Interatomic-Coulombic-decay-induced recapture of photoelectrons in helium dimers

We investigate the onset of photoionization shake-up-induced interatomic Coulombic decay (ICD) in ${\mathrm{He}}_2$ at the ${\mathrm{He}}^{+*}(n=2)$ threshold by detecting two ${\mathrm{He}}^{+}$ ions in coincidence. We find this threshold to be shifted towards higher energies compared to the same threshold in the monomer. The shifted onset of ion pairs created by ICD is attributed to a recapture of the threshold photoelectron after the emission of the faster ICD electron.

Figure 1

(a) Kinetic energy release (KER) of ${\mathrm{He}}^{+}$ ion pairs as a function of photon energy; the red arrow shows the threshold for creating ${\mathrm{He}}^{+*}$ ($n=2$) for a helium atom. (b) Red solid line: projection of (a) onto the $y$ axis for a photon energy range of 65.41–65.42 eV. Black dotted line: theoretical KER distribution. The two curves are normalized to the maximum.

Figure 2

Photon energy dependence of the count rate for two ${\mathrm{He}}^{+}$ ions [solid circles; projection of data in Fig. 1 onto the $x$ axis] and ${\mathrm{He}}^{+*}$ ($n=2$) ions from ionization of the monomer (open circles). Black dotted step function: threshold for ${\mathrm{He}}^{+*}$ ($n=2$); black dashed line: step function convoluted with a Gaussian function ($\sigma$ = 4.5 meV) for the energy resolution of the beam line. Green solid line: expected onset of ICD without taking PCI into account (see text); red dash-dotted line: model including recapture of photoelectrons by PCI using calculated widths from Table 1 (see text) as well as the experimental resolution of $\sigma =4.5$ meV. The data points are normalized at the highest points.