M. Raoult

Calculation of vibrational preionization in H2 by multichannel quantum defect theory: Total and partial cross sections and photoelectron angular distributions

Multichannel quantum defect theory has been used to calculate the effect of vibrational preionization on the total and partial oscillator strength distributions and photoelectron angular distribution in H2 for excitation between 790 and 760 A. The total oscillator‐strength distribution obtained agrees well with the high‐resolution photoionization data of Dehmer and Chupka. The partial oscillator strength resonance profiles are predicted to have different shapes in different vibrational ionization channels, while their widths change little with channel. The preionization resonances are also predicted to affect the angular distribution asymmetry parameters b over a broader range than they affect the oscillator strength distribution. The gross features of the preionization resonances are discussed in terms of approximate solutions of the MQD equations.

A unified treatment of 2Π–2Π Rydberg–valence state interactions in NO

Generalized quantum defect theory is used to study the 2Π–2Π Rydberg–valence state perturbations in the NO molecule. The work is a generalization of the two‐step quantum defect treatment used previously by Giusti‐Suzor and Jungen for the calculation of competing preionization and predissociation processes in NO. The existing high‐resolution spectroscopic data, mainly from Miescher and co‐workers, are used to extract a set of quantum defect parameters relating to the pπ Rydberg channel associated with the NO+ X 1Σ+ state, and to determine potential energy curves for the B 2Π and L 2Π states as well as Rydberg–valence interaction parameters. Based on these parameters, the observed term values including rotational and spin–orbit effects are reproduced with a mean deviation of 4 cm−1 for all the 2Π levels from 45 500 to 71 600 cm−1 (i.e., to within 200 cm−1 of the dissociation limit). The fit is better than that previously obtained by Gallusser and Dressler in a vibronic perturbation approach, particularly fo...

A,C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the obser...

Spectroscopy in the ionisation continuum. Vibrational preionisation in H2 calculated by multichannel quantum-defect theory

Multichannel quantum-defect theory has been used to calculate the effect of vibrational–rotational preionisation on the total and partial oscillator-strength distributions and photoelectron angular distribution in H2 for excitation between 800 and 750 A. The total oscillator-strength profiles obtained agree well with the high-resolution photoionisation data of Dehmer and Chupka. The results of these authors concerning the final vibrational state distributions obtained by exciting preionisation resonances are also in excellent agreement with the present calculations. Particular attention is given to Rydberg levels which preionise viaΔv < –1 processes. Preionisation is found to affect all partial vibrational cross-sections σv+ near narrow Δv < –1 resonances, while near broad Δv=–1 peaks only σv+=v– 1 is perturbed (v+ and v are the vibrational quantum numbers of the final state and of the preionised Rydberg level, respectively). Further, it is found that peaks which preionise viaΔv < –1 and fall among the higher members of a series with less vibrational energy tend to appear as “complex” resonances, consisting of a sharp central peak surrounded by an extensive structure of broader satellites.

Rydberg states of NO in a magnetic field: Multichannel quantum defect approach of the linear Zeeman effect

The linear Zeeman effect for the nf and np Rydberg states of NO has been analyzed in the intermediate energy region (n=7 and n=15). This analysis is based on a multichannel quantum defect treatment (MQDT), including the magnetic interaction, as proposed by Monteiro and Taylor [T. S. Monteiro and K. T. Taylor, J. Phys. B: At. Mol. Phys. 22, L191 (1989)]. The nonpenetrating nf states exhibit almost no channel mixing, but they are significantly perturbed by the Zeeman interaction within each rotational channel in a moderate field of 1 T. An excellent agreement has been found between the experimental results obtained in a 1 T magnetic field and the calculations, showing in particular the first manifestation of the Paschen–Back effect in a molecule. The penetrating np states of NO exhibit channel interaction, but, on the other hand, they are less perturbed by a 1 T magnetic field than the nf states are. Theoretical predictions have been made for a larger field strength of 5 T in the case of the 7p and 15p states.

Vibrational structure of the ArNO+ van der Waals cation

Abstract We report two different assignments of the vibrational structure of the ArNO + cation observed by Takahashi. On one hand, the ab initio potential energy surface (PES) of Robbe et al. (Chem. Phys. Letters 210 (1993) 170) allows us to interpret the main observed features as arising from a Fermi-type interaction between the stretching progression ( ν s , ν b = 0) and the ( ν s − 1, ν b = 2) combination band. On the other hand, the more recent ab initio PES of Wright et al. (J. Chem. Phys. 100 (1994) 5403) allows us to interpret this structure as arising from a Fermi-type interaction between the stretching progression ( ν s , ν b = 0) and two bending combination bands, namely ( ν s − 1, ν b = 0) and ( ν s − 2, ν b = 3). Only a subsequent intensity analysis could help decide between these two interpretations.

Application of generalized quantum defect theory to van der Waals complex bound state calculations

Bound states of atom–diatom van der Waals complexes are calculated in the generalized quantum defect theory framework (GMQDT). Due to very strong interchannel couplings, the diabatic GMQDT formalism is extended in order to account for the strongly closed channels (associated with potential curves lying above the total energy E) in the asymptotic analysis of the wave function. An alternative GMQDT treatment combining diabatic and adiabatic representations is also presented. The influence of these strongly closed channels on the level positions and oscillator strengths is analyzed.

Quadratic Zeeman effect in Rydberg states of NO

We analyse a resonant-enhanced multiphoton ionization (REMPI) spectrum of the high n (35–50) members of the Rydberg series of NO molecules recorded in a 1 T external magnetic field. In this range the quadratic Zeeman interaction takes over the Coulomb interaction and gradually scrambles the Coulomb structure of the Rydberg series. A perturbative quantum treatment of the diamagnetic interaction, giving rise to the so-called l-mixing and n-mixing, shows that the apparent complexity of the higher range of the experimental spectra results essentially from the interplay of two Rydberg series associated with different rotational core states. This perturbative treatment is expected to gradually break down and a semiclassical approach is proposed to understand the underlying physical pattern governing this higher spectral range.

Above threshold accumulated phase in molecular potentials

A complex accumulated phase above the threshold is defined in the quantum defect theory framework. It is the analytic continuation of the familiar accumulated phase defined below the threshold. Our phase is related to the usual parameters of the quantum defect theory (QDT). Moreover it is shown that the main part of this phase is deduced from the reflection coefficient of a wave function propagating along the external part of the potential, from the inner part to the asymptotic part, even in absence of a centrifugal barrier. Examples concern essentially the potential scattering in s partial wave and the ultracold collisions of alkali-metal atoms. It is shown how resonances or virtual states can be calculated like the bound states as poles of the cotangent of this phase. A physical interpretation of this quantity is given in a simple multichannel case.

Frame transformation theory for heavy particle scattering: Application to the rotational predissociation of Ar–H2

The frame transformation concept, which is standardly used in discussions of atomic and molecular Rydberg spectra, is applied to heavy particle scattering theory. It is shown how this approach permits the use of an optimal reference frame in the various different ranges of the scattering coordinate. It is further argued that the accuracy of the coupled states or p‐helicity decoupling approximations may be significantly improved without substantial additional computational effort especially for low energy scattering processes, through the use of frame transformation procedures. The method is then applied to the calculation of the photodissociation line shapes for the rotational predissociation of Ar–H2 within the framework of generalized multichannel quantum defect theory (MQDT). It is shown how the use of the frame transformation procedure yields reasonably correct product rotational state distributions in situations where the standard coupled states or body‐fixed decoupling procedure fails even at the qu...