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

Abstract

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. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. For Mg clusters larger than five atoms we observe stable doubly ionized clusters. Thus, ETMD provides an efficient pathway to the formation of doubly ionized cold species in doped nanodroplets.

Electron transfer mediated decay of alkali dimers attached to He nanodroplets

“…The low energy peaks (2 − 9 eV) in the electron spectrum of figure 1 (a) are indicative for ETMD of alkali metal molecules attached to He droplet. For He doped with Mg metal clusters, we had previously observed a low energy peak in the electron spectrum at about 1 eV, which was due to double ionization of the dopants by ETMD [15]. In contrast, the low kinetic energy features seen here (figure 1 a)) are at higher energies due to the lower ionization potentials, , of alkali metals.…”

“…ETMD, by contrast, proceeds by electron transfer from a donor to the initially created ion, such that the released energy leads to emission of a second electron either from the donor, ETMD(2), or from a second neighboring atom or molecule, ETMD(3), [5,11,12,13]. It was shown to be the leading decay pathway if ICD is energetically forbidden [3,14,15]. In contrast to ICD where the final charge state of the electronically excited species remains constant, in an ETMD step its charge decreases by one electron charge.…”

“…Helium (He) has a simple electronic structure and the highest ionization energy amongst all elements. Therefore, pure or doped He nanodroplets offer a unique medium where interatomic/molecular decays can be studied [14,15,[20][21][22][23][24][25][26][27][28][29]. Owing to its chemical inertness and low temperature, attached dopant molecules are only weakly perturbed and tend to aggregate into weakly bound complexes inside the droplet or at the droplet surface.…”

“…Owing to its chemical inertness and low temperature, attached dopant molecules are only weakly perturbed and tend to aggregate into weakly bound complexes inside the droplet or at the droplet surface. Recently, it was found that ETMD between a He + or a He 2 + and a dopant attached to the droplet leads to the double ionization of the dopant [14,15]. In particular, experiments in He nanodroplets doped with magnesium (Mg) clusters demonstrated that such a single photon double ionization process mediated by ETMD is highly efficient [15].…”

“…Recently, it was found that ETMD between a He + or a He 2 + and a dopant attached to the droplet leads to the double ionization of the dopant [14,15]. In particular, experiments in He nanodroplets doped with magnesium (Mg) clusters demonstrated that such a single photon double ionization process mediated by ETMD is highly efficient [15]. Unlike Mg clusters which are embedded inside the droplet, alkali dimers are adsorbed on its surface [25].…”