Interatomic Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process wherein an excited atom transfers its excess energy to a neighboring species leading to the ionization of the latter. In helium clusters, ICD can take place, for example, after simultaneous ionization and excitation of one helium atom within the cluster. After ICD, two helium ions are created and the system undergoes a Coulomb explosion. In this work, we investigate theoretically ICD in small helium clusters containing between two and seven atoms and compare our findings to two sets of coincidence measurements on clusters of different mean sizes. We provide a prediction on the lifetime of the excited dimer and show that ICD is faster for larger clusters. This is due to (i) the increased number of neighboring atoms (and therefore the number of decay channels) and (ii) the substantial decrease of the interatomic distances. In order to provide more details on the decay dynamics, we report on the kinetic-energy distributions of the helium ions. These distributions clearly show that the ions may undergo charge exchange with the neutral atoms within the cluster, such process is known as frustrated Coulomb explosion. The probability for these charge-exchange processes increases with the size of the clusters and is reflected in our calculated and measured kinetic-energy distributions. These distributions are therefore characteristics of the size distribution of small helium clusters.

Interatomic Coulombic decay (ICD) is an ultrafast energy transfer process. Via ICD, an excited atom can transfer its excess energy to a neighboring atom which is thus ionized. On the example of the NeHeNe cluster, we recently reported [Phys. Rev. Lett. 119, 083403 (2017)] that the total ICD widths are substantially enhanced in the presence of an ICD inactive atom. The enhancement occurs due to the coupling of the resonance state to intermediate virtual states of the bridge atom-a mechanism named superexchange ICD. In this followup work, we analyze the partial ICD widths in the NeHeNe cluster and show that only some channels are affected by the superexchange ICD process. Furthermore, we consider superexchange ICD in NeHeAr. We show that in this system the enhancement is still present but the energy transfer mediated by the superexchange mechanism is less efficient than in NeHeNe owing to the different ionization potentials of Ar and Ne. The behavior of the computed ICD widths is explained with a simple model based on first-order perturbation theory and a Hartree-Fock-like description of the states.

Inner-valence ionized states of atoms and molecules live shorter if these species are embedded in an environment due to the possibility for ultrafast deexcitation known as interatomic Coulombic decay (ICD). In this Letter we show that the lifetime of these ICD active states decreases further when a bridge atom is in proximity to the two interacting monomers. This novel mechanism, termed superexchange ICD, is an electronic correlation effect driven by the efficient energy transfer via virtual states of the bridge atom. The superexchange ICD is discussed in detail on the example of the NeHeNe trimer. We demonstrate that the decay width of the Ne^{+}(2s^{-1})  ^{2}Σ_{g}^{+} resonance increases 6 times in the presence of the He atom at a distance of 4 Å between the two Ne atoms. Using a simple model, we provide a qualitative explanation of the superexchange ICD and we derive analytical expressions for the dependence of the decay width on the distance between the neon atoms.

Interatomic Coulombic Decay (ICD) is a general mechanism in which an excited atom can transfer its excess energy to a neighbor which is thus ionized. ICD belongs to the family of Feshbach resonance processes, and, as such, states undergoing ICD are characterized by their energy width. In this work, we investigate the computations of ICD widths using the R-matrix method as implemented in the UKRmol package. Helium dimer is used here as a benchmark system. The results are compared with those obtained with the well established Fano-Algebraic Diagrammatic Construction method. It is shown that the R-matrix method in its present implementation provides accurate total and partial widths if the kinetic energy of the ICD electron is lower than 10 eV. Advantages and limitations of the R-matrix method on the computations of ICD widths are discussed.

In this paper we present an ab initio approach to the computation of decay widths of Fano resonances. The method relies on Fano theory, in which a resonance is described as a bound state embedded in and interacting with a continuum of states. In our approach, we use the Configuration Interaction (CI) method to describe the bound-like and continuum-like parts of the resonance wave function. The aim of this Fano-CI method is to provide decay widths of resonances at a low computational cost such that large systems can be treated. Along with the implementation of the method, we present benchmark calculations of decay widths of Auger and ICD processes in Ne atom, and Ne2 and NeAr dimers. Our results are in good agreement with the decay widths from other theoretical and experimental works. This makes the Fano-CI approach a promising method for the treatment of Fano resonances.

Submitted for the DAMOP17 Meeting of The American Physical Society On the computations of decay widths of Fano resonances1 TSVETA MITEVA, SÉVAN KAZANDJIAN, NICOLAS SISOURAT, Université Pierre et Marie Curie, Paris — We present a novel approach to the calculation of decay widths of Fano resonances in singly-ionized systems. In our approach, the bound part of the resonance is approximated at the zeroth order as a one-hole configuration. The final states of the decay are obtained after diagonalization of the Hamiltonian matrix in the space of all two-hole-one-particle (2h1p) configurations with a fixed virtual orbital. The Fano-CI method can be applied to the computation of both total and partial decay widths. Furthermore, it has fairly low computational costs and can thus be employed for investigating medium-sized atomic and molecular systems. To check the validity of our method, we carried out benchmark calculations of Auger and ICD widths of small rare-gas and hydrogen-bonded clusters. Comparison with available theoretical and experimental data shows that a satisfactory estimate of the decay width can be achieved with a relatively small basis set, which is of importance for the application of the method to larger systems. [1] [1] T. Miteva, S. Kazandjian and N. Sisourat, accepted in Chem. Phys. (2016) 1European Unions Horizon 2020 research and innovation programme Grant agreement No 705515 Tsveta Miteva Université Pierre et Marie Curie, Paris Date submitted: 24 Jan 2017 Electronic form version 1.4

We report a new method to compute the Interatomic Coulombic Decay (ICD) widths for large clusters which relies on the combination of the projection-operator formalism of scattering theory and the diatomics-in-molecules approach. The total and partial ICD widths of a cluster are computed from the energies and coupling matrix elements of the atomic and diatomic fragments of the system. The method is applied to the helium trimer and the results are compared to fully ab initio widths. A good agreement between the two sets of data is shown. Limitations of the present method are also discussed.

Benzene dimer is a prototype to study intermolecular interactions between aromatic systems. Owing to the weak interactions between the molecules within the dimer, several conformational geometries are nearly isoenergetic and thus coexist even at low temperatures. Furthermore, standard spectroscopies are unable to distinguish between them. In this work, we study the electronic relaxation processes following inner-valence ionization of benzene and the lowest conformers of benzene dimer. We show that the kinetic energy distributions of the secondary electrons emitted via two autoionization mechanisms, namely, the Auger and the intermolecular coulombic decay (ICD) effects, provide a means to probe the conformers of benzene dimer. The proposed spectroscopy opens the way to a better characterization of weakly bound molecular clusters.