Service de Chimie Quantique et Photophysique
Atomes, Molécules et Atmosphères
Home Members Teaching Quantum Chemistry
& Atomic Physics
Experimental Spectroscopy Atmospheric
Spectroscopy
M. Godefroid
N. Vaeck
E. Cauët
J. Liévin
J. Loreau
L. Filippin
A. Gibbons
N. Iacobellis
G. Jumet
M. Kas
T. Launoy
L. Santos

Structure and dynamics of atoms and molecules

The “Quantum Chemistry and Atomic Physics” unit focuses on the calculation of the structure and dynamics of atomic and molecular species that are relevant in various fields such as spectroscopy, atmospheric chemistry, astrophysics and astrochemistry, cold molecules, nanoelectronics, or quantum computing. We are active in many research areas, some of which are outlined below. A description of previous research performed in our group over the years can be found here.

A searchable list of publications authored by members of the group is available here.


Computational Atomic Structure

Calculation of atomic structures and data including energy levels, electron affinities, radiative and non-radiative transition rates, lifetimes of excited levels, fine and hyperfine parameters, isotope shifts, polarizabilities of neutral atoms, negative and multicharged ions. These atomic data are relevant in astrophysics, plasmas physics, thermonuclear fusion research programs and in the study of earth and planetary atmospheres. We contribute to the development of methods and computer codes for calculating electronic wave functions and spectroscopic properties, including correlation and relativistic effects. The group is involved in developing state of the art computer codes for atomic calculations in the non-relativistic scheme with relativistic corrections in the Breit-Pauli approximation ATSP2K as well as in the fully relativistic scheme GRASP2K. The codes rely on multiconfiguration methods and the wave function for an atomic state is expanded in configuration state functions. Some of the group's research are highlighted in this "Lab Talk" article in Journal of Physics B.
Collaborations: Charlotte Froese Fischer (NIST), Alan Hibbert (Queen's U), Ian Grant (Oxford), Per Jönsson (Malmö U.), Gediminas Gaigalas (Vilnius U.), Paul Indelicato (Sorbonne Universités), and many others: see the "International collaboration on Computational Atomic Structure" (CompAS).

Astrochemistry

We calculate potential energy surfaces by means of high level quantum chemistry methods in order to describe elementary astrophysical reactions, like the interstellar C + H3+ → CH+ + H2 reaction, recently studied by collision experiments. We participate in the development of models describing the chemistry of various astrophysical environments. Our current research is centered on planetary and cometary atmospheres, with the aim to explain the observed densities and compositions through chemical models, to understand the coupling of the upper atmosphere to the solar wind, and to describe atmospheric erosion. These models rely on quantities such as reaction cross-sections and rate constants that can be subject to large uncertainties. Due to the large number of reactions that have to be considered, it is challenging to obtain a reliable set of cross sections or rate constants, together with their energy/temperature dependence. We identify key reactions in these models and compute the reaction cross sections and rates using quantum-mechanical methods. We are currently working in collaboration with the BIRA-IASB on the modelling of the composition coma of the comet 67P, which is being measured using the ROSINA instrument aboard Rosetta. More information on this project is available at the dedicated website.
Collaborations: J. De Keyser, R. Maggiolo (BIRA-IASB), X. Urbain (UCL), P. Quinet, P. Palmeri (UMons).

Cold Molecules

We investigate the dynamics of reactive and non-reactive collisions in the cold regime (T<1K). The study of cold collisions provides insights into the intermolecular potentials as well as into the fundamental mechanisms that are governing atomic and molecular collisions. At these low temperatures, quantum effect such as resonances or tunnelling become important, and chemistry can be controlled at the quantum level. From the theoretical point of view, one of the main challenges is to obtain reliable potential energy surfaces to perform dynamical calculations as low-energy collisions are extremely sensitive to the details of the PES as well as to the long-range potential. We are currently involved in several projects, from the simulation of the buffer gas cooling of ammonia to the study of cold chemistry with molecular anions.
Collaborations: T. Softley (Oxford University), M. Weidemüller (Heidelberg).

Non-Reactive Collisions

The mechanism of rotational energy transfer through atom-molecule or molecule-molecule collisions is important in many astrophysical environments, from planetary atmospheres to the interstellar medium. Chemical models of these environments require accurate rate coefficients for these processes at low temperature (T<300 K), which can be obtained though quantum-mechanical methods. We are working on the various aspects that are necessary to compute cross sections and rate coefficients for rotationally-inelastic collisions. We use post-Hartree-Fock methods to calculate the potential energy surface of the ground state of the collision complex. Using a fitted form of the PES, we then utilize the time-independent close-coupling method to solve the Schrodinger equation, and obtain integral and differential cross sections that can be compared to experimental data.
Collaborations: A. van der Avoird (Radboud University Nijmegen).

Molecular Spectroscopy

We perform accurate calculations of the spectra of polyatomic molecules that can be compared to experimental measurements. We focus in particular on the rovibrational structure of small molecules of astrophysical interest. We developed for this purpose an original ab initio method of calculation combining variational and pertubation theories. We also contribute by means of large scale ab initio calculations to the interpretation of the electronic spectra of transition metal containing diatomics and of the photelectron spectra of cyanoacetylenic species.
Collaborations: P. F. Bernath (Old Dominion University), P. Cassam-Chenaï (Nice Sophia Antipolis University), S. Boyé-Péronne, D. Gauyacq, B. Gans (ISMO, University Paris-Sud, Orsay).

Excited Electronic States

Highly excited electronic states participate in a large range of reactions such as charge transfer, molecular photodissociation, ion-pair production and neutralization, isomerization,… which are important in astrophysical environments as well as in plasma physics. We investigate the structure and dynamics of excited valence and Rydberg electronic states of diatomic and small polyatomic molecules. We use post-Hartree-Fock quantum chemistry methods with large basis sets to determine accurate potential energy surfaces of these species. The topography of these surfaces (stationary points, reaction paths, conical intersections) is investigated. For molecular complexes we develop adapted basis sets containing atomic orbitals that are optimized to reproduce the excited states of the fragments. Non-adiabatic radial and rotational couplings between the excited PECs are calculated and used to define the diabatic representation. We also compute other molecular parameters such as dipole matrix elements, spin-orbit couplings, Born-Oppenheimer corrections,… Different methodologies (wave packet propagation, R matrix method, semi-classic approaches) are used to solve the nuclear Schrodinger equation and elucidate the dynamics involving Rydberg states.
Collaborations: M. Desouter-Lecomte (LCP, University Paris-Sud, Orsay), X. Urbain (UCL), K. Béroff, M. Chabot, S. Boyé-Péronne, D. Gauyacq, B. Gans (ISMO, University Paris-Sud, Orsay).

Quantum Control & Quantum Computing

In collaboration with the Laboratoire de Chimie Physique of the University Paris-Sud, we are exploring the laser control of molecular systems. The manipulation of reactive processes using external fields has proven to be a very useful tool to control the outcome of reactions. We are developing methodological tools based on time-dependent methods combined with local control and optimal control algorithms with additional constraints to obtain pulses that can be realized experimentally. We are studying processes such as the optimization of specific channels in the photodissociation of molecular ions, or the control of the vibrational redistribution in acetylene. We are also interested in the potential applications of laser control to quantum computing by encoding qubits on systems such as vibrational states of trapped ions or on hyperfine states of ultracold polar molecules.
Collaborations: M. Desouter-Lecomte (LCP, University Paris-Sud, Orsay), C. Meyer (Université Toulouse III Paul Sabatier).

Surface Reactions

Heterogeneous chemistry at the solid/gas interface plays a crucial role for the formation of molecules in many astrophysical environments, such as the interstellar medium (ISM), the Earth’s atmosphere, or the outgassing in the cometary coma. We focus in particular on the study of adsorption of atoms by methane ices surrounding interstellar dust grains and on the desorption of small species such as methanol from grains under the action of UV radiation. We employ a QM/MM approach in which the grain surface is described classically, while its interaction with the adsorbed molecules is treated quantum-mechanically.
Collaboration: J. De Keyser (BIRA-IASB).

Nanoelectronics

The reduction of silicon transistor size allowed the incredible increase of computing capacity over the past fifty years, in accordance with Moore’s law. As this miniaturization is becoming increasingly difficult to achieve, it is important to look at alternative to silicon transistors. The aim of nanoelectronics is to explore new methods and materials that could provide solutions to this problem. Quantum dots represents a potential candidate, as these nanocrystals can operate as single electron transistors (SET). The properties of the SET can be manipulated with external fields, which requires knowledge of structural variables and modelling of transport phenomena. Furthermore, we are also investigating the possibility of using exciton states as an implementation of qubits for quantum computing. Another project focuses on using Transition metal dichalcogenides (TMDC) monolayers. These semiconductors provide a promising alternative to silicon and can be used as transistors as they have a great electronic mobility and a direct band gap. We study the electronic, optical and transport properties of TMDC using various methods and explore their capacity to make a good quantum computing device.
Collaborations: M. Desouter-Lecomte (LCP, University Paris-Sud, Orsay), C. Meyer (Université Toulouse III Paul Sabatier).

List of active projects


Université Libre de Bruxelles – Faculté des Sciences