Table of contents

The problem of the determination of the diatomic centrifugal distortion constants (CDC) is considered. The previous use of the Rayleigh-Schrodinger perturbation approach (by Hutson) gives analytical expressions of D_nu ,H_nu ,L_nu ,M_nu in terms of psi _nu ⁽⁰⁾ (vibrational wavefunction) and psi _nu ⁽¹⁾, psi _nu ⁽²⁾ (first two rotational harmonics). A similar use (by Kobeissi) gives expressions up to N_nu ,O_nu ,. . . In both formulations the analytical expression increases in complexity with increasing order. The present work shows that all CDC of any order are given by one single expression E⁽ⁿ⁾=( psi _nu ⁽⁰⁾ mod R-B_nu mod psi _nu ^(n-1)) where R=1/r², B_nu is the rotational constant; psi _nu ⁽ⁿ⁾ is the solution of the equation y"+f(r)y=s_n(r) where just s varies with n, the psi _nu ⁽ⁿ⁾ initial values at an arbitrary origin r₀(0<r₀< infinity ) being given by psi _nu ⁽ⁿ⁾(r₀)=0, d psi _nu ⁽ⁿ⁾(r₀)/dr= integral _nu ^r0s_n(r) psi ⁽⁰⁾ (r)dr/ psi ⁽⁰⁾(r₀); s_n(r) is given by an exact analytical expression depending on psi _nu ⁽⁰⁾, psi _nu ⁽¹⁾,. . ., psi _nu ^(n-1).

The possibility of the joint decay of two holes and one excited electron is discussed as one way many-electron Auger transitions can take place. It is shown that existing experimental decay spectra of reasonably excited states in krypton and xenon exhibit weak lines which may be associated with this new type of Auger process.

Quantum deformation of the U(4) supset SO(4) supset SO(3) chain is discussed. The deformed version of this chain is then applied to describe the rotation-vibration spectra of diatomic molecules. The results show that the six-parameter formula obtained from the U(4) supset SO_q(4) supset SO_q'(3) algebra chain, where q, and q' are two independent parameters, can be used to sum up the Dunham expansion fully.

The strong-coupling asymptotic expansions of the Bessel function of first kind J_nu (z) recently reported in connection with some two-state models are shown to be incorrect since they neither have the correct limits nor the functional properties required. The correct asymptotic expansions of J_nu (z) are derived in the case of v=+or-ffi alpha , alpha real and z real and positive, and shown to be very accurate in a large (z, alpha ) region. They are used in the derivation of the strong-coupling form of the transition probability in the generalized Demkov model for which a good agreement is obtained with numerical calculations. An interesting effect of failure of the attempt to derive the asymptotic expansion of a sum of products of Bessel functions by using only the leading terms of their asymptotic expansions is found and discussed.

In a Lyman alpha photon-scattered particle coincidence experiment, the angle-differential H(2p) alignment has been determined for 1 keV H⁺ impact on Na. At small scattering angles, large preferential population of the H(2p, m=0) substate is found. The results are in good agreement with coupled-channel calculations by Shingal.

The authors have measured angular distributions of binary encounter electrons resulting from collisions of 0.6 MeV amu^-1 Au¹¹⁺ projectiles with H₂ and He between 0' and 45'. For the hydrogen target a splitting of the binary peak into two well resolved components was observed in the large angular interval from 5' to 30'. On the other hand the He target produced cross sections in which the BE peak splitting is essentially confined within the angular range from 20' to 30'. The early onset of the double-peak feature at very forward angles in the H₂ case is interpreted as the experimental manifestation of a deep diffraction minimum, located at a laboratory angle of 12.5', in the model cross section for the elastic scattering of 327 eV electrons (i.e. electrons possessing the velocity equivalent of 0.6 MeV amu^-1) in the screened field of the Au¹¹⁺ projectile.

The energy loss spectrum of electrons scattered at 90' has been measured for Xe emanating from a 30 mu m diameter nozzle, in the 8.0-8.9 eV energy loss range, at 2 eV residual energy, and at stagnation pressures between 0.2 and 1.3 bar. Weak satellite bands of the ³P₂ and ³P₁ atomic lines have been observed at a stagnation pressure of 0.6 bar and assigned to electronic transitions of the van der Waals dimer Xe₂. An additional shoulder emerging at even higher stagnation pressures has been attributed to larger clusters. The energy dependence of the excitation of the Xe ³P₂ and ³P₁ atomic lines exhibits a number of narrow doubly excited Feshbach resonances. These features are much less pronounced or absent in the excitation functions of the Xe₂ and Xe_n bands, indicating energy broadening and lifetime shortening of the doubly excited resonances in the clusters.

The authors have developed a method to calculate core potentials for quasi-one-electron systems and the corresponding single electron orbitals. It is shown that the approximate inclusion of exchange effects between the valence electrons and the core removes the unphysical structure in the potential function that is characteristic for potentials calculated by only including the effect of core polarization due to the valence electrons. Excellent agreement with experimental ionization potentials is achieved, and example results for various systems such as Na, Cs, Ba⁺, In and Tl are presented.

An optogalvanic detection method has been combined with either CW monomode dye laser excitation or frequency doubled pulsed dye laser excitation, to investigate the even parity spectra of atomic krypton from levels of the 4p⁵4d and 4p⁵5s configurations. The energies of 81 levels belonging to 4⁵nf (J=1 to 5) Rydberg series and 61 levels belonging to 4p⁵np (J=1 to 3) Rydberg series have been determined. Most of these levels had never been observed previously. A multichannel quantum defect theory treatment of J=1 and J=2 even parity spectra has been performed, completing previously similar studies on odd parity spectra. The first ionization limit position has been redetermined. Taking into account the more recent values of krypton resonance line wavenumbers, this position results in: I₃2/=112914.41+or-0.02 cm^-1; it is in good agreement with previous determinations.

With regard to experimental investigations of electric-field anticrossings, the Stark splitting of 1snl levels of He I is evaluated for n=3, 4 and 5. Anticrossings of 1snl ¹ Lambda and ³ Lambda Stark sublevels with l>or=2 and Lambda = mod M_L mod <or=2 occur at fields F approximately 80 kV cm^-1, 20 kV cm^-1 and 7 kV cm^-1 for n=3, 4 and 5, respectively. The electric-field positions and the energy separations of closest approach of the anticrossings are calculated. Using the Liouville matrix formalism, a theory of anticrossing signals is formulated allowing a systematic analysis of signals arising from anticrossings of twofold degenerate Stark sublevels. The signal amplitudes are calculated as functions of the excitation parameters. The investigations show that these anticrossing signals are well suited for determining relative cross sections for the excitation of singlet and triplet substates by atom-atom and ion-atom collisions.

Electric-field anticrossings of 1snl ¹ Lambda and ³ Lambda Stark levels of He I with n=4 to 7, l=2 to 5 and Lambda = mod M_L mod <or=2 are investigated experimentally using 12 keV He-He collisions for excitation. For supplementary measurements also Ne atoms were used as projectiles. The measured electric-field positions of the anticrossing signals are in accord with calculated values. The dependence of the signal amplitudes on the polarization of the detected impact radiation is investigated. These measurements corroborate the identification of the signals. Some signal amplitudes are strongly affected by weak magnetic fields. Most anticrossing signals are Lorentz-shaped. However, significant deviations from the Lorentz shape are found for the anticrossings of the 1s4d configuration. These deviations are caused by the high-velocity He atoms used as projectiles. The findings are discussed and compared with theoretical results.

Experimental results on the ionization of calcium atoms following resonant pumping of the 4s²¹S₀-4s4p ³P₁ transition with a CW ring dye laser are reported. Absorption profiles of the 4s²¹S₀-4s4p ³P₁ transition around 6572.78 AA are measured under different conditions of temperature (T) and buffer gas pressure (p) where ionization was observed. Voigt profile fits are seen to adequately describe the observed absorption profiles. The results on collisional widths are in agreement with some previously published measurements. At low buffer gas pressures, collisional widths are found to scale as T^0.5.

Non-perturbative, exact, analytical photodetachment spectra for a model negative ion including a single bound state and two continua are calculated with continuum-continuum transitions taken into account. Asymmetric two-peak structures of significant widths are shown to be the typical spectra both for l=1 and for l=2 photoelectrons. The population trapping in the initial state is demonstrated as due to a creation of a bound state of the dressed system.

The Green function is constructed for the electron moving in the superposition of uniform static and linearly polarized monochromatic electric fields. It is employed to obtain general integral expressions for electron flux distributions and partial widths in multiphoton detachment or ionization in presence of uniform static electric field. The photodetachment in weak static field is analysed in detail for arbitrary strength of the laser field. The separation of the uniform field and laser field effects allows one to obtain simple and appealing expressions for photodetachment partial widths via static-field-free amplitudes in case of arbitrary number of absorbed photons and quantum numbers of the initial state. The interference oscillations of the photoelectron current are described uniformly for arbitrary direction of the electric vector of the laser field with respect to the static field axis.

The photoionization of a sodium atom from the inner and outer shells is studied in the electric dipole plus quadrupole approximation. It is found that the quadrupole contribution which is small at photoelectron energies close to the photoionization threshold becomes important at higher photoelectron energies. Significant interference effects between dipole and quadrupole are obtained in the cross section ratio R(= sigma _C(M)/ sigma _L(M)) of photoionization from a magnetic substate (M) with circularly ( sigma _C(M)) and linearly polarized ( sigma _L(M)) photons of energies 1.4 keV and above.

Relative abundances of the various ionic charges resulting from photoionization of argon by monochromatized synchrotron radiation at energies between 3150 and 4900 eV have been measured with low statistical error. Despite the fact that already, right at the onset of K excitation (3203.5 eV), almost the full 1s binding energy is deposited in the atom, the relative abundances change smoothly over the region of np excitation up to 6 eV above the limit. While this behaviour, caused by Rydberg shake-off (about 0.5 probability at the 1s to 4p resonance) and by recapture of the photoelectron via post-collision interaction, appears to be understood there is no explanation for a gradual change of relative abundances extending over tens of eV below the lowest K excitation. K+M and K+L single photon double excitations both show up as a smooth rise of the average ionic charge q from the respective threshold to a plateau at considerably higher energy, the overall increase of q being 0.206+or-0.02 and 0.13+or-0.01.

The one-dimensional delta-function atom subject to a laser field is investigated in the regime of ultrahigh laser intensity. Stabilization is found in the sense that for sufficiently high intensity, the ionization probability becomes a decreasing function (on average) of the laser amplitude and may reach a level below 90%. Detailed intensity scans reveal the relevance of ponderomotive effects and allow a discussion of the criteria determining the onset of stabilization. A simple high-intensity approximation compares well with the exact numerical results, enabling an estimate of the optimal pulse turn-on time. Its success points to a mainly classical origin of stabilization and leads to the conclusion that the survival probabilities will be much lower for three-dimensional atoms. Its application to hydrogen leads to a significantly lower ground-state probability than obtained in a recent numerical study by Horbatsch, Tang and Basile (1991) confirming that localization is more likely to occur in 3D than in 1D.

For pt.I see ibid., vol.25, no.23, p.5141-8 (1992). In Ne-Xe mixtures molecular states responsible for the second continuum of xenon ( lambda =170 nm) are shown to be created by homonuclear and heteronuclear reactions from the ³P₁ and ³P₂ states of xenon. In Ar-Xe mixtures, at lambda =826 nm, two lines of argon and xenon are superimposed; it is observed that the atomic states responsible for the two transitions undergo decay (for argon) or creation (for xenon). The creation of the 2p₁₀ xenon state is measured from the transition at lambda =980 nm which it produces. In the Ne-Xe mixture, at lambda =882 nm, it is the decay, through homonuclear and heteronuclear mechanisms, of the precursor 2p₈ state of xenon which is demonstrated. In each case, the rate constants of the corresponding reactions are reported.

A thick target of Ta was used to determine the L₃ subshell alignment, induced by proton bombardment, in the energy range 0.35-1.8 MeV, through the measurement of the L alpha ₂ X-ray line angular distribution. A plane crystal spectrometer with reflection geometry, equipped with an X-ray linear position sensitive proportional counter detector, was used to resolve the L alpha ₂ and L alpha ₁ lines. Experimental results are compared to theoretical SCA values obtained with hydrogenic and Herman-Skillmann wavefunctions.

Asymmetric and/or symmetric double-charge exchange in fast collisions of bare nuclei with helium-like atomic systems is considered. The author emphasizes the need for going beyond the standard independent particle model and consequently employs the four-body version of the boundary corrected first Born (CB1) approximation with a full account of the long-range Coulomb effects arising from the relative motion of the scattering aggregates. A method is devised which is capable of reducing the transition amplitudes T_if^(CB1)+or- to two-dimensional real integrals. As a test of the proposed theory, the author investigates the double-electron capture from helium by completely stripped lithium. The prior and post total cross sections obtained are found to be in satisfactory agreement with the available experimental data at impact energies ranging from 250 to 2000 keV.

The complex Kohn method, using correlated target states and a projected optical potential, is applied to elastic s-wave scattering of electrons from atomic helium in the energy range of the (1s2s²)²S Feshbach resonance. A projected optical potential is a very useful construct in the study of electron-molecule scattering, but the convergence of this approximation has not been systematically investigated. In this study, the projected optical potential approximation is shown to be convergent and yields an energy and width of the He ²S resonance of 19.357 eV and 14 meV, respectively, in excellent agreement with experiment.

The authors report measurements of the two linear polarization correlation parameters P₁ and P₂ for excitation of the Ar 4s'(1/2)_J=1 state by 25 eV electrons and scattering angles up to 55'. Two of the four independent natural parameters required for a complete description of the excitation process, the alignment angle gamma and the linear polarization P_lin⁺ of the angular part of the collisionally induced excited state charge cloud, were extracted from the P₁ and P₂ parameters. A comparison with theoretical predictions and with similar data obtained at impact energies in the range 50 eV to 30 eV support the previously observed discrepancy between measured data and theoretical predictions which increases systematically and gradually with decreasing impact energy. The authors also carried out P₁ and P₂ measurements for the excitation of the 5s(3/2)_J=1 state in Kr at 30 eV and scattering angles up to 55' and extended the previous measurements at 50 eV to 40'.

The authors have performed high resolution electron beam transmission experiments in O₂ over the energy range 12 meV to 1 eV. They estimate an approximate trend in widths of the v'=4, 5 and 6 resonance states of the O₂^- intermediate and again the approximate trend in the relative contributions of the l=2, 4 and 6 partial waves. Widths lie in the meV range as found previously for v'=6 by Field et al. (1991) and higher partial waves contribute less at lower energies. Analysis of the non-resonant elastic scattering data below 100 meV shows that scattering takes place through an interaction dominated by s-waves.

The results of relativistic calculations on the elastic positron scattering from mercury atoms for incident positron energies of up to 100 eV are presented. The polarization potential applied in the calculations was obtained by solving the coupled Dirac-Hartree-Fock equations. The values of scattering length, phaseshifts, total elastic cross sections and elastic momentum transfer cross sections are compared with theoretical results of other authors.

The population of a three-level system in the lambda configuration interacting with three classical fields is studied. The system is essentially a Raman coupled system with pump, Stokes and anti-Stokes fields. One channel between the two ground states involves the pump and Stokes field while the other involves the pump and anti-Stokes field. Since exact solutions are not generally available the author studies the dynamics numerically. But when the excited state is highly detuned, it may be adiabatically removed so that the system becomes effectively a two-level system but with an effective Rabi frequency that depends on the relative phases of the driving fields. For some choices of the phases this frequency can be made to vanish, an effect of the interference between the two channels. For the special case of equal and opposite detunings, an exact solution of the full three-level equations is presented. The effects of ionization of the excited state are also briefly considered.