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

Abstract

We investigate the onset of photoionization shakeup induced interatomic Coulombic decay (ICD) in He2 at the He + * (n = 2) threshold by detecting two 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. PACS numbers:Excited ions can get rid of their excess energy via the emission of a photon or an electron. If, however, the excited atom is spatially close to other atoms and the excitation energy is above the ionization threshold of this neighbor, the excess energy can also be transfered to the neighbor where it leads to emission of an electron. This energy transfer process is termed interatomic Coulombic decay (ICD). It was introduced by Cederbaum and coworkers in 1997 [1] and was demonstrated experimentally first for Neon clusters [2] and Neon dimers [3]. The related interatomic Auger transitions in solid matter have ofen been discussed, but broad valence bands, surface/bulk differences and significant electron energy-loss processes do typically preclude a clear assignment of this process for solids. Many studies have shown since then that ICD is a very general phenomenon occuring in van der Waals bound (see e.g. [2][3][4][5]) and hydrogen bound systems (see e.g. [6][7][8]). It can be induced by photoionization (see e.g. [2,3]), photoexcitation [9][10][11][12], Auger decay [13,14], ion impact [15][16][17], and electron impact [18] or as in the present case after shakeup [19]. The most extreme system for which ICD has been reported is the Helium dimer [20,21]. The neutral He dimer is very weakly bound (about 95 neV) and the internuclear distance extends to very large distances, with the mean * Electronic address: doerner@atom.uni-frankfurt.de distance of about 52Å. ICD in He 2 can occur when one of the Helium atoms is ionized and its remaining electron is shaken up to any excited state (He + * (n=2,3...)). In the next step, the He + * (n=2)He contracts and during that nuclear motion it undergoes ICD. The electron of the exited He + relaxes to the ground state and the energy is transfered to the neutral neighbor where the ICD electron is emitted. Finally, the two He + ions Coulomb explode back-to-back: He-He hν −→ He + * -He + e ph ICD − −− → He + + He + + e ph + e ICD Usually, ICD and the subsequent Coulomb explosion is discussed in a two step picture, where the decay is independent from the initial ionization/excitation process. In the present work, we show that close to the ionization/excitation threshold this two step approximation breaks down. Photoelectron and ICD electron interaction can lead to recapture of the photoelectron into a bound state of one of the two ions, which quenches the Coulomb explosion. A direct link between the ionization process and ICD has been discussed in two contexts in the literature so far. The first is the recoil effect, where it has been shown ex...

Ultrafast temporal evolution of interatomic Coulombic decay in NeKr dimers

“…Furthermore, even the two measured curves exhibit some disagreement, whereas they were expected to follow the same ICD dynamics. The reason for this difference is the fact that the probability for recapture of the photoelectron [36] strongly depends on the photoelectron energy: the lower the photoelectron energy, the more likely a recapture can occur, which consequently truncates the decay-time measurement at a certain minimal time value.…”

“…The theoretical approach employed in this work accounts only for the ICD process, thus neglecting the PCI effect. While it was shown that the correlation between the photoelectron and the Auger electron may affect the spectra in the Auger decay of atoms [60], it should be noted that the two-step approach used in this work has been successfully applied to describe the experimental observations in a related experiment on He dimers [36].…”

“…Thus, by measuring the energy shift of the photoelectron, information on the time delay between the excitation and decay can be retrieved. For very short decay times and for photoelectron energies very close to the ionization threshold, the energy loss can be so large that the electron is recaptured to a Rydberg state [36].…”

“…For very short decay times and for photoelectron energies very close to the ionization threshold, the energy loss can be so large that the electron is recaptured to a Rydberg state. 36 In the following, we report on a detailed experimental investigation of ICD in NeKr dimers aer Ne + (2s À1 ) photoionization, where we apply the method of PCI-streaking and measure the energy shi of the slow photoelectron in order to infer the time of the decay. The reaction proceeds as follows: g(48.60 eV/48.68 eV) + NeKr / Ne + (2s À1 )Kr + e photo / Ne + + Kr + + e photo + e ICD .…”

Time-resolved studies of interatomic Coulombic decay

“…It turns out, that the range of accessible decay times and in particular the minimum accessible decay time is determined by the initial energy of the photoelectron prior to PCI. This is due to the fact that electrons experiencing a strong decelaration may be recaptured into the ion [45]. As the initial photoelectron energy E p is increased, the minimum accessible decay time decreases.…”