ICD/Auger

We develop methods and associated numerical tools for the theoretical description of the electronic and nuclear dynamics during ultrafast non-radiative electronic decay processes, such as Auger decay and Interatomic Coulombic Decay (ICD).

 

Scheme of Auger decay in neon

Fig 1. Scheme of Auger decay in Neon

Fig 1. Scheme of ICD in Neon dimer

 

The objective of this development is twofold:

 

- first, we aim at describing ab initio these processes in more and more complex systems. General quantum mechanical equations for describing these processes are known and have been successfully used for atomic and diatomic systems. However, a full quantum approach becomes unfeasible for larger systems and one has to search for an approximate method.

The first issue in modeling Auger and ICD processes in complex systems is the simulation of multi-dimensional nuclear dynamics. In order to treat polyatomic systems, we have developed a semiclassical method, inspired by the surface hopping approach of J. Tully widely employed in non-adiabatic dynamics. This method has proven to provide quantitative estimates of the main experimental observables and allows to treat systems with several thousands of atoms.

The second issue is the computations of energetics (potential energy surfaces) and rates. In particular, the rate is an important property to characterize the resonance effect but its accurate determination is very challenging. In our group we have developed and benchmarked two methods to compute the rates of complex systems. The first one relies on the Diatomic-In-Molecules approximation and is specifically designed for treating large rare-gas clusters. This method is currently used to investigate ICD in helium clusters. The aim of this study is to evaluate the analytical power of ICD to predict the size of helium nanodroplets. The latter are employed as nanolaboratories. Our project should help to achieve a better control of these systems. The second method is based on the Fano description of a resonance as “a discrete state embedded in a continuum”. A Configuration Interaction (CI) method is used to describe the discrete and continuum part of the resonance. This method has been successfully benchmarked on simple atomic and molecular systems and can be used to investigate medium-size systems.

 

-second, the tools developed in the group have been used to help the interpretation of several experiments. In this context, we have continuous collaborations with a group within the LCPMR and with the German group lead by Pr. Reinhard Dörner. As an example of these works, we have investigated the ultrafast dissociation dynamics in the Auger cascade of HCl after K-shell excitation. Our calculations helped the interpretation of the measured Auger spectra. This combined experimental-theoretical work has demonstrated that multi-step dissociation of a molecule can take place within a few femtoseconds.

 

More on 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.

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.