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Accueil > Productions scientifiques > Séminaires à PHENIX > 2019 > Séminaire Benoît Gervais (CIMAP/GANIL, Caen) - 11/10/2019 à 11h

Séminaire Benoît Gervais (CIMAP/GANIL, Caen) - 11/10/2019 à 11h

par Pierre Illien - 26 août 2019

Séminaire de Benoit Gervais (CIMAP, Caen) :

Molecular dication stability observed in tomographic atom probe

le vendredi 11 octobre 2019 à 11h, salle des conseils de l’UFR de Chimie (32-42.101).

Résumé

Molecular dication stability observed in tomographic atom probe

Benoit Gervais

CIMAP, Caen

Tomographic Atom Probe (TAP) is a unique experimental apparatus
dedicated to 3D chemical analysis of materials. Like the field ion
microscopes (FIM), it is based on field emission principle combined
with a time-of-flight and localization detection. Its operation
consists in the preparation of the analyzed sample to give it a needle
shape with a curvature of the order of a few tens of nanometers at the
apex. Applying an electric potential difference of the order of 10 kV
between the sample and the detector generates thus a large electric
field of a few V/nm at the apex. Such a field is comparable to the
electric field experienced by valence electrons in a molecule. As a
result, the material is brought close to its limit of stability and a
small additional perturbation, often generated with a short-pulse
laser, triggers the emission of ions from the surface. Though the
details of emission process are not clear, it is used to analyze
materials by reconstruction of the ion trajectories with a nanometer
resolution.

The emission of single atomic ions in Tomographic Atom Probe (TAP)
experiments is often associated to correlated emission leading to
multiple events, which can be observed in a correlated event map,
where the mass-over-charge ratio of the second fragment is plotted
versus the massover-charge ratio of the first fragment, as illustrated
below. In such a map, some specific features appear as correlation
tracks, which have been explained as a result of the in-flight
dissociation of aparent molecular ion into charged fragments [1].

After an introduction of TAP principles, I will present a theoretical
analysis of the fragmentation of dicationic dimers for materials
emitted from corresponding metal-oxide or metalnitride alloys [2] [3]
Our analysis is based on the simulation of the ion dynamics in the
field of a paraboloid tip for a model internal energy of the molecule
deduced from accurate ab initio calculation of the electronic
structure. We show that the spin-orbit coupling between electronic
states of different spin symmetries is at the origin of the in-flight
dissociation. We shall see however, that the specificity of each
molecular dication, and in particularits electronic structure, is
essential to analyze the experimental observations, because it
conditions the dissociation dynamics and the nature of the
dissociating products.


[1D. Saxey, Ultramicroscopy 111, (2011) p. 473.

[2D. Zanuttini, et al., Phys. Rev. A 95, (2017) p. 061401(R).

[3D. Zanuttini, et al., J. Chem. Phys. 147, (2017) p. 164301.