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. 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.

Figure 1

Ion signal intensity of ${\mathrm{Mg}}^{+}$ (black), ${\mathrm{Mg}}_2^{+}$ (red), ${\mathrm{Mg}}_3^{+}$ (green), ${\mathrm{Na}}^{+}$ (cyan), and ${\mathrm{He}}_2^{+}$ (blue) as a function of photon energy. The droplet size is 5000 He atoms with either an average of two to three Mg atoms or one Na atom attached.

Figure 2

(a) Mass spectra of He droplets doped with Mg clusters at photon energies of 40 (black) and 21.5 eV (red). The droplet size is 50 000 He atoms with an average of five to six Mg atoms attached. (b) Electron kinetic energy distributions measured in coincidence with single ions for ${\mathrm{Mg}}_{n=7,9,11}^{2+}$ (black line), ${\mathrm{Mg}}_2^{+}$ (red line), and ${\mathrm{Mg}}_3^{+}$ (blue line).

Figure 3

(a) Ion-ion coincidence time-of-flight spectrum and (b) electron spectrum correlated to ${\mathrm{Mg}}_m^{+}\text{−}{\mathrm{Mg}}_n^{+}$ ion-ion coincidences (black line). The photon energy is 40 eV. The droplet size is 50 000 He atoms with an average of six Mg atoms attached. Electron spectra are correlated to a single Mg atom (red line) embedded in a droplet of size 5000 atoms.