Highly sensitive method for detecting ion pairs in aqueous solution developed

Scientists of the Helmholtz-Zentrum Berlin, Freie Universität Berlin, Universität Heidelberg, and the University of Chemistry and Technology Prague have empirically detected a very specialised type of electron transfer in an aqueous salt solution, one which had only been predicted theoretically up to now. Based on these results, they now expect to have an extremely sensitive method for detecting ion pairs in solutions.

The scientists were successful in acquiring empirical evidence for what is referred to as electron-transfer-mediated decay (ETMD). “ETMD is a decay channel that becomes operative when a hole in the backbone of a molecule is filled by an electron from a neighbouring molecule. The energy released by this process is then utilized for ionisation of this or an additional neighbouring molecule”, explains Prof. Emad Flear Aziz.

“The decay is non-local and therefore is in competition with the much more frequently occurring processes of Auger emission and intermolecular Coulomb decay (ICD)”, explains co-author Dr. Robert Seidel. An electron hole in both these processes is filled by an electron from within their own respective molecules. The ETMD process was predicted earlier in 2001 and first detected in clusters of gas in 2011, according to the physicist.

The group utilised lithium chloride salt in an aqueous solution to detect the ETMD process, since neither Auger emission nor ICD decay are believed to occur with lithium ions in water. In this way, they increased the probability of the ETMD process and its detection.

The measurements took place at the Helmholtz-Zentrum BESSY II synchrotron in Berlin using the LiquidJet PES facility there. The lithium chloride solution to be investigated was injected as an extremely fine stream of liquid into a vacuum chamber and analysed with soft X-ray emissions.

“Since the strength of the ETMD process is strongly influenced by the separation between the host and neighbouring molecules, statements about the ion pairing can be made from the distribution and intensity of the ETMD spectrum”, explains Prof. Aziz. This means that ETMD puts a spectroscopic tool in the hands of scientists with which they can ascertain the thickness of a solvent envelope immediately surrounding an ion in an aqueous solution. The results of the study have been published in the renowned peer-reviewed journal Nature Chemistry.

The publication: Observation of electron-transfer-mediated decay in aqueous solution

Isaak Unger, Robert Seidel, Stephan Thürmer, Marvin N. Pohl, Emad F. Aziz, Lorenz S. Cederbaum, Eva Muchová, Petr Slavíček, Bernd Winter, and Nikolai V. Kryzhevoin.

Nature Chemistry (2017). DOI: 10.1038/nchem.2727