Institut des Sciences Moléculaires d'Orsay



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Home > Research Teams > Dynamics and Interactions: Radiation, Atoms and Molecules (DIRAM) > Photoionisation of atomic and molecular ions

Photoionisation of atomic and molecular ions

Jean-Marc Bizau, Denis Cubaynes, Ségolène Guilbaud

The activity of this group is based on the study of photoionization processes, in the XUV photon energy range, in various ionic species (atomic multiply-charged ions, molecular ions…). The measurements of the physical parameters which characterized these processes (absolute photoionization cross sections, excitation energies…), performed at the SOLEIL synchrotron radiation facility, are compared to the theoretical predictions obtained from various models. This comparison, done in close collaboration with several groups of theoreticians, enables the improvement of the quantum mechanics models developed for the modeling of astrophysical plasmas (interstellar medium, planetary atmospheres…) and laboratory plasmas (laser produced plasmas, tokamak…).

Nébuleuse planétaire
Nébuleuse planétaire

Ninety-nine percent of the matter in the Universe is present in the form of plasma, a mixing of ions, electrons and photons in interaction. The goal of our experimental studies is to obtain accurate laboratory data on the interaction processes between photons and different ionic targets, including multiply-charged atomic ions, negative ions and small molecular ions. Interests for such studies are multiple. Fundamental first, since they allow to describe with great details the electron correlation effects in the atomic and molecular many-body targets. A good knowledge of these effects in simple systems is a prerequisite to the studies of photoionisation processes on more complex targets.

In addition, spectroscopy is the favored tool used to quantify the thermodynamic state and chemical composition of plasma, the so called fourth state of the matter. Laboratory data on multiply-charged ions, encountered in hot astrophysical or laboratory produced plasmas, or small molecular ions participating to the complex chemistry of planetary ionospheres, is essential to interpret the spectra sent by the X-ray observatories. Knowledge of photoabsorption cross sections is also of prime importance to describe radiation transfer in hot and dense plasmas, since photoabsorption is one of the main ways for energy transport in these mediums. This process is important also for inertial confinement fusion, and it plays a role in astrophysical plasmas, controlling, for example, the radiative-gravitational equilibrium in stars.


Our first studies have been performed at the LURE synchrotron radiation facility in Orsay (see the poster “de SILURE à MAIA”) until the shutting down of this laboratory in 2003.


We use now two different experimental setups: a merged beam setup (MAIA for Multi-Analysis Ion Apparatus) and a small FT-ICR (Fourier Transform Ion Cyclotron Resonance) ion trap (MICRA). Both use the synchrotron radiation emitted by the storage ring SOLEIL in St-Aubin. MAIA is permanently installed on one branch of the PLEIADES beam line. It allows the measurement in absolute values of the photoionisation cross sections for various ionic targets. It is optimized now for ionic spectrometry, corresponding to the detection of the ions charge stage after their interaction with the photons. More precise spectroscopic tools, like electron spectroscopy, are now under development.
MICRA can be installed on the DESIRS or PLEIADES beam lines. It allows measurements on cooled down ions to their ground state. Due to its very high mass resolution, it is particularly well adapted to the study of molecular ions.

Spectres de photoionisation de Fe VAs an example, the opposite figure shows photoionisation spectra recorded for Fe V ion. The experimental curve, recorded with the merged beam setup, is given by the black line. It offers a stringent test of the predictions of the models developed for the description of spectral opacities, as the ones show here : the OPAS code for hot and dense plasmas (red curve) and the Opacity Project code for stellar envelops (blue curve).

Available datas

De SILURE à MAIA - 184.9 kb