Enhanced Ionization of Embedded Clusters by Electron-Transfer-Mediated Decay in Helium Nanodroplets

 We report the observation of electron-transfer-mediated decay (ETMD) involving magnesium (Mg) clusters embedded in helium (He) nanodroplets.. A. C. LaForge et al.; PRL 116, 203001 (2016)

 We report the observation of electron-transfer-mediated decay (ETMD) involving magnesium (Mg) clusters embedded in helium (He) nanodroplets. ETMD is initiated by the ionization of He followed by removal of two electrons from the Mg clusters of which one is transferred to the He ion while the other electron is emitted into the continuum.
Electron-transfer-mediated decay (ETMD), theoretically predicted and recently experimentally observed, is an interatomic decay mechanism accessible upon ionization in weakly bound systems. Recently, Stumpf et al. [Phys. Rev. Lett. 112, 193001 (2014)] predicted that ETMD dramatically enhances (by ∼3 orders of magnitude) the single photon double ionization of a magnesium (Mg) atom in the vicinity of a He atom. ETMD proceeds by the initial ionization of a He atom followed by ETMD of the neighboring Mg atom yielding Mg2+ and neutral He. The figure shows the schematic of double ionization of species S directly (left pathway) and via ETMD from a neighboring ion N (right pathway).  Surprisingly, due to ETMD, the theoretical cross section for double ionization of Mg is even higher than that of direct single ionization and is comparable to that of He.


As a model system, we performed a set of experiments on Mg clusters embedded in He nanodroplets and indeed observed a high ionization efficiency of Mg as well as doubly ionized Mg clusters due to ETMD. In the experiment the photon energy was tuned from 20 to 26 eV, which covers energies below the lowest dipole-allowed excitation energy (21.2 eV) to above the atomic ionization threshold (24.6 eV) of He. There are several mechanisms leading to single ionization of Mg atoms and clusters embedded in He droplets. In addition to direct single and double ionization the He environment opens new pathways. When the droplet is resonantly excited ionization proceeds through Penning or ICD processes. When the droplet is ionized, electron transfer can ionize the dopant. Below 21 eV photon energy no signal was observed for the Mg+ ion, indicating that direct ionization is negligible.At photon energies above 23 eV the Mg ion signals closely follow the He2+ ion signal pointing to He-mediated ionization of Mg. We observe the peaks of several Mg oligomers including doubly ionized Mg clusters and cluster fragments in coincidence, a signature of ETMD, in the mass spectra. The analysis of electron spectra correlated with selected ions and ion-ion coincidences confirm the interpretation.


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Enhanced Ionization of embedded clusters by electron-transfer mediated decay in helium nanodroplets
A. C. La Forge, V. Stumpf, K. Gokhberg, J. von vangerow, F. Stienkemeir, N. V. Kryhevoi, P. O'Keeffe, A. Ciavardini, S. R. Krishnan, M. Coreno, K. C. Prince, R. Richter, R. Moshammer, T. Pfeifer, S. Cederbaum, M. Mudrich; Phys. Rev. Lett. 116 (2016) 203001


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