David Zanuttini

Spectroscopic properties of alkali atoms embedded in Ar matrix.

We present a theoretical investigation of visible absorption and related luminescence of alkali atoms (Li, Na, and K) embedded in Ar matrix. We used a model based on core polarization pseudopotentials, which allows us to determine accurately the gas-to-matrix shifts of various trapping sites. The remarkable agreement between our calculated results and the experimental spectra recorded by several authors allows us to establish a clear assignment of the observed spectra, which are made of contributions from crystalline sites on the one hand, and of grain boundary sites on the other hand. Our study reveals remarkably large Stokes shifts, up to 9000 cm(-1), which could be observed experimentally to identify definitely the trapping sites.

An accurate model potential for alkali neon systems

We present a detailed investigation of the ground and lowest excited states of M-Ne dimers, for M=Li, Na, and K. We show that the potential energy curves of these Van der Waals dimers can be obtained accurately by considering the alkali neon systems as one-electron systems. Following previous authors, the model describes the evolution of the alkali valence electron in the combined potentials of the alkali and neon cores by means of core polarization pseudopotentials. The key parameter for an accurate model is the M(+)-Ne potential energy curve, which was obtained by means of ab initio CCSD(T) calculation using a large basis set. For each MNe dimer, a systematic comparison with ab initio computation of the potential energy curve for the X, A, and B states shows the remarkable accuracy of the model. The vibrational analysis and the comparison with existing experimental data strengthens this conclusion and allows for a precise assignment of the vibrational levels.

Solvation of Na2+ in Arn clusters. I. Structures and spectroscopic properties

We present a theoretical study of Na(2) (+) solvation in an argon matrix Ar(n) for n=1 to a few tens. We use a model based on an explicit description of valence electron interaction with Na(+) and Ar cores by means of core polarization pseudopotential. The electronic structure determination is thus reduced to a one-electron problem, which can be handled efficiently. We investigate the ground state geometry and related optical absorption of Na(2) (+)Ar(n) clusters. For n<or=5, the lowest energy isomers are obtained by aggregation of Ar atoms at one single extremity of Na(2) (+), leading to moderate perturbation of the optical transition. For 6<or=n<or=15, the Ar atoms aggregate at both extremities. This structural change is associated with a strong blueshift of the first optical transition (X (2)Sigma(g) (+)-->A (2)Sigma(u) (+)), which reveals the confinement of the excited A (2)Sigma(u) (+) state. The Na(2) (+) energy spectrum is so strongly perturbed that the A (2)Sigma(u) (+) state becomes higher than the B (2)Pi(u) (+) states. The closure of the first solvation shell is observed at n=17. Above this size, the second solvation shell develops. Its structure is dominated by a pentagonal organization around the Na(2) (+) molecular axis. The optical transitions vary smoothly with n and the A (2)Sigma(u) (+) and B (2)Pi(u) states are no longer inverted, though the first optical transition remains strongly blueshifted.

Structure and photoabsorption properties of cationic alkali dimers solvated in neon clusters

We present a theoretical investigation of the structure and optical absorption of M(2)(+) alkali dimers (M=Li,Na,K) solvated in Ne(n) clusters for n=1 to a few tens Ne atoms. For all these alkali, the lowest-energy isomers are obtained by aggregation of the first Ne atoms at the extremity of the alkali molecule. This particular geometry, common to other M(2)(+)-rare gas clusters, is intimately related to the shape of the electronic density of the X  (2)Σ(g)(+) ground state of the bare M(2)(+) molecules. The structure of the first solvation shell presents equilateral Ne(3) and capped pentagonal Ne(6) motifs, which are characteristic of pure rare gas clusters. The size and geometry of the complete solvation shell depend on the alkali and were obtained at n=22 with a D(4h) symmetry for Li and at n=27 with a D(5h) symmetry for Na. For K, our study suggests that the closure of the first solvation shell occurs well beyond n=36. We show that the atomic arrangement of these clusters has a profound influence on their optical absorption spectrum. In particular, the XΣ transition from the X  (2)Σ(g)(+) ground state to the first excited (2)Σ(u)(+) state is strongly blueshifted in the Frank-Condon area.

Nonadiabatic molecular dynamics of photoexcited Li 2+ Ne n clusters

We investigate the relaxation of photoexcited Li(2)(+) chromophores solvated in Ne(n) clusters (n = 2-22) by means of molecular dynamics with surface hopping. The simplicity of the electronic structure of these ideal systems is exploited to design an accurate and computationally efficient model. These systems present two series of conical intersections between the states correlated with the Li+Li(2s) and Li+Li(2p) dissociation limits of the Li(2)(+) molecule. Frank-Condon transition from the ground state to one of the three lowest excited states, hereafter indexed by ascending energy from 1 to 3, quickly drives the system toward the first series of conical intersections, which have a tremendous influence on the issue of the dynamics. The states 1 and 2, which originate in the Frank-Condon area from the degenerated nondissociative 1(2)Π(u) states of the bare Li(2)(+) molecule, relax mainly to Li+Li(2s) with a complete atomization of the clusters in the whole range of size n investigated here. The third state, which originates in the Frank-Condon area from the dissociative 1(2)Σ(u)(+) state of the bare Li(2)(+) molecule, exhibits a richer relaxation dynamics. Contrary to intuition, excitation into state 3 leads to less molecular dissociation, though the amount of energy deposited in the cluster by the excitation process is larger than for excitation into state 1 and 2. This extra amount of energy allows the system to reach the second series of conical intersections so that approximately 20% of the clusters are stabilized in the 2(2)Σ(g)(+) state potential well for cluster sizes n larger than 6.

Electronic structure and stability of the SiO2+ dications produced in tomographic atom probe experiments

The molecular electronic states of the SiO2+ dication have been investigated in a joint theoretical and experimental analysis. The use of a tip-shaped sample for tomographic atom probe analysis offers the unique opportunity to produce and to analyze the lifetime of some excited states of this dication. The perturbation brought by the large electric field of the polarized tip along the ion trajectory is analyzed by means of molecular dynamics simulation. For the typical electric fields used in the experiment, the lowest energy triplet states spontaneously dissociate, while the lowest energy singlet states do not. We show that the emission process leads to the formation of some excited singlet state, which dissociates by means of spin-orbit coupling with lower-energy triplet states to produce specific patterns associated with Si+ + O+ and Si2+ + O dissociation channels. These patterns are recorded and observed experimentally in a correlated time-of-flight map.

Dissociation of GaN2+ and AlN2+ in APT: Electronic structure and stability in strong DC field

We investigate from a theoretical point of view the stability of AlN2+ and GaN2+ dications produced under high static electric fields like those reached in Atom Probe Tomography (APT) experiments. By means of quantum chemical calculations of the electronic structure of these molecules, we show that their stability is governed by two independent processes. On the one hand, the spin-orbit coupling allows some molecular excited states to dissociate by inter-system crossing. On the other hand, the action of the electric field lowers the potential energy barrier, which ensures the dication stability in standard conditions. We present a detailed example of field emission dynamics in the specific case of the 11Δ states for a parabolic tip, which captures the essentials of the process by means of a simplified model. We show that the dissociation dynamics of AlN2+ and GaN2+ is completely different despite the strong resemblance of their electronic structure.

Dissociation of GaN2+ and AlN2+ in APT: Analysis of experimental measurements

The use of a tip-shaped sample for the atom probe tomography technique offers the unique opportunity to analyze the dynamics of molecular ions in strong DC fields. We investigate here the stability of AlN2+ and GaN2+ dications emitted from an Al0.25Ga0.75N sample in a joint theoretical and experimental study. Despite the strong chemical resemblance of these two molecules, we observe only stable AlN2+, while GaN2+ can only be observed as a transient species. We simulate the emission dynamics of these ions on field-perturbed potential energy surfaces obtained from quantum chemical calculations. We show that the dissociation is governed by two independent processes. For all bound states, a mechanical dissociation is induced by the distortion of the potential energy surface in the close vicinity of the emitting tip. In the specific case of GaN2+, the relatively small electric dipole of the dication in its ground 13Σ- and excited 11Δ states induces a weak coupling with the electric field so that the mechanical dissociation into Ga+ + N+ lasts for sufficient time to be observed. By contrast, the AlN2+ mechanical dissociation leads to Al2+ + N which cannot be observed as a correlated event. For some deeply bound singlet excited states, the spin-orbit coupling with lower energy triplet states gives another chance of dissociation by system inter-system crossing with specific patterns observed experimentally in a correlated time of flight map.

Spin-orbit coupling in the dissociative excitation of alkali atoms at the surface of rare gas clusters: A theoretical study

We analyze the role of the spin-orbit (SO) coupling in the dissociative dynamics of excited alkali atoms at the surface of small rare gas clusters. The electronic structure of the whole system is deduced from a one-electron model based on core polarization pseudo-potentials. It allows us to obtain in the same footing the energy, forces, and non-adiabatic couplings used to simulate the dynamics by means of a surface hopping method. The fine structure state population is analyzed by considering the relative magnitude of the SO coupling ξ, with respect to the spin-free potential energy. We identify three regimes of ξ-values leading to different evolution of adiabatic state population after excitation of the system in the uppermost state of the lowest np (2)P shell. For sufficiently small ξ, the final population of the J=12 atomic states, P12, grows up linearly from P12=13 at ξ = 0 after a diabatic dynamics. For large values of ξ, we observe a rather adiabatic dynamics with P12 decreasing as ξ increases. For intermediate values of ξ, the coupling is extremely efficient and a complete transfer of population is observed for the set of parameters associated to NaAr3 and NaAr4 clusters.

Reconstructing APT Datasets: Challenging the Limits of the Possible

Atom probe tomography (APT) has been successfully used in materials science for several decades for probing local compositional variations. In metals, the capacity to measure reliable composition at the nanoscale in 3D was successfully demonstrated in the early nineties [I, 2]. By using a few geometric ingredients, a reconstruction recipe was proposed at this time that surprisingly produced on selected materials, 3D images with an atomic scale precision of the distribution of atoms with elemental identification [3). The ability to localize precisely the 3-D coordinates of individual atoms in direct space of materials is expected to find important applications in materials science and engineering, nanoscience, physics and chemistry. It is particularly crucial in semiconductor industry, to design optical devices or electronic devices, such as light-emitting diodes (LEDs), or transistors. This ideal picture of near-perfect 3D microscope however faces two issues. First, the precision of images may vary strongly from one material to another. Compared to an optical or an electron microscope, the spatial resolution loss is not diffraction limited, since image is produced from the inverse projection of ion trajectories, ions been produced by field evaporation of surface atoms. Precision is perturbed by slight deviations of the trajectories of ions from their initial positions at the specimen surace to the ion detector. A real understanding of this trajectory taking into account first the quantum interaction of atoms/ions with the specimen surface under the presence of the strong electric field existing on the surface is mandatory. The interplay between quantum processes, and more classical deviations induced by the distribution of the electric field close to the surface is extremely complex. In metals, most of the deviations thought to be induced by the field, as it was recently demonstrated experimentally by field ion microscopy in tungsten (fig. I). In oxides, quantum effects such as in-flight dissociation of molecules may degrade significantly the spatial precision (fig. 2) [4]. The second issue concerns the global distortions induced by the presence in the sample of phases very different in term of the critical fields required to extract surface atoms. This is a current case when analyzing devices composed of complex structures of metals, oxides and semiconductors materials. The surface of the sample ideally modelled with a constant curvature radius evolves dynamically under the process of field evaporation in a complex 3D surface. The direct consequence is the presence of distortions in the projected image. After the evaporation of each atom, the field distribution around the sample must be evaluated, to understand the projection law that should necessary be taken into account to generate an accurate reconstruction [5, 6). To overcome this cumbersome calculation, an alternative model, much simpler, was developed to reproduce the imaging process in simple configurations. In multilayers systems, it was demonstrated that most of the distortions could be reduced using a fast and efficient reconstruction algorithm that predicts the analytical shape of the emitting surface all along the evaporation process [7]. If a complementary technique can be used on the same sample to verify the morphology or provide complementary information before APT analysis, then this can also greatly…