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A rather systematic study of the influence of strong electromagnetic fields on a schematic collisional process is undertaken. In this paper we start on investigating the role of the source intensity in inducing transitions in a two-state description of the collisional event. Classical impact-parameter dynamics is used to model the nuclear motion.

The second-order polarisation propagator approximation has been used to calculate the generalized oscillator strength f ( K ) for the 1 S→ 1 P transition of Be for K 2 from 0 to 4 au. The inelastic electron scattering cross section has also been calculated for incident energies from 50 to 1000 eV. Agreement with published calculations and experimental results is good.

Of 5,327 pregnant women who were screened between July 1981 and July 1983, 5.2% were found to be hepatitis Bs antigen (HBsAg) carriers. This high percentage of carriers accentuates the importance of vertical transmission of hepatitis B virus in our area. In view of this we sought to verify the effectiveness of hyperimmune immunoglobulins in the prevention of infection. We studied 201 babies born to HBsAg-positive mothers. These babies had regular checkups until 12 months of age. One hundred forty-nine received prophylactic hyperimmune globulin (two to seven doses according to risk ranking), while 52 did not. A higher percentage of infection was found in the babies who had no prophylaxis (25%) than in the babies who had prophylaxis (7.4%). Furthermore, hepatitis appeared earlier (from the 2nd to the 6th month) in the babies who had no prophylaxis than in the babies who had prophylaxis (from the 9th to the 12th month). No infection was observed during prophylaxis or up to 6 months.

Bound negative atomic ions are frequently described in terms of very-short-range pseudopotentials. While in the case of hydride-ion H − remarkably accurate results are obtained in terms of drastically simplified pseudopotentials, for alkali ions the simplest asymptotic descriptions are inadequate. The extended asymptotic model introduces a one-parameter r −2 -type pseudopotential: we reinvestigate the photodetachment cross section of this model by resorting to a recently proposed approach, founded on a time-dependent picture of the ionization process.

A recently proposed approach to the photoionization of atoms according to a formalism based on the use of time-dependent correlation functions is investigated in terms of a possible implementation of the procedure via a time-dependent variational principle. A simple application to the hydrogen atom suggests that the method is worth further consideration.

The two-photon ionization probability of a simple model of diatomic molecule has been exactly evaluated under the assumption that a perturbative treatment is valid. The model consists of a simple electron in one-dimensional motion in the field of two fixed Dirac delta wells. The exact results are compared with a number of approximations arising from simplifications introduced at the level of final state and/or the intermediate states involved in the evaluation of the relevant ionization amplitude.

A simple model mimicking the collision between two atomic partners in the presence of an intense radiation field is investigated according to a two-state, non perturbative approach, to put in quantitative evidence inadequacies in both transition probability and cross section arising from the use of transition amplitudes which are not gauge-invariant.

Gauge-invariant transition and survival probabilities for a Morse oscillator initially prepared in its ground state are evaluated for the case of intense and short-duration pulses by a nonperturbative approach founded on the solution of the dynamical problem in terms of a Magnus formalism. False probabilities built up from «conventional”,i.e. gaugedependent, amplitudes are also obtained and contrasted to the true transition probabilities as a test for the difference between them.

One- and two-photon ionization cross sections for the negative hydrogen ion H/sup -/ are evaluated by using a one-electron model where the effective intra-atomic potential energy is assumed to be of a very-short-range character and approximated by a ''spherical delta shell.'' The adjustment of the two parameters involved in the model potential so as to reproduce either the photodetachment energy or the behavior of the one-photon ionization cross section over a substantial range of frequencies beyond the ionization threshold leads to conjecture of a reasonable estimate of the two-photon ionization cross section over the proper range of frequencies of the ionizing field.

Fourier transform of two-center charge distributions corresponding to arbitrary Slater-type orbitals are evaluated by a Gaussian quadrature procedure without any preliminary series expansion of the integrand. Convergence and accuracy of the method are discussed and illustrated.

Dynamic polarizabilities obtained by various standard approximate methods at both real and imaginary frequencies are shown to be related by inequality relationships. This appears to hold, under definite assumptions, for coupled and uncoupled time-dependent Hartree-Fock schemes and Tamm-Dancoff approximation. The use of such inequalities suggest some remarks about van der Waals dispersion coefficients which can be evaluated in these various approximations.

Inelastic scattering of fast electrons from the electronic ground state of H2 is investigated in the context of the first Born approximation plus exchange a la Ochkur. Excited molecular states of s...

Ab initio N 2 + ( 2,4,6 Σ g,u , 2,4,6 Π g,u ) potential splittings have been calculated in the internuclear distance range 2–5 A. The results for the Σ potentials have been obtained in the Heitler-London formulation either retaining all relevant integrals (method 1), or by neglecting some terms (method 2), while the Π splittings have been calculated only within the more approximate method 2. The results have been used for calculating the total charge transfer sections Q tr of the process N( 4 S) + (N + ( 3 P) = N 3 P) + N( 4 S). A reasonably satisfactory agreement with the corresponding Q tr values obtained from previous potentials can be pointed out, with the exception of the results based on the extrapolation of spectroscopic potential curves. A good agreement has also been found with the experimental Q tr values of Belyaev as well as with the Q tr cross sections derived by Duman and Smirnov by means of a completely different theoretical approach.

The inelastic collision of fast electrons with ground state closed shell atoms is investigated within the context of First Born Approximation and Random Phase Approximation. Generalized oscillator strengths and total cross sections for 11 S → n 1 D transitions in He and 11 S → n 1 S in Be are evaluated and discussed.

The action of strong radiation field from a laser source is expected to influence collision processes, frequently to a substantial degree, through the enhancement of channels already accessible and/or the opening of new ones. An introductory excursion in this area is offered, which emphasizes the role of “dressed” electron-field potential energy surfaces and transitions between them as a key for understanding the occurrence of processes in the domain of Laser Assisted Chemistry.

Inelastic scattering of fast electron from the electronic ground state of the H 2 molecule is investigated within the first Born approximation. The relevant transition properties have been obtained in some approximation corresponding so ??? order solutions of the equations-of-motion formation put forward by Rowe-Tamm-???approximation, random phase approximation. Generalized oscillator strengths and ??? of different symmetry are evaluated and discussed.

Dynamic multipole polarizabilities and the magnetic dipole susceptibility of N2 and H2O are evaluated in the framework of the equation‐of‐motion method corresponding to the random phase (RPA) and Tamm–Dancoff approximations (TDA). The results are presented in the form of a Cauchy expansion truncated after the ω4 terms. Nuclear magnetic shielding constants of N, H, and O are obtained as by products of the main calculations. While the RPA results for the magnetic properties compare very well with available experimental data, the computed values for the electric observables are only in moderately good agreement with the experiments, suggesting an inadequacy of the employed basis sets and a different relevance of the correlation effects as well. The dipole–dipole van der Waals dispersion coefficients for the pairs N2–N2, H2O–H2O, and N2–H2O obtained by RPA values of the dipole polarizability at imaginary arguments compare only approximately well with semiempirical estimates.

Inelastic scattering cross sections for collisions of electrons with helium atoms in metastable 2 1,3 S states are calculated for some final states of S and P symmetry by using the First Born Approximation and the Random Phase Approximation. The results for the total cross sections compare reasonably well with previous accurate values in the case of singlet-singlet transitions.

A reformulation of photoionization in phase space in terms of Wigner distribution functions along similar lines to that suggested for photodissociation by Heller is proposed and numerically explored for the hydrogen atom.

One- and two-photon ionization of a simple model of diatomic molecule, consisting of a single electron moving one-dimensionally in the field of two attractive, fixed Dirac-delta wells, has been investigated in terms ofL2-integrable wave functions. Emphasis is placed on the calculation of the two-photon ionization amplitude as far as the representability of the resolvent operator by means of aL2-basis set is concerned.

A computational scheme for frequency-dependent polarizabilities of open-shell systems by coupled Hartree-Fock perturbation theory is presented. It allows the simultaneous approximate prediction of the involved electronic transition energies and can trivially be extended to imaginary values of the frequency. As a simple application, the dipole polarizability and some related properties of Li atoms are evaluated, the agreement with previous estimates being excellent.

The second virial coefficient of refractivity of He gas at room temperature is calculated by approximating the incremental polarizability of the diatom (He) 2 , at various internuclear distances, by its RPA value. The virial coefficient of refractivity can be expressed in the Cauchy form, at sufficiently low frequencies. The values evaluated for the first three Cauchy moments at T = 322 K are B 2,0 = −0.040 cm 6 /mole 2 , B 2,1 = −0.010 × 10 −32 cm 6 /mole 2 s −4 , respectively. As far as our value for the dielectric virial coefficient B 2,0 is concerned, it compares favourably with the experimental one. B cxpt 2,0 = −0.06 = 0.04 cm 6 /mole 2 , and other previous theoretical estimates as well, while the other two moments, for which no estimates are presently available, are argued to be reasonably reliable.

A very simple one-dimensional model for a one-electron diatomic molecule, under the influence of a weak static electric field, is investigated by perturbation theory. The dipole polarizabilities for the two only bound states supported by this molecular model are evaluated as a function of the internuclear distance.

The response to an ionizing radiation field of a simple one-electron atomic system modelled by an attractive Fermi-type pseudopotential is investigated under the assumption that the source intensity is sufficiently low for a perturbation theory treatment at appropriate order to be adequate in dealing with the radiation field-matter interaction. One- and two-photon ionization cross-sections are evaluated either in the length or in the velocity forms by using an approach recently put forward by the authors, which stresses the nature of such quantities as Fourier transforms of relevant time-dependent correlation functions. Some remarks concerning the merits of different gauges in approximate two-photon ionization cross-section calculations conclude the paper.

N-photon ionisation cross sections obtained by perturbation theory treatments are expressed in terms of Fourier transforms of appropriate autocorrelation functions. As expected, the N=1 case is related to the well known electric dipole-dipole autocorrelation function, while correlation of increasingly complicated operators is involved as N=1, 2, 3, . . .. A simple application of the formalism to the evaluation of one- and two-photon cross sections for a schematic model of the negative ion H- is presented.

Proton magnetic shielding constants in H2O, NH3, CH4, and CH3F molecules have been evaluated by the perturbed HF scheme, by employing extended basis sets of Slater orbitals. Explicit formulas for both diamagnetic and paramagnetic contributions to the observable are given, for a gauge transformation corresponding to any change of the origin of the external vector potential. The calculated results are in fairly good agreement with the experimental data, when the chosen gauge is coincident with the central nucleus of the molecule.

Long‐range dispersion energies for the atom pairs He–He, He–Be, and Be–Be in their ground states have been calculated in the form E disp=C6/R6+C8/R8+C10/R10+C11/R11+C12/R12, the coefficients Cn being obtained from the appropriate dynamic polarizabilities at imaginary frequencies. The needed polarizabilities of imaginary argument have been calculated within the time‐dependent coupled Hartree‐Fock approximation. Three‐body nonadditive contributions to the interaction energies have been also evaluated.

The SCF MO's of CH3F have been calculated by using bases of up to 47 STO's. The best molecular energy obtained is − 139.0612 a.u. The ground‐state wavefunctions have been utilized to compute several observables corresponding to one‐electron operators, viz., the electric dipole, quadrupole, and octopole moments, the electric field gradient at the carbon atom and at the proton, and the diamagnetic part of the susceptibility. In addition, by using the coupled HF perturbation theory, the electric polarizability and the paramagnetic part of the susceptibility have been computed.

Small magnetic terms in the molecular Hamiltonian are treated in a SCF perturbation scheme, using H.F. molecular orbitals as basis; some approximations are discussed and compared. In particular, the paramagnetic contributions to the susceptibility and nuclear shielding constants for LiH and N2 are investigated and discussed.

SCF MO's of H2O molecule have been calculated for a geometry very close to the experimental one, using a basis set of 29 STO's. The evaluated HF energy is −76.0384 a.u. Several ground‐state one‐electron properties have also been computed, viz., the first three electric multipole moments, electric field and gradient field at the nuclei, electric polarizability, and magnetic susceptibility.

Dynamic multipole polarizabilities of a number of simple atomic closed-shell systems in their ground states have been calculated within the coupled time-dependent Hartree-Fock scheme. The systems here investigated include He, ${\mathrm{Li}}^{+}$, ${\mathrm{Be}}^{2+}$, ${\mathrm{B}}^{3+}$, ${\mathrm{C}}^{4+}$, Be, ${\mathrm{B}}^{+}$, and ${\mathrm{C}}^{2+}$. In order to acquire some feeling about the obtained results, the dipole polarizabilities have been evaluated either by using a scalar- or a vector-potential representation for the oscillating external field: these two choices give rise to the well-known polarizability expressions in terms of "length" and "velocity" oscillator strengths, respectively. Furthermore, a unitary transformation of the Hartree-Fock Hamiltonian is devised, which allows one to use a perturbation operator corresponding to that involved in the "acceleration" oscillator-strength expression. Estimates of transition energies to low-lying excited states have been also obtained from the frequency values at which the various polarizabilities display discontinuities.

The magnetic susceptibility of H2O, NH3, CH4, and H2O2 molecules has been evaluated by the perturbed HF scheme. Several wavefunctions, all of LCAO SCF MO type, have been considered. By using a double perturbation technique we derive some useful quantities which allow one to evaluate diamagnetic and paramagnetic contributions to the susceptibility for any gauge. The results corresponding to the largest basis sets are in satisfactory agreement with experimental data. The dependence of the evaluated quantities upon the gauge transformation corresponding to a change of the origin of the vector potential has been examined in detail.

Electric polarizabilities of some polyatomic molecules are evaluated, within the HF pertubative scheme, by using LCAO SCF MO's wave functions.