Interatomic Coulombic Decay following Photoionization of the Helium Dimer: Observation of Vibrational Structure

Using synchrotron radiation we simultaneously ionize and excite one helium atom of a helium dimer (${\mathrm{He}}_2$) in a shakeup process. The populated states of the dimer ion [i.e., ${\mathrm{He}}^{*+}(n=2,3)-\mathrm{He}$] are found to deexcite via interatomic Coulombic decay. This leads to the emission of a second electron from the neutral site and a subsequent Coulomb explosion. In this Letter we present a measurement of the momenta of fragments that are created during this reaction. The electron energy distribution and the kinetic energy release of the two ${\mathrm{He}}^{+}$ ions show pronounced oscillations which we attribute to the structure of the vibrational wave function of the dimer ion.

Figure 1

Bottom: He dimer potential (left scale) (from 23). dotted green line in right axis: ${\psi }^2{R}^2$ (from 24). The expectation value of internuclear distance is 52 Å [8], which is off scale. Upper panel: measured internuclear distance at instant of ICD from ${\mathrm{He}}^{*+}(n=2)-\mathrm{He}\to {\mathrm{He}}^{+}+{\mathrm{He}}^{+}+e_{\mathrm{ICD}}$. Calculated from KER (see Fig. 3) at a photon energy of 68.86 eV using the reflection approximation $R=1/\mathrm{KER}$.

Figure 2

Kinetic energy release of the ${\mathrm{He}}^{+}-{\mathrm{He}}^{+}$ fragments versus the energy of one of the two electrons from photoionization excitation of ${\mathrm{He}}_2$ at photon energies of 68.86 and 77.86 eV. The arrows indicate the expected position of photoelectron (horizontal line) and the corresponding ICD electron (diagonal lines) for excitation to the intermediate state ${\mathrm{He}}^{*+}(n)-\mathrm{He}$.

Figure 3

Kinetic energy release of the ${\mathrm{He}}^{+}-{\mathrm{He}}^{+}$ fragments for intermediates excited states with principal quantum $n=2$ [${\mathrm{He}}^{*+}(n=2)-\mathrm{He}$]. Data are obtained by projecting the respective diagonal line in Fig. 2 onto the KER axis.

Figure 4

Distribution of the kinetic energy release for intermediates excited states ($n$) at a photon energy of 77.86 eV. The principal quantum number $n$ as given in the figure.