Thomas Carette

Resonance Effects in Photoemission Time Delays

We present measurements of single-photon ionization time delays between the outermost valence electrons of argon and neon using a coincidence detection technique that allows for the simultaneous measurement of both species under identical conditions. The analysis of the measured traces reveals energy-dependent time delays of a few tens of attoseconds with high energy resolution. In contrast to photoelectrons ejected through tunneling, single-photon ionization can be well described in the framework of Wigner time delays. Accordingly, the overall trend of our data is reproduced by recent Wigner time delay calculations. However, besides the general trend we observe resonance features occurring at specific photon energies. These features have been qualitatively reproduced and identified by a calculation using the multiconfigurational Hartree-Fock method, including the influence of doubly excited states and ionization thresholds.

Measurements of relative photoemission time delays in noble gas atoms

We determine relative photoemission time delays between valence electrons in different noble gas atoms (Ar, Ne and He) in an energy range between 31 and 37 eV. The atoms are ionized by an attosecond pulse train synchronized with an infrared laser field and the delays are measured using an interferometric technique. We compare our results with calculations using the random phase approximation with exchange and multi-configurational Hartree-Fock. We also investigate the influence of the different ionization angular channels.

Diagrammatic approach to attosecond delays in photoionization

We study laser-assisted photoionization by attosecond pulses using a time-independent formalism based on diagrammatic many-body perturbation theory. Our aim is to provide an ab initio route to the delays for this above-threshold ionization process, which is essential for a quantitative understanding of attosecond metrology. We present correction curves for characterization schemes of attosecond pulses, such as streaking, that account for the delayed atomic response in ionization from neon and argon. We also verify that photoelectron delays from many-electron atoms can be measured using similar schemes if, instead, the so-called continuum-continuum delay is subtracted. Our method is general and it can be extended also to more complex systems and additional correlation effects can be introduced systematically. DOI: 10.1103/PhysRevA.86.061402

A Theoretical study of C- 3S^o_3/2 and 2D^o_3/2,5/2 bound states and C ground configuration: fine and hyperfine structures, isotope shifts and transition probabilities

This work is an ab initio study of the 2p{sup 3} {sup 4}S{sub 3/2}{sup o}, and {sup 2}D{sub 3/2,5/2}{sup o} states of C{sup -} and 2p{sup 2} {sup 3}P{sub 0,1,2}, {sup 1}D{sub 2}, and {sup 1}S{sub 0} states of neutral carbon. We use the multiconfiguration Hartree-Fock approach, focusing on the accuracy of the wave function itself. We obtain all C{sup -} detachment thresholds, including correlation effects to about 0.5%. Isotope shifts and hyperfine structures are calculated. The achieved accuracy of the latter is of the order of 0.1 MHz. Intraconfiguration transition probabilities are also estimated.

Relativistic effects on the hyperfine structures of 2 p(4)(P-3)3 p D-2(o), D-4(o), and P-4(o) in F-19 I

The hyperfine interaction constants of the 2p(4)(P-3)3p D-2(3/2,5/2)o, D-4(1/2-7/2)o, and P-4(1/2-5/2)o levels in neutral fluorine are investigated theoretically. Large-scale calculations are carried out using the multiconfiguration Hartree-Fock (MCHF) and Dirac-Hartree-Fock (MCDHF) methods. In the framework of the MCHF approach, the relativistic effects are taken into account in the Breit-Pauli approximation using nonrelativistic orbitals. In the fully relativistic approach, the orbitals are optimized using the Dirac-Coulomb Hamiltonian with correlation models inspired by the nonrelativistic calculations. Higher-order excitations are captured through multireference configuration interaction calculations including the Breit interaction. In a third (intermediate) approach, the Dirac-Coulomb-Breit Hamiltonian matrix is diagonalized in a relativistic configuration space built with nonrelativistic MCHF radial functions converted into Dirac spinors using the Pauli approximation. The magnetic dipole hyperfine-structure constants calculated with the three relativistic models are consistent and reveal unexpectedly large effects of relativity for 2D(5/2)(o), P-4(3/2)o, and P-4(5/2)o. The agreement with the few available experimental values is satisfactory. The strong J dependence of relativistic corrections on the hyperfine constants is investigated through the detailed analysis of the orbital, spin-dipole, and contact relative contributions calculated with the nonrelativistic magnetic dipole operator.

Theoretical study of the isotope effects on the detachment thresholds of Si

The isotope effects in Si- bound levels are studied using the multiconfiguration Hartree-Fock ab initio approach. Large-scale calculations are carried out for the 3p34So, 2Do, and 2Po multiplets of Si- and the 3p23P multiplet of Si. We predict an anomalous isotope shift on the electron affinity, dominated by the specific mass shift, with a value of -0.66(6) m-1 for the 30-28 isotope pair. We also report hyperfine-structure parameters for the studied multiplets. We provide the values of level electric-field gradients at the nucleus that could be of interest in a study of the metastable silicon isotopes. Relativistic corrections are estimated using nonrelativistic orbitals in the Breit-Pauli and fully relativistic frameworks.

From atoms to biomolecules: A fruitful perspective

We present a summary of the research activities of the “Quantum Chemistry and Atomic Physics” theoretical group of the “Chimie Quantique et Photophysique” Laboratory at Université Libre de Bruxelles. We emphasize the links between the three orientations of the group: theoretical atomic spectroscopy, structure, and molecular dynamics and list the perspectives of our collaboration.

Ab initio calculations of 14N and 15N hyperfine structures

Hyperfine structure parameters are calculated for the 2p2( 3P)3s 4PJ, 2p2(3P)3p 4PoJ and 2p2(3P)3p 4DoJ levels, using the ab initio multiconfiguration Hartree-Fock method. The theoretical hyperfine coupling constants are in complete disagreement with the experimental values of Jennerich et al deduced from the analysis of the near-infrared Doppler-free saturated absorption spectra.

Erratum: Resonance Effects in Photoemission Time Delays [Phys. Rev. Lett. 115 , 133001 (2015)]

This corrects the article DOI: 10.1103/PhysRevLett.115.133001.

Isotope shift on the chlorine electron affinity revisited by an MCHF/CI approach

Today, the electron affinity is experimentally well known for most of the elements and is a useful guideline for developing ab initio computational methods. However, the measurements of isotope shifts on the electron affinity are limited by both resolution and sensitivity. In this context, theory is of great help to further our knowledge and understanding of atomic structures, even though correlation plays a dominant role in negative ions’ properties and, particularly, in the calculation of the specific mass shift contribution. This study solves the longstanding discrepancy between calculated and measured specific mass shifts on the electron affinity of chlorine (Berzinsh et al 1995 Phys. Rev. A 51 231). (Some figures may appear in colour only in the online journal)