Table of contents

We propose a criterion to determine the numerical degree of global (non-operator-specific) correlation of a multiparticle quantum system and apply it to several situations, including electron-atom scattering and strong-field photoionization. We show that the degree of correlation tends to increase in the presence of the field.

The effect of external time-dependent electric fields (TDF) on the radiative recombination (RR) process near the zero energy threshold and for capture to high Rydberg states is examined in detail. The momentum coherence phenomenon discovered recently causes a large enhancement in the cross section, with the maximum located near the principal quantum number n_g approximately=Z/g, where Z is an effective charge of the ion and g is the field momentum defined by the time integral of the electric field perturbation. The sensitivity of the RR cross sections on TDF may be used to study the field conditions in the interaction region where the recombination reaction takes place. Dependence of the enhancement on field parameters is presented in a form suitable for data analyses. Calculated cross sections are compared with recent experiments on Ar¹⁵⁺ and U²⁸⁺, Au²⁵⁺ and on ions with Z<25, where sizable enhancements were observed.

The NO²⁺ molecule is prototypical in that its lowest two electronic states, the X² Sigma ⁺ and the A² Pi , exist in local minima and offer a real model of low-lying embedded vibrational spectra, where both the discrete and the continuous parts couple via the electric dipole interaction. By applying an ab initio quantum mechanical theory, we computed the above and below threshold dissociation probabilities induced by laser pulses of h(cross) omega =0.24-0.41 eV and peak intensity l₀=8.75*10¹³ W cm^-2. If the two states were assumed to be uncoupled by the field, the multiphoton dissociation of NO²⁺ via the ² Sigma ⁺ state by such a pulse would have an insignificant probability. The coupling enhances the dissociation through this channel by six orders of magnitude. Actually the multiphoton dissociation occurs overwhelmingly via the A² Pi state, which controls the dynamics of the dissociation.

High pressure nitrogen targets excited by energetic alpha particles show dominant emission of the Gaydon-Herman green system (transition). Bands from the Herman IR transition are also observed. The bands appear at pressures up to 900 kPa and under extremely high purity conditions only, which explains why the bright green emission has not been seen previously from nitrogen targets at irradiation. The two transitions seem to appear always at the same time, which supports previous suggestions of a common upper state.

First measurements of the angular distributions of autoionization electrons from 2p⁵3s3p ²D_5/2 state of Na atom excited by electron impact from the ground and laser excited 3p ²P_3/2 state are reported. Strong anisotropy of the angular distribution is revealed, which has different signs for the excitation from the ground and excited states. Theoretical analysis based on the Born approximation is given.

The validity of the Kroll-Watson approximation to treat electron-atom collisions in the presence of a laser field is discussed. Numerical calculations have been carried out using classical mechanics and a model potential to describe electron-helium scattering for conditions of experimental interest. The effect of an additional polarization term, arising from the interaction between the laser field and that due to the electron, is shown to be negligible.

Resonances appearing above the ionization threshold in coupled-state calculations of positron-atom scattering are discussed. Calculations in the six-state approximation Ps(1s, 2s, 2p)+H(1s, 2s, 2p), which show such resonance structure are compared with a more extensive 18-state approximation Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+(H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) in which channels other than 1s, 2s and 2p are represented by pseudostates. The results strongly indicate that the above ionization threshold resonances observed in the six-state approximation, and in other small basis set calculations, are not real. It is suggested that they are a consequence of the neglect, or inadequate representation in other approximations, of ionization channels. In the six-state approximation the positronium component of the system wavefunction attempts to represent the missing ionization channels but in so doing produces unreal resonances above the ionization threshold. More generally, it is suggested that, in coupled-state calculations of positron-atom scattering, the atom part of the system wavefunction will try to compensate for defects in the positronium component and vice versa. When the defects are serious, for example, the omission of important ionization channels, unusual spurious behaviour is to be expected.

In this letter we present measurements of total cross sections for 1-10 eV positrons scattered from sodium (Na) atoms. Comparisons of these measurements and other recent measurements of total and positronium (Ps) formation cross sections for positron-Na and potassium (K) scattering by our group with the close-coupling approximation calculations by Hewitt et al. (1993) provide rather clear evidence that coupling effects between Ps formation and other scattering channels are playing a very important role in positron scattering by Na and K.

Relative cross sections for double and triple ionization of argon by positron impact are reported. The measured cross section ratios for double and triple relative to single ionization display multiple peak structures of which the high-energy maxima are attributed to inner-shell processes.

The basis and workings of a very useful technique in the treatment of atomic collisions is explained, which is the introduction of a common translation factor in the framework of close-coupling expansions. A historical review of the subject is presented, together with a description of the properties of the factor, and a detailed illustration of its performance.

The atomic d shell is explored from the perspective of the quark model of the atom. In this scheme, all the states are generated by four statistically independent fictitious particles (the quarks), each with angular momentum 3/2, augmented by two parity labels. The detailed quark expansions are given for the two ²D terms of d³, and the results are compared to the classic analysis of Condon and Shortley (1935). Matrix elements of single-electron tensor operators and of the Coulomb interaction are calculated in the quark scheme, and the connection to the conventional approach is made. The diagonal sum method of Slater is applied to the quark states and extensions to many-electron operators are discussed. The correspondence between the quark model and the traditional method implies relations between various n-j symbols, and this is illustrated by two special cases.

The problem of the rotational effect in the rovibrational matrix elements M_{upsilon J}^{upsilon 'J'}=( Psi _{upsilon J} mod f(r) mod Psi _{upsilon 'J'}) is considered. Rayleigh-Schrodinger perturbation theory is used to show that, for any diatomic electronic potential U(r) (even of the RKR type) and for any operator f(r), the rotational factor G_{upsilon upsilon '}(J,J')=M_{upsilon J}^{upsilon 'J'}/M_{upsilon 0}^{upsilon '0} is given by G_{upsilon upsilon '}(J, J')=G_{upsilon upsilon '}⁽⁰⁾+G_{upsilon upsilon '}⁽¹⁾m+G_{upsilon upsilon '}⁽²⁾m²+... with m=(J'(J'+1)-J(J+1))/2. The coefficients G_{upsilon upsilon '}⁽⁰⁾, G_{upsilon upsilon '}⁽¹⁾ and G_{upsilon upsilon '}⁽²⁾ are simply deduced from the integrals ((Y_upsilon mod f mod Y_{upsilon '}), in which Y denotes the pure vibrational wavefunction Psi _upsilon ⁽⁰⁾, or a successive perturbative correction Psi _upsilon ⁽¹⁾, Psi _upsilon ⁽²⁾,... Numerical application to the RKR potential of the X 0_g⁺ state of I₂ (for the transitions upsilon =0 and Delta upsilon <or=8 with m=-5, -10, -15) and to the Dunham potentials of the ground states of HCl (for the transitions upsilon =0 and Delta upsilon <or=7 with m=-2, -4, -6, -8) and HF molecules (for the transitions upsilon =0 and Delta upsilon <or=5 with m=-2, -4, -6, -8) shows the satisfactory accuracy of the present formulation.

Recent accurate calculations by Magnier et al. (1994) have demonstrated that for some symmetries the long-range excited adiabatic potential curves of the Na₂ molecule display many avoided crossings, some of them being due to competition between ionic and covalent dissociation. We have used those molecular data to estimate cross sections for the energy pooling reaction: Na(3p)+Na(3p) to Na(3s)+Na(5s, 4d, 4f) in the framework of a semiclassical multicrossing Landau-Zener model. All the pseudocrossings of the adiabatic potential curves from the Na(3s)+Na(4p) up to the Na(3s)+Na(4f) dissociation limit have been considered. At thermal collision energy (T approximately=500 K) cross sections are in good agreement with available experimental data. At lower collision energy (T approximately=200 K), we show that a ¹ Sigma _g⁺ and a ³ Sigma _u⁺ channel correlated to Na(3s)+Na(5s) dissociation limit become dominant. This illustrates the crucial role of long range crossings in the dynamics of collisions between excited sodium atoms at very low energies.

To improve on the behaviour of usual close coupling expansions at intermediate energies it has been recently proposed to enlarge the basis with the addition of probability absorbers or doorway states. Application to atomic expansions requires the regularization of the Coulomb potential interactions, either by using model potentials or projection techniques. A preliminary application of the latter method to the benchmark He⁺(1s)+H⁺ collisional system is presented, using a one-centre atomic expansion, in the range of 10-500 KeV impact energies. Use of different regularizations, together with an analysis on the shape of the probability absorber functions, permits electron loss, ionization, charge exchange and excitation cross sections to be obtained.

A fast intersecting beam technique similar to that used previously in this laboratory has been used to study He²⁺ formation in He⁺-He⁺ collisions at CM energies within the range 54-229 keV. Absolute total cross sections for He²⁺ formation have been determined and the separate contributions from charge transfer and ionization have also been obtained. The process of simultaneous electron loss by both He⁺ ions has been shown to be relatively unlikely even at the highest energies considered. The measurements usefully extend previous lower energy data and, while agreement with these is generally satisfactory, some discrepancies have also been noted. The energy dependence of the measured cross sections for charge transfer indicate an unusually broad structure in general accord with one theoretical prediction. Comparisons with a number of theoretical descriptions have been made for both charge transfer and ionization.

A crossed beam technique incorporating time-of-flight spectroscopy and coincidence counting of fast ion (atom)-slow ion collision products has been used to obtain for the first time separate cross sections for simple electron capture (charge transfer) and for transfer ionization in collisions involving Fe atoms. We have studied processes involving Feⁿ⁺ formation in one-electron capture by 70-500 keV amu^-1 H⁺ ions for n between 1 and 4 and both one and two electron capture by 37.5-360 keV amu^-1 He²⁺ ions for n between 1 and 6. It is shown that, for the most part, total electron capture cross sections are dominated by contributions from transfer ionization processes rather than from simple charge transfer. The energy dependence of some of the measured cross sections exhibits interesting structure and the formation of the observed multicharged Feⁿ⁺ ions is considered in terms of a model involving electron capture of either 4s, 3d or 3p target electrons together with electron removal through binary collisions at high velocities.

Projectile angular scattering is investigated following electron capture by protons from circular Rydberg atoms oriented along a quantization axis perpendicular to the incident projectile direction. We find strong left-right scattering asymmetry as a function of the orientation angle at collision speeds comparable to the electron orbital speed. The capture cross section is dramatically enhanced due to velocity matching when the angular momenta of the proton and the electron are aligned. The scattering asymmetry exhibits characteristic impact parameter dependencies suitable for studying three-body dynamics influenced by both long and short range forces of comparable strength.

A closed form expression for the scattering amplitude in the Coulomb-Glauber approximation for an arbitrary nlm to n'l'm' excitation of the hydrogen-like ions by the impact of structureless charged particles in the limit of infinitely large target nuclear charge Z_T is obtained. Cross sections for a few excited-state to excited-state transitions by electron impact are presented.

The average deflection of a laser excited, divergent sodium beam with a broad velocity distribution is measured by means of a Langmuir-Taylor detector and exploited for determining the absolute density of the excited state in the interaction area. Simulations of the excitation and deflection process point out that the knowledge of the velocity distribution of the sodium atoms plays a crucial part in obtaining reliable information from deflection measurements. From the comparison of simulated and experimental results we find that in our set-up typically 30% of the sodium atoms are in the excited state when both ground state levels are pumped with an intensity of 35 mW cm^-2 from an electro-optically modulated CW laser beam; the corresponding excited state density is 1.5*10⁹ cm^-3.

The emission spectrum of the ⁷¹GaI molecule, lying in the 3800-4500 AA region and attributed to the A ³ Pi ₀-X ¹ Sigma ⁺ and B ³ Pi ₁-X ¹ Sigma ⁺ transitions has been reinvestigated. The spectrum of the single isotopic species ⁷¹GaI was excited using an electrodeless discharge lamp run at 160-180 W microwave power (2450 MHz). More than 130 bands have been photographed on a 3.4 m Jarrell-Ash grating spectrograph in second and third orders and assigned to the vibrational schemes of the two systems. In both the systems reversal of shading has been observed in the bands of the same sequence and also secondary heads have been observed in several of the bands at high J. The A-X vibrational scheme has been extended to include bands involving upsilon '<15 and upsilon "<25. More precise vibrational constants have been determined which include the omega _ey_e term for the A ³ Pi ₀ state.

An ab initio calculation of the hyperfine splitting (HFS) intervals in the 2s_1/2^(mu)1s_1/2^(e) state of the exotic (⁴He²⁺ - mu^-e^-)⁰ and (³He²⁺ - mu^-e^-)⁰ atoms is carried out. The smallness of beta = m_e/m_mu approximately 1/200 (the electron and the mu-meson mass ratio) enables us to take into account the non-relativistic interparticle correlational interaction perturbatively. Corresponding corrections, which give the dominant contribution to the HFS, are calculated to logarithmic accuracy analytically and exactly. Together with the leading relativistic and radiative corrections the HFS values obtained turn out to be in accord with the results of other calculations.

A broad collection of experimental tunnel ionization data obtained using intense 10.6 mu m CO₂ laser radiation, is presented. The variety of species studied includes three rare gas atoms (Xe, Kr and Ar), three homonuclear diatomic molecules (H₂, O₂ and N₂), two heteropolar diatomic gases (CO and NO) and one triatomic molecule (CO₂). The ionization behaviour going from the neutral particle to its associated singly charged ionic state, for all of the above species, is compared with a quasistatic tunnel ionization model. Good agreement between this theoretical model and the complete range of experimental results is found.

The Kramers-Kronig (K-K) relations are verified for the third-order non-linear optical susceptibilities which are described by known expressions, obtained by the density matrix formalism. Possible applications of the K-K relations in non-linear Raman spectroscopy techniques are also discussed. Coupling of the real and imaginary parts of CARS excitation and probe profiles, as well as a relation between the spectra of two variants of OHD-RIKE are demonstrated. Some cases are shown when the susceptibility does not obey the K-K relations. This result is not a consequence of a break of the causality principle.