[en] The dissociative recombination (DR) of molecular ions: AB⁺ + e → A + B has been presented and discussed by B. Mitchell, in this same volume. The current chapter will comment on some interesting features of this process and present recent theoretical developments in this field. Examples and extension to related processes will close this chapter. 21 references, 12 figures

[en] Most of the rules for atomic or molecular autoionization have been derived within a two-channel model, involving one open channel (the continuum) and one closed channel (the discrete states). Very often, especially in molecules, autoionization processes involve additional coupled channels, open or closed, which complicate the resonance features by violating the two-channel rules. As an example the authors discuss the case of complex resonances, due to the autoionization of a Rydberg series close to its ionization threshold, perturbed by an isolated discrete state. 14 references, 5 figures

No abstract available

[en] A theory of Rydberg and continuum states in intense laser fields is developed based on the observation that the effect of laser radiation can be described in a scattering formulation as a finite-volume interaction coupling Coulomb-type fragmentation channels. In particular, laser-induced couplings may be incorporated in a multichannel quantum-defect treatment, in which a set of dressed channels corresponding to different photon numbers is defined, with intensity-dependent quantum defects and mixing angles. These quantities may be obtained by solving a system of close-coupling equations for the electron wave function, in a frame where the asymptotic electron oscillations are transformed away. This allows us to read off at a finite distance a radiative reaction matrix, which is a smooth function of energy for an energy range small compared with the photon frequency, and contains the net effect of radiative couplings. Calculations are presented for ionization of hydrogen s states in circularly polarized laser light, with allowance for above-threshold photon absorption

[en] The key features of the generalized multichannel quantum defect theory are outlined and applications to two processes involving both ionization and dissociation channels are described: the competition between preionization and predissociation in the NO molecule, the dissociative recombination of the H₂⁺ ion. In both cases, it is found that apparent vibrational couplings between electronic continuum and Rydberg series result mainly from an indirect electrostatic interaction via the nuclear continuum. (Auth.)

[en] The application of molecular quantum defect theory to the molecules H² and NO is examined. In these cases the competition between ionisation and dissociation processes has been observed experimentally in detail. It is shown that it is possible to extend MQDT so that it accounts for experimental results. (U.K.)

[en] Nonlinear variations affecting the widths and positions of laser-induced resonances in the molecular-dissociation process are discussed as a function of the electromagnetic field intensity. It is shown how the close-coupled equations of quantum scattering theory, taking into account multiphoton absorption and emission processes through the time-independent Floquet Hamiltonian, go beyond the widely used weak-field Fermi golden rule approximation. For the intermediate-field regime adiabatic electron-field channels can, in some cases, serve as a guide for the interpretation of nonlinearities, while for higher laser intensities very large field-induced mixings between molecular states predominate, requiring the introduction of an increasing number of Floquet blocks for converged calculations. The formalism is applied to the photodissociation of H₂⁺(1sσsub g/, v = 0, J = 1→2pσsub u/). Nonlinear effects appear for intensities larger than 10/sup 11/ W/cm², for which experiments begin to be available

[en] The theory of configuration interaction in molecules is extended to permit the simultaneous treatment of doubly excited electronic states and vibrationally excited Rydberg states which can decay by autoionization and predissociation. The method is applied to dissociative recombination and compared with the corresponding multichannel quantum-defect theory. Calculations using both techniques are reported for collisions of electrons of energy below 0.5 eV with H²⁺ ions in the lowest three vibrational states. The Rydberg states lead to narrow structures in the cross sections, mostly in the form of dips. The cross section and recombination rate for ground-state ions are anomalously low, being smaller than those for v = 1 and 2 by a factor of approx.6

[en] Rydberg-free and free-free transitions (above threshold ionization) of hydrogen in circularly polarized laser fields are discussed from a Quantum Defect Theory (QDT) point of view. An outline is given of how the full time dependence of transition probabilities from low-lying bound states to Rydberg states close to threshold may be calculated within a QDT. 18 refs.; 4 figs.; 1 table

[en] The usual approach to calculating multiphoton collision and half-collision processes uses the interaction picture and introduces a classical time-dependent field into the Hamiltonian for the scattered particles, which is further simplified using the Floquet ansatz. In particular, the laser-induced decay of an initial bound state is derived from a half-collision loquet ansatz that utilizes a complex quasienergy, whose imaginary part is identified with the laser-induced decay constant of the bound state. This interpretation presupposes a pure exponential decay of the initial-state population and yields a Lorentzian distribution of product-state energies in the infinite-depletion limit. Here we demonstrate how a full-collision Floquet ansatz can be derived from a fully quantal wave packet constructed to represent scattering in the presence of a coherent state of the laser field. The wave packet utilizes the set of the time-independent close-coupled wave functions for scattering in the field generated by quantized radiation number states. The resultant Floquet scattering states are energy normalized and stationary, and the quasienergy is real. We show how to use these states to construct a coherent-state wave packet that describes the decay of an arbitrarily prepared bound state, and yields a half-collision Floquet ansatz without any commitment to a complex quasienergy. The product energy and quantum-state distribution in the infinite-depletion limit are obtained without approximation to exponential decay. Further we show how the Fourier transform of the infinite-depletion line shape gives the temporal behavior of the initial bound-state population, often with distinctly nonexponential behavior. Such results are demonstrated for the above-threshold ionization of H, and the above-threshold dissociation of H₂⁺. In the following paper [R]