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.
[1] D. Saxey, Ultramicroscopy 111, (2011) p. 473.
[2] D. Zanuttini, et al., Phys. Rev. A 95, (2017) p. 061401(R).
[3] D. Zanuttini, et al., J. Chem. Phys. 147, (2017) p. 164301.
