Sequential Penning Ionization: Harvesting Energy with Ions

We report the observation of the ejection of electrons caused by collisions of excited atoms with ions, rather than neutrals, leading to the production of doubly charged ions. Doping superfluid He droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable He atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization. In some cases, depending on the neutral target, the process also leads to a subsequent Coulomb explosion of the dopant.

Figure 1

High resolution mass spectrum with methyl iodide as the dopant. The spectrum was recorded with (see mirrored spectrum) and without blocking the He beam and at an electron energy of 70 eV. For electron energies higher than twice the excitation energy of He, an enhanced signal of the doubly charged iodine cation ${\mathrm{I}}^{2+}$ and a new series of the adduct ions $({\mathrm{IHe}}_n{)}^{2+}$ appear. The (red) arrows designate the observed $({\mathrm{IHe}}_n{)}^{2+}$ cluster ions.

Figure 2

Scheme of a sequential Penning-like ionization. Two metastable He ($2{}^{3}S$) atoms are formed by an electron. (a) Both He ($2{}^{3}S$) atoms are weakly attracted via dipole-induced dipole interactions to the methyl iodide. One excited helium atom transfers its internal energy to the embedded methyl iodide, (b) and a molecular or atomic cation is produced according to reaction (2). Ion-induced dipole interaction attracts further excited He atoms efficiently. In a final step the internal energy of the second metastable He ($2{}^{3}S$) is transferred to the atomic iodine cation (c), and this energy transfer can lead to a Penning-like ionization of ${\mathrm{I}}^{+}$, according to reaction (3). Please note that the sizes of the electrons, He ($2{}^{3}S$), and the neutral and charged dopants are strongly exaggerated compared to the size of the droplet for better visualization.

Figure 3

Electron energy onsets measured for ${\mathrm{I}}^{2+}$ and $({\mathrm{IHe}}_n{)}^{2+}$. The early onset of ${\mathrm{I}}^{2+}$ (unblocked) can be attributed to dissociative double ionization of methyl iodide. The late rise in ${\mathrm{I}}^{2+}$ (unblocked) is due to double Penning ionization. The onset curves for $({\mathrm{IHe}}_n{)}^{2+}$ (bottom) may arise from He pickup by ${\mathrm{I}}^{2+}$ produced from double Penning ionization at 40 eV and also by $(\mathrm{IHe}{)}^{2+}$ produced from associative ionization.

Figure 4

Observed ion intensity distribution for doubly charged helium cluster ions $({\mathrm{IHe}}_n{)}^{2+}$. The ions are produced in helium droplets doped with methyl iodide and exposed to electrons at an electron energy of about 70 eV. The yield of the doubly charged cations $({\mathrm{IHe}}_n{)}^{2+}$ shows a clear drop in intensity with an increasing number of He atoms attached. This drop can be assigned to a shell closure.