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Interatomic Coulombic Decay in nanodroplets: towards a novel spectroscopy

Periodic Reporting for period 1 - ICDSpec (Interatomic Coulombic Decay in nanodroplets: towards a novel spectroscopy)

Reporting period: 2016-05-01 to 2018-04-30

• What is the problem/issue being addressed?
Currently, the structure of nanodroplets is probed using mass spectrometry. Droplets are ionized (usually by electron impact) and undergo fragmentation. However, the fragmentation dynamics is fairly complex and determining the structure of the droplets from the mass spectrum is not straightforward. Several theoretical works have focused on the structure and the dynamics of pure and doped droplets, either in the description of the droplets formation and pick-up processes or on understanding the fragmentation dynamics. An obstacle in modeling the latter is that the number of charges and the distances between them are not known. In the case of multiple ionization the explosion of the droplets depends strongly on the distance between the charges. Models have thus to rely on some kind of statistical averaging over the distances. Even in the case of singly-ionized droplets, simulating the fragmentation dynamics is difficult since important quantum effects are expected. A better control of the number of charges and their location is therefore desirable. One of the goals of this project is to investigate Interatomic Coulombic Decay (ICD) as means to control the charges in nanodroplets.

• Why is it important for society?
The project is primarily of fundamental importance and no direct impact for society is expected. However, helium nanodroplets are widely used in many applications. Our project establishes a new spectroscopic tool to measure the size of nanodroplets. A better characterization of the droplets contributes to the progress in these applications.

• What are the overall objectives?
The aim of the project is to assess the applicability of Interatomic Coulombic decay as a spectroscopic tool for probing the structure of nanodroplets. To answer this question, the electronic decay and subsequent fragmentation dynamics of pure helium nanodroplets of different sizes have been simulated from first principles. From these simulations, the kinetic-energy distributions (KER spectra) of the ionic fragments, which are the main observables, are obtained and compared with available experimental data in order to establish a relationship between the observables and the size of the initial droplets. It is expected that this project provides enough detailed information to propose a new spectroscopic tool for characterizing nanodroplets. Moreover, this investigation will be a first step towards the study of doped nanodroplets. ICD spectroscopy may help to determine the number of the dopants as well as their location and thus lead to a better control of the isolation technique.
A set of methods and numerical tools were developed during the project, which are available in the webpage of the project. Furthermore, the results of the ICDSpect project demonstrate that ICD can be used to probe the size of helium clusters Hen for n = 2 - 112. For n = 2 - 7 we show that the kinetic energy distribution of the ions produced in the ICD process ions are characteristic of the size of the cluster. For larger clusters, our work shows that the photoelectron spectra are a powerful probe of cluster size. Nanodroplets have been partially studied and they are still under investigation.
Our project has provided the first complete theoretical description of ICD in polyatomic systems. The methods and codes developed are currently the state-of-the-art in describing ICD. The most significant results of the project are the following: the theoretical KER spectra of small clusters showed a hitherto unexplored feature the presence of ions of low kinetic energy. This led to the discovery of a new process following ICD, namely charge transfer after the Coulomb explosion of the ions, which we termed frustrated Coulomb explosion. This process gave us yet another tool to determine the cluster size from the KER spectra. Additionally, we discovered a novel ICD mechanism, termed superexchange ICD, which occurs in the presence of ICD-inactive neighbors. This process was reported in Phys. Rev. Lett. 119, 083403 (2017) and has evoked broad scientific interest which led to two new collaborations with the group of Dr. Kolorenč and Dr. Buhmann.