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Neutralization and relaxation of cations in an environment driven by interatomic decay processes

Stumpf, Vasili

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Non-local electronic decay mechanisms constitute important pathways for the relaxation of cations produced by the action of ionizing radiation in van-der-Waals or hydrogen bonded chemical environment. Electronically excited cations may undergo the ultrafast Interatomic Coulombic Decay or ICD process, whereby the excess electronic energy is transferred to the environment and used to ionize it. It has been extensively studied by computational and experimental techniques during the last two decades and shown to operate in a variety of systems from rare gas dimers to large biomolecules. In this thesis we investigate using ab initio methods the Electron Transfer Mediated Decay or ETMD process which is responsible for the charge redistribution in environment, whenever atomic cations with a low excess energy and high electron affinity are produced. In ETMD electron transfer to the cation leads to the emission of an electron from the neighboring species. The net result is partial neutralization of the cation and the increase of the charge of the environment by two.

The light rare gas atoms He and Ne have a high ionization potential and, in the presence of a suitable neighbor are likely to undergo ETMD when they are singly ionized, e.g. by photoionization. In particular, we showed that a HeMg cluster efficiently decays by ETMD whenever He is photoionized and a ground state He+ ion is produced. The joint process of photoionization and ETMD corresponds to a one-photon double ionization of Mg. Remarkably, we found that the cross section of this process is three orders of magnitude higher than the cross section of the atomic one-photon double ionization, which demonstrates the prominent role of the neighboring He species in the double ionization. This mechanism of the ETMD driven one-photon double ionization was recently demonstrated experimentally in doped He nanodroplets and is proposed as a method for the experimental production of cold molecular dications.

Multiply charged rare gas cations have higher electron affinities and undergo ETMD with a larger variety of neighboring atoms or molecules. Such cations are naturally produced by the Auger decay following core ionization of rare gases in the X-Ray absorption. The ETMD process reduces their positive charge by one, i.e. leads to their partial neutralization and serves as a purely electronic alternative to neutralization mechanisms driven by the movement of the nuclei. Our calculations show that in small Ne2+Xe and Ne2+Kr2 clusters the ETMD process takes place on a picosecond timescale. The ETMD in these systems is accompanied by nuclear dynamics which in turn enhance the rate of the electronic decay. We show that for such systems ETMD is an important mechanism responsible for the fast redistribution of the localized charge produced in the Auger decay process.

We also demonstrated that multiply charged hydrated metal cations are likely to decay via complicated cascades comprising both ETMD and ICD steps. Our calculations in the Mg2+(H2O)6 microsolvated cluster showed that such a cascade proceeds on a timescale of few hundreds of femtoseconds and leads to a massive degradation of the imetal’s solvation shell through its multiple ionization and emission of slow electrons. Repulsive nuclear dynamics at later stages of the cascade, which were not taken into account explicitly, are expected to considerably reduce its duration. We expect that studying interatomic decay cascades of metal cations is important for understanding mechanisms of the damage caused by X-Rays to metal containing biomolecules such as DNA, metalloproteins etc. For the latter of particular importance is the knowledge of the duration of different interatomic decay steps, since it determines the timescale at which proteins become damaged by X-Rays and beyond which their structure becomes compromised.

These considerations led us to investigate the dependence of ICD lifetimes on atomic charge in excited microhydrated Na2+ and Mg3+ cations. Our ab initio results reproduce within the numerical error the experimental ICD lifetimes of the respective ions in aqueous solutions. We show that the microsolvated Mg3+ cations decay faster than the Na2+ ones, in accordance with experiments on aqueous solutions. The detailed analysis reveals that at characteristic metal-water separations the polarization of the water neighbor enhances ICD the stronger the higher the charge of the metal is. This, together with the shorter Mg-water equilibrium distances, leads to the observed ordering of the ICD rates. We also showed that polarizing the neighbors causes sub-linear growth of ICD rates with the number of water molecules in the first solvation shell. This investigation of ICD in microsolvated metal cations demonstrated the prominent role the cation’s charge and the consequent polarization of the medium have on the decay rate. It also leads to a reasonable expectation that even faster, sub-femtosecond decay lifetimes might be achieved for highly charged solvated metals ions.

Item Type: Dissertation
Supervisor: Cederbaum, Prof. Dr. Lorenz S.
Date of thesis defense: 9 December 2016
Date Deposited: 19 Jan 2017 06:59
Date: 2017
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 000 Generalities, Science
500 Natural sciences and mathematics
530 Physics
540 Chemistry and allied sciences
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