Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay


As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly bound complexes such as He droplets offer an environment where local excitations can interact with neighbouring embedded molecules leading to new intermolecular relaxation mechanisms. Here, we report on a new decay mechanism leading to the double ionization of alkali dimers attached to He droplets by intermolecular energy transfer. From the electron spectra, the process is similar to the well-known shake-off mechanism observed in double Auger decay and single-photon double ionization1,2, however, in this case, the process is dominant, occurring with efficiencies equal to, or greater than, single ionization by energy transfer. Although an alkali dimer attached to a He droplet is a model case, the decay mechanism is relevant for any system where the excitation energy of one constituent exceeds the double ionization potential of another neighbouring molecule. The process is, in particular, relevant for biological systems, where radicals and slow electrons are known to cause radiation damage3.

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Fig. 1: Coincidence spectra for discriminating possible involved decay mechanisms applied while measuring the energetics of the constituent ions and electrons. The ionization process is triggered by energy transfer from the excited 1s2s1S He atom (Ee = 20.6 eV).
Fig. 2: The potential energy curve of K–Rb dimers in the ground (black) and dicationic (red) state.
Fig. 3: Electron kinetic energy distributions from the ionization of small, homogeneous clusters of alkali metals attached to the surface of a He droplet.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on request.


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This work was financially supported by the Carl-Zeiss-Stiftung and the Deutsche Forschungsgemeinschaft (project MU 2347/10-1). The authors thank L. Cederbaum, K. Gokhberg, N. Kryzhevoi and N. Berrah for stimulating discussions.

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A.C.L. and M.M. conceived the experiment. A.C.L., M.S. and R.R. conducted the experiment. A.C.L., M.S. and M.M. analysed the data. M.M. performed the FCF calculations. A.C.L. interpreted the data with help from R.R., F.S., R.M., T.P. and M.M. A.C.L. wrote the paper. All authors reviewed the manuscript.

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Correspondence to A. C. LaForge.

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LaForge, A.C., Shcherbinin, M., Stienkemeier, F. et al. Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay. Nat. Phys. 15, 247–250 (2019). https://doi.org/10.1038/s41567-018-0376-5

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