Hélène Lefebvre-Brion

Partial photoionization cross section and photoelectron angular distribution for the X 2Σg+ state of N2+ in the static‐exchange approximation

The partial photoionization cross section of N2 leading to the production of N2+ in its X 2Σg+ ground state is calculated in the single‐center frozen core static‐exchange approximation. The initial and final states are antisymmetrized products using SCF molecular orbitals of either N2 or N2+ ground states. All these calculations reproduce well the σu shape resonance in the partial cross section. The photoelectron angular distribution is also calculated and the average over the vibrational motion is performed. The R‐averaged results are in agreement with recent measurements using synchrotron radiation. The branching ratio of the cross sections leading N2+ in its two lowest vibrational states compares rather well with the experimental values obtained recently by West et al.

Ab initio approach to the multichannel quantum defect calculation of the electronic autoionisation in the Hopfield series of N2

The two-step formulation of the multichannel quantum defect theory, applied to molecular electronic autoionistion by Giusti-Suzor (1980) and Lefebvre-Brion (1973) is used to calculate the total and partial photoionisation cross sections in the region of the Hopfield series on N2, the 700-730 AA wavelength range. The electronic quantities necessary for this treatment are obtained from ab initio calculations. The cross sections and the photoelectron angular distribution parameters are in satisfying agreement with experiment. The results clearly show that the absorption series correspond to the (B2 Sigma u+)nd sigma g series and the apparent emission series to the (B2 Sigma u+)nd pi g series.

Theoretical study of shape resonances in the partial photoionization cross sections for the b 4Σg− and B 2Σg− states of O2+

Using the single‐center static‐exchange approximation, the 3σg photoionization cross section of O2 is calculated. The R‐averaged results are obtained for the production of O+2 in the b 4Σ−g and B 2Σ−g states. Shape resonances, a few electron volts wide, are found at 3 and 2.5 eV from the threshold for these two states, respectively. We find reasonable agreement with the experimental cross sections for the B 2Σ−g state and with the β values for the b 4Σ−g state. For this last state, comparison between theory and experiment leads us to a possible interpretation of the data, but a definite answer can not be given without taking into account the autoionized Rydberg states.

Comparison between predissociation mechanisms in two isoelectronic molecules: CO and N2

A correspondence between the observed electronic states of N2 and CO is presented, in order to compare the perturbations and predissociations of these two isoelectronic molecules. The homogeneous perturbations between the 1Σ+ (or ) and the 1Π (or 1Π u ) states are reviewed, and comparisons are made between the indirect predissociations of the 1Π states due to the 3Π states in both N2 and CO.

Chapter 1 – Simple Spectra and Standard Experimental Techniques

The field of diatomic molecule electronic spectroscopy is very old, broad, rich, and mature, yet astonishingly active and innovative. Even systems as seemingly simple as diatomic molecules often act as complex, many-body systems. Mechanistic understanding and insight, as opposed to mere empirical description, are based on the existence and discovery of patterns that owe their existence to approximate constants of motion. An approximate constant of motion is the eigenvalue of an operator that commutes with most, but not all, terms in the exact molecular Hamiltonian. The simple spectra described in this chapter provide the language, concepts, and intuitive framework required for the design and interpretation of even the most elaborate and innovative modern molecular structure and dynamics experiments. A spectrum, I(ω), is a plot of some sort of signal strength versus frequency. The signal can be in the form of either a decrease or an increase from a constant baseline level, can be due to either an absorption or emission of light, can be based on detection of photons, parent molecules, or photofragment species, and is recorded as an electrical current or voltage or the darkening of a photographic plate. The frequency is always expressed by spectroscopists in cm -1 units.

Theoretical approach to the Cooper minimum in hydrogen iodide

Abstract Calculations of the photoionization cross section corresponding to the X 2 Π (5pπ) −1 ion state and of the angular asymmetry parameter β are performed for the HI molecule in the 10.4–92 eV range. Within the frozen-core static exchange approximation, no Cooper minimum is found in the cross section, contrary to experiment. Interchannel coupling with the continuum states corresponding to ion states associated with a 4d hole is also partially taken into account. Two energy points are calculated within this coupling scheme, showing better agreement with experiment, especially for β at high energies.

Adiabatic versus diabatic descriptions of the lowest Rydberg and valence Σ+1 states of HCl

In this contribution we first report new ab initio self-consistent field configuration interaction calculations of the first excited adiabatic potential of (1)Σ(+) symmetry, the 2(1)Σ(+) or B(1)Σ(+) state, which presents two minima and can thus be seen as made up of the Rydberg E(1)Σ(+) and the valence V(1)Σ(+) states. Based on the computed 2(1)Σ(+) potential, we devised a theoretical procedure to compute the vibronic structure in order to try to explain the energy levels observed in the region above 76 254.4 cm(-1) which display an irregular vibrational structure, indicative of spectral perturbations. We try to find out which representation of the electronic states, the diabatic or the adiabatic one, is best suited to replicate the lowest observed vibronic levels of the E and V states. To this end, we deduce, from the 2(1)Σ(+) potential and its complementary adiabatic potential, two diabatic potentials. We then carry out a coupled equation treatment based on these diabatic potentials. The results of this treatment indicate that, in the present case, the adiabatic representation is better than the diabatic one to describe the observed vibronic levels. This is due, as expected, to the existence of a strong electrostatic interaction between the two diabatic potentials.

Ab initio and semiempirical estimates of PN valence state interactions

Abstract Hartree-Fock wavefunctions for the valence states of PN arising from the lowest energy open shell configurations, 2π47σ3π and 2π37σ23π, are generated and used in ab initio calculations of diagonal spin-orbit, off-diagonal spin-orbit, and rotation-electronic matrix elements. These results are compared with those from two semiempirical methods, one based on atomic orbital populations and the other dependent solely on atomic spin-orbit splittings. The latter method is found to be surprisingly successful in predicting the 3Δ spin-orbit constant for a series of isovalent molecules. Semiempirical estimates of the 3Δ and 3Π spin-orbit constants of AsN are given. The Hartree-Fock values of the a and b perturbation parameters are found to be 50 cm−1 and 0.60 (unitless), respectively. A previously reported perturbation in the A1Π-X1Σ+ band system is shown to arise from a 3 Σ − ∼ 1 Π spin-orbit interaction.

Multichannel quantum defect calculation of the phase lag in the coherent control of HI

Multichannel quantum defect theory (MQDT) is applied within a unified framework to compute the ionization and dissociation channel phases of HI. Our numerical results illustrate the mathematical origin of a channel phase within the MQDT formalism, and are consistent with the existing theory of this phenomenon, based on the collision formalism and with experimental measurements. The present study explains why previous MQDT calculations predicted that the channel phase vanishes identically.

On the vibrational autoionization in NO2

The vibrational autoionization for NO2 is evaluated by ab initio calculations and compared to recent experimental results of Grant et al. It is concluded that the resonances are broader for symmetric stretching than for asymmetric stretching as in the case observed, but this conclusion depends of the nature of the Rydberg orbital.