Simulation

RESEARCH

Hollow molecules probed by single photon double-ionization

In collaboration with F. Penent and P. Lablanquie, we are studying the formation of double core-ionized hollow molecules by a single-photon excitation[7,8] in small molecules (ex : C2H2).
Two different ionization events can be considered: (i) both the inner-shell electrons are ejected from the same carbon atom; (ii) both the inner-shell electrons are each coming from one (different) carbon center of the C2H2 molecule. To support these experimental measurements, we have calculated the core binding energies corresponding to these two different channels, as well as the full core ionized/excited spectra including multi-electron (shake-up) processes. For this, a home code enabling calculations of double-core ionized and core-ionized/core-excited states at a CI(SDTQ) level of theory coupled with a Monte Carlo-Simulated annealing optimization basis set routine* has been written. *Accurate core electron binding energy calculations using small 6-31G and TZV core hole optimized basis sets, Carniato, S., Millié, P., Journal of Chemical Physics, 116, 3521 (2002).
[7] Unveiling residual Molecular Binding in Triply Charged Hydrogen Bromide, Penent F. et al., Phys. Rev. Lett., 106, 103002 (2011) [8] Properties of Hollow molecules probed by Single-Photon double ionization, Lablanquie P. et al., Phys. Rev. Lett., 106, 63003 (2011).

Probing IR-Raman vibrationally excited molecules with X-ray spectroscopies

This topic consists of modelling the combination of IR-Raman laser technique (Raman chirped adiabatic passage, RCAP) with x-ray spectroscopy providing a way for time control bond dissociation of molecules. We have suggested an experimental scheme where selected vibrational states are populated by infrared (IR) laser pulses, and x-ray spectroscopy probes the vibrational state through the evolution of the Cl(1s) x-ray photoelectron spectrum. The advantage of the use of x-rays relies particularly on its site selectivity (chemical shift) which enables detailed studies in specific regions of large molecules. This makes RCAP combined with x-ray spectroscopy a unique opportunity for a time dependent tracking of bond dissociation by step-by-step selective inversion of vibrational level and core-level ionization imaging through modulation of core hole chemical shift with high resolution.
The first calculations performed on a benchmark diatomic molecule (HCl) have shown that the vibrational excitation from the lowest to higher vibrational levels by IR fields not only influences the x-ray ionization energy but the cross-section of each band is also drastically affected making of this technique a very efficient tool to explore the dynamic of bond elongation in excited molecular systems, through the control (ex: time evolution) of the binding energy.

Caption : Calculated XPS spectra for different population of the vibrational levels of the electronic ground state of HCl reached by chirped pulse technique

Intermolecular Coulombic Decay (ICD)

Interatomic (molecular) Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process for excited atoms or molecules embedded in a chemical environment. Via ICD, the excited system can get rid of the excess energy and this excess energy is transferred to one of the neighbors and ionize it. As an example, after inner-valence ionization of an atom in a cluster, this atom can relax by ionizing another unit of the cluster. It should be stressed that whereas the same excited atom when isolated relaxes only by emitting a photon in a time range of picoseconds to nanoseconds, ICD takes place in the femtosecond range. Thus, ICD is generally the most favorable decay process.

The ICD process was predicted in the late 90’s by L. S. Cederbaum et al. and since then it has been intensively studied both theoretically and experimentally. It has been shown that ICD is a general process, taking place in a large variety of systems, like hydrogen bonded [e.g. (H2O)n, (HF)n] or van der Waals [e.g. Ne(n), Ne(n)Ar(m), etc.] clusters. The ICD and related processes have been studied by an experimental group in our institute (Lablanquie, and co-workers). Most recently, the group of Hergenhahn reported the first clear experimental confirmation for the ICD in water clusters and Dörner et al. have performed a coincidence measurements of ICD in water dimers.

In this context, we develop ab-initio methods to give a full description of the ICD processes. This includes demanding calculations of the decay rates and quantum treatment of the nuclear dynamics during and after the electronic decay processes.

Caption : scheme of the ICD process in Ne dimer


Dernière modification : January 12 2015 17:27:04.