonclassical states are defined by exciting Hillery's even and odd coherent states. Their physical properties are investigated numerically. It is found that they do not exhibit the squeezing effect but exhibit remarkable characteristics of sub-Poissonian distribution and antibunching. In sharp contrast to the case of excited Glauber coherent states, it is observed that the uncertainty of the field quadrature in excited even and odd coherent states increases with the degree of excitation. It is also seen that the Wigner functions associated with them take negative values for wider regions of parameters compared to those of Hillery's even and odd coherent states.

In this paper we present a theory which shows that the optical force on atoms due to spontaneous processes depends upon the quantum state of the atom at the time it begins interacting with the optical field. The momentum is treated as a classical variable and the theory used to model the interaction between the atom and radiation is based on a quantum electrodynamic technique. The formalism developed is quite general and, in principle, can be applied to the determination of initial atomic states prepared by a variety of mechanisms. An example of an application of the spontaneous force is presented which shows how it is possible to use the momentum absorbed by an atom from a coherent light field to determine the density matrix of the atomic state created by a collision process. For completeness, the derivation of a stimulated force model is included where it is shown that the resulting force on atoms also depends on their initial state.

The intensity dependent absorption was measured on the line (6 transition) in atomic cesium. The magnetic field applied to the vapour and the spatial cross section of the laser beam were controlled and varied during data collection. A three-level rate equation model is presented in an attempt to explain the results. We show that this well known approach does not sucessfully model the data obtained in the absence of a magnetic field. Hence, a more complex and complete model that explicitly includes all of the hyperfine magnetic sublevels (a multilevel model) is presented. This approach accurately models all of the data collected. The good agreement betweeen this model and the data allows the determination of the transit relaxation rate (due to atomic time of flight through the laser beam), , where is the two-dimensional root-mean-squared speed of the atom and D is the FWHM of the Gaussian laser beam spatial profile.

Due to an unfortunate error which occurred in the printing of this article the title was changed. The correct title should be:

Vibrational excitation of N₂ by low energy electrons