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 (He_2) in a shakeup process. The populated states of the dimer ion (i.e. He^[*+](n = 2; 3)-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 He^+ ions show pronounced oscillations which we attribute to the structure of the vibrational wave function of the dimer ion.

💡 Research Summary

In this work the authors investigate interatomic Coulombic decay (ICD) in the helium dimer (He₂), the most weakly bound van‑der‑Waals molecule known. By exposing a cold He₂ beam to synchrotron radiation around 63 eV they induce a simultaneous ionization and shake‑up of one helium atom, creating the excited ionic state He⁺* (n = 2 or 3)–He. The excited ion and its neutral partner form a transient dimer‑ion complex that can relax by ICD: the excess energy of the excited ion is transferred to the neutral atom, causing emission of a low‑energy secondary electron (≈0.5–2 eV) from the neutral site. The emission of this electron leaves two He⁺ ions that repel each other and undergo a Coulomb explosion.

The experiment employs a COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) reaction microscope, which records the three‑dimensional momenta of the emitted electron and both He⁺ fragments in coincidence. From these data the authors extract the electron kinetic‑energy distribution and the kinetic‑energy release (KER) spectrum of the ion pair with sub‑eV resolution. Both spectra display pronounced oscillatory structures rather than smooth continua. The oscillations are interpreted as a direct fingerprint of the vibrational wave function of the He⁺*–He dimer ion at the moment of decay.

To substantiate this interpretation the authors calculate the potential energy curves of the He⁺* (n = 2, 3)–He system using ab‑initio methods that include electron correlation and the long‑range polarization interaction. The ground‑state He₂ wave function (a very diffuse bound state at ≈0.4 K) is used to evaluate Franck‑Condon overlaps with the vibrational eigenstates of the excited ionic potential. The calculated vibrational level spacings (≈0.2–0.3 eV) reproduce the observed periodicities in both the electron‑energy and KER spectra. Moreover, a correlation analysis between electron energy and KER allows the authors to infer the internuclear distance at which ICD occurs (≈3–5 Å), precisely the region where the potential curves cross and the ICD rate is maximal.

The study yields several key insights. First, ICD is shown to be operative even in a system bound by only 10⁻⁴ eV, confirming that non‑radiative energy transfer does not require strong chemical bonding. Second, the vibrational structure of the transient dimer ion is preserved in the final kinetic observables, providing a rare experimental window on nuclear wave‑packet dynamics during an ultrafast electronic decay. Third, the combination of synchrotron‑induced shake‑up ionization and high‑resolution COLTRIMS detection proves to be a powerful tool for probing electron‑nuclear coupling in weakly bound clusters.

These results have broader implications for the understanding of ICD in more complex environments such as rare‑gas clusters, biological water clusters, and low‑temperature plasmas, where similar non‑radiative decay channels can dominate energy redistribution. Future work may extend the methodology to heavier rare‑gas dimers, mixed clusters, or to explore the influence of external fields on ICD rates and vibrational branching, thereby deepening our knowledge of ultrafast interatomic processes across chemistry and physics.