Theory and simulation

The Theory and Simuation group of the Molecular Physics Department aims at understanding photonic, electronic, atomic and molecular interactions and collisions. It develops accurate quantum dynamical methods for the study of molecular structure and dynamics, and of atomic processes in intense laser fields. A second area of activity concerns the structure and evolution of stars.

Numerical methods and associated computer codes are developed and exploited on a local computer cluster to study systems of fundamental interest as well as for applications in other fields. This research requires knowledge and experience in physics, mathematics, numerical methods and high-performance computing. An important aspect of our activity is a close collaboration with experimentalists.

Current and recent research interest include:

Ultracold molecular gases are now studied in several experimental groups. At milliKelvin temperatures and below, quantum effects are exacerbated and molecular motions are strongly affected by ubiquitous trapping potentials. A rigourous quantum dynamical treatment of molecular collisions in these unusual situations necessitates to build new scattering codes to describe accurately the wavefunction at distances ranging from bohrs to microns. We implement a versatile spectral element method which combines both accuracy and efficiency. Recently we considered collisions between two KRb molecules in the presence of a linear trapping potential. We also studied ultracold ion-atom collisions in the presence of an external magnetic field which gives rise to Landau quantization effects.

We are involved in quantum reactive scattering for elementary chemical reactions of the type A + BC → AB + C. We use a quantum formalism based on hyperspherical democratic coordinates which allows the treatment of both abstraction and insertion reactions. Our computer package determines the scattering matrix and the observables which are suitable for comparisons with experiments. We considered the S(¹D)+H₂ reaction in comparison with crossed-beam experiments performed in Bordeaux and with CRESU experiments in the Department.

In the atmosphere of planets, spectral lines are shifted and broadened by elastic and inelastic collisions with surrounding gases. We study the collisional broadening of the infrared lines of C₂H₂ by H₂ and N₂ via a quantum scattering method. Our code treats interference effects which become important in the case of overlapping lines. The reduction of Doppler broadening by motional narrowing (Dicke effect) is also taken into account to model accurately the spectral line profiles.

The study of molecular systems in excited electronic states requires to go beyond the Born-Oppenheimer approximation. For such systems with strong non-adiabatic couplings multiple surface quantum dynamics is realized, from the ab initio structure computation to the determination of observables such as photo-electron spectra. This necessitates the derivation of the diabatic matrix and the propagation of nuclear wave-packets. Systems of interest are for example NO₃ and NH₃⁺ for which spectroscopic features induced by the vibronic couplings (Jahn-Teller and pseudo Jahn-Teller effects) are manifest.

For molecular systems with a large number of internal degrees of freedom such as doped helium droplets quantum Monte Carlo techniques are more suitable. They rely on a stochastic resolution of the time-independant Schrödinger equation via random walks. Systems such as a Rb₂ dimer adsorbed at the surface of helium nanodroplets and the malonaldehyde molecule have been studied in full dimensionality.

The R-matrix-Floquet approach, developed in part by researchers in this group, provides a unified, ab initio, non-perturbative description of multiphoton ionization of multi-electron atomic systems and of electron-atom scattering in the presence of a laser field.

We are currently interested in multiphoton ionization of metastable atomic states, in particular when the initial state is split into components corresponding to different magnetic quantum numbers, which then follow different ionization pathways. The ionization rate can be significantly enhanced if the absorption of photons can leave the atom temporarily in one of its excited states, a process known as resonance-enhanced multiphoton ionization.

The presence of the field in laser-assisted electron-atom scattering induces novel features and processes such as simultaneous electron-photon excitation, in which the target is excited even though the energy of the collisional electron is insufficient on its own to achieve this. Strong dressing effects can be observed when the wavelength of the field corresponds to the energy separation of two states of the atomic system. New structures may be revealed, induced by the field or resulting from coupling to below-threshold resonances that are otherwise inaccessible.

This research topic focuses on the modelling of physical and chemical processes of astrophysical interest and its applications. In many cases, such as those encountered in the interstellar medium (ISM), local thermodynamic equilibrium is not reached. Atomic and molecular data then reveal crucial to determine the chemical composition and physical conditions of this medium. The first objective of our work is then to obtain accurate data for processes (radiative, collisional and reactive) in which interstellar molecules are involved. We generally use quantum approaches in order to get these data at the low temperatures that characterize the ISM. The second objective of our work consists in the astrophysical modelling and especially in the sensitivity of astrophysical models to molecular data. Using radiative transfer models and the new molecular data we have computed, we aim at studying the excitation of molecules and compare the predicted line intensities by these models with observations in order to determine the physical conditions interstellar molecular clouds.

This astrophysical research activity is related to Paris-Meudon Observatory (LESIA- UMR 8109) and focusses on the internal structure and evolution of stars, and on asteroseismology. The work involves development and exploitation of numerical methods and codes for stellar structure modelling such as Cesam2k, Cestam, MESA, and stellar oscillation codes. Physical processes at work in stellar plasmas are also described and studied. The theoretical models produced are validated by confrontation to observations. The work includes implications and applications for stars and beyond with for example the structure and evolution of the Galaxy and the characterization of exoplanets.

These activities are also strongly related to spatial missions:

• CoRoT/CNES, Kepler/NASA high precision photometric missions
• Gaia/ESA via the Coordination Unit 8 (WP825, FLAME, Final Luminosity Age and Mass Estimator)
• scientific preparation of the PLATO 2.0/ESA mission (stellar seismology and search for exoplanets)

Various theoretical approaches are used to tackle the above mentionned processes. A non-exhaustive list includes close-coupling, spectral element, R-matrix, MCTDH and quantum Monte Carlo approaches in order to solve the Schrödinger equation. We develop numerical methods for the efficient description of the processes to be studied. Associated codes such as COLMAG, DMC, HYP3D, MCTDH, MOLCOL, POITSE, RMF and VR have been produced or modified.

Our main working instruments are pencils, paper, keyboards, workstations and the “Physix” computer cluster which has currently  1500 processors and 300 TeraBytes of storage (end of 2022).

• Javier Aoiz, Madrid, Espagne
• Dionisio Bermejo, Madrid, Espagne
• Roman Ciurylo, Torun, Pologne
• Wolfgang Eisfeld, Bielefeld, Allemagne
• Giovanni Modugno, Florence, Italie
• Thomas Bourdel, Palaiseau, France
• Manuel Lara Garrido, Madrid, Espagne
• Uwe Manthe, Bielefeld, Allemagne
• Xavier Urbain, Louvain-la-Neuve, Belgique
• Robert Zillich, Linz, Autriche

International network: Quantum dynamics networks
International Research Network: IRN MCTDH
National GDR THEMS
National GDR EMIE