Mixed Cluster Ions of Magnesium and C60

Abstract

Magnesium clusters exhibit a pronounced nonmetal-to-metal transition, and the neutral dimer is exceptionally weakly bound. In the present study, we formed pristine Mg_n^z+ (n = 1-100, z = 1-3) clusters and mixed (C₆₀)_mMg_n^z+ clusters (m = 1-7, z = 1, 2) upon electron irradiation of neutral helium nanodroplets doped with magnesium or a combination of C₆₀ and magnesium. The mass spectra obtained for pristine magnesium cluster ions exhibit anomalies, consistent with previous reports in the literature. The anomalies observed for C₆₀Mg_n⁺ strongly suggest that Mg atoms tend to wet the surface of the single fullerene positioning itself above the center of a pentagonal or hexagonal face, while, for (C₆₀)_mMg_n^z+, the preference for Mg to position itself within the dimples formed by fullerene cages becomes apparent. Besides doubly charged cluster ions, with the smallest member Mg₂²⁺, we also observed the formation of triply charged ions Mg_n³⁺ with n > 24. The ion efficiency curves of singly and multiply charged ions exhibit pronounced differences compared to singly charged ions at higher electron energies. These findings indicate that sequential Penning ionization is essential in the formation of doubly and triply charged ions inside doped helium nanodroplets.

Figure 1

Mass spectrum of low-mass ions ejected from neutral HNDs doped with magnesium upon multiple electron bombardment (upper diagram). At mass per charge values below 240 Thomson, the mass spectrum is dominated by He_k⁺ ions that exhibit an almost biexponential decrease of the ion yields as a function of k. At higher mass per charge values, Mg_n⁺ is the prevailing ion series. Pronounced intensity anomalies can be seen in this semilogarithmic plot. The inset shows a small section of the same mass spectrum, and individual mass peaks were assigned to the isotopic patterns of Mg₁₂⁺ (blue circles), Mg₂₄²⁺ (red squares), and Mg₃₆³⁺ (green triangles). In the lower diagram, the Mg_n⁺ distribution is shown with black circles, the solid ones were deduced with the software IsotopeFit from the mass spectrum above, and the resulting cluster size distribution is compared to a Mg_n⁺ cluster size distribution from the literature (open circles). The Mg_n²⁺ distribution is depicted with red squares: the solid ones represent data obtained via IsotopeFit from the mass spectrum shown in the upper diagram, and the open squares are taken from Diederich et al.

Figure 2

Mass spectrum of low-mass ions ejected from neutral HNDs doped with C₆₀ and magnesium upon multiple electron bombardment (upper diagram). At mass per charge values below 720 Thomson, the mass spectrum is dominated by He_k⁺ ions, exhibiting a biexponential decrease in ion yields as a function of k. At higher mass per charge values, the prevailing ions are (C₆₀)_mMg_n⁺. Pronounced intensity anomalies can be seen in this semilogarithmic plot. The lower diagram shows a section of the mass spectrum above (solid black line) and the resulting fit from IsotopeFit (red dashed line). The isotopic patterns of selected ions are included by taking the peak shape of the mass spectrometer into consideration. The most intense ions are indicated in the diagram. The detailed assignment of all ions can be found in Figure S1 of the Supporting Information (SI).

Figure 3

Cluster size distributions of (C₆₀)_mMg_n⁺ (a) and (C₆₀)_mMg_n²⁺ (b) as a function of the number of magnesium atoms (n) for m = 1–3. The ion yields were obtained by fitting the mass spectrum of Figure 2 with IsotopeFit taking into consideration the isotopic patterns and potential background signal, as well as the contributions from other ions in the mass range of individual mixed C₆₀ magnesium cluster ions. The main contribution of C₆₀⁺ (gray solid circle in the upper, left diagram) originates from the electron ionization of C₆₀ molecules of an effusive fullerene beam emitted from the oven. Subtraction of these ions leads to a small contribution from doped HNDs that is prone to large error. To increase the visibility of the ion abundance of low-intensity cluster ions, the values were multiplied by a factor of 10 (open red triangles).

Figure 4

Normalized ion efficiency curves of selected monocations formed upon multiple electron bombardment of neutral HNDs doped with C₆₀ and Mg. The singly charged dimer ions of He and Mg exhibit a pronounced shoulder at an electron energy of 22 eV that is missing for doubly and triply charged magnesium cluster ions. The ion efficiency curve of Mg₂⁺ (blue solid line) was multiplied by a factor of 5 (dashed blue line) for better visibility of the 22 eV shoulder.

Figure 5

Ion efficiency curves for selected monocations (upper diagram) and dications (lower diagram) formed upon multiple electron bombardments of neutral HNDs doped with C₆₀ and Mg. All singly charged ions except for He₁₄⁺ exhibit a threshold at 20 eV that indicates an ionization mechanism that involves He*. In contrast, the dications are predominantly formed at electron energies higher than 24.6 eV, and all curves exhibit a pronounced signal rise at 40 eV, which indicates an additional ionization mechanism at electron energies higher than 40 eV.