Single Photon Double Ionization of the Helium Dimer

Abstract

We show that a single photon can ionize the two helium atoms of the helium dimer in a distance up to 10 Å . The energy sharing among the electrons, the angular distributions of the ions and electrons, as well as comparison with electron impact data for helium atoms suggest a knockoff type double ionization process. The Coulomb explosion imaging of He 2 provides a direct view of the nuclear wave function of this by far most extended and most diffuse of all naturally existing molecules. DOI: 10.1103/PhysRevLett.104.153401 PACS numbers: 36.40.Àc, 34.80.Dp The helium dimer ( 4 He 2 ) is an outstanding example of a fragile molecule whose existence was disputed for a long time because of the very weak interaction potential [1] (see black curve in Fig. 1). Unequivocal experimental evidence for 4 He 2 was first provided in 1994 in diffraction experiments [2] by a nanostructured transmission grating. Subsequently, the average dimer bond length and dimer binding energy could be determined to be 52 Å and 10 À7 eV (0:9 Â 10 À3 cm À1 or 1.3 mK) [3]. This very large bond length, a factor 100 larger than the hydrogen bond length, goes along with a prediction of very widely delocalized wave function, unseen in any other molecule [4] (see R 2 É 2 i function in Fig. 1). It is because of these exotic properties that ''as the hydrogen molecule in the past, the helium dimer today became a test case for the development of new computational methods and tools '' [5] in quantum chemistry. Despite this fundamental nature of the diffuse helium dimer wave function, it has escaped direct experimental observation until now, as the diffraction grating experiment measures the mean value and not the shape of the wave function itself. Our experiment provides a direct view of this diffuse object.The large distance between the two helium atoms and the minuscule binding energy make the helium dimer a unique model system to explore electron correlations over large distances. The most sensitive tool for such studies is multiple photoionization. Since photoabsorption is described by a single electron operator the photon energy and angular momentum is best thought of as being initially given to one electron of the atom only. In the absence of electron correlation the ejection of a single electron would be the only possible outcome of the photoabsorption process. Because of the ubiquity of electron correlation, however, the ejection of electron pairs by a single photon is a wide spread phenomenon seen in atoms [6], molecules [7,8], and solids [9]. This two electron process poses at least two central questions: what is the correlation mechanism by which the photon energy is distributed among the two electrons and over which distance are such correlations active? In the present Letter we report the surprising observation that a single photon can lead to nonsequential ejection of two electrons from two atoms which are separated by many atomic radii and where the overlap of the electronic wave functions is negligible. By measuring the internuclear dist...

Interatomic Coulombic decay in helium nanodroplets

“…For comparison, the ion KE spectrum measured for free He 2 at hν = 68.86 eV is shown in Fig. 2 d) [2]. The distribution peaked around 4.2 eV is attributed to KER from Coulomb explosion of the pair of He + ions generated by ICD [2,3].…”

“…Thus, ICD offers a new, ultrafast decay path where energy is exchanged with a neighboring atom leading to its ionization. Since its discovery, ICD has been observed in a wide variety of weakly-bound systems from He dimers [2,3] and rare-gas clusters to biologically relevant systems such as water clusters; for reviews see [4,5]. Today, the focus is on condensed-phase systems where ICD is involved in complex relaxation mechanisms [6][7][8], which can generate genotoxic low-energy electrons and radical cations [9].…”

“…By detecting either electrons or ions with the VMI detector in coincidence with the corresponding particles of opposite charge with the TOF detector, we obtain either ion mass-correlated electron spectra or mass-selected ion kinetic energy (KE) distributions by Abel inversion of the VMIs [20]. The XUV photon energy is tuned near the first excited level of He + , hν E(He + * , n = 2) = 65.4 eV [2].The elementary ICD process He 2 + hν → HeHe + * + e − sat → He + + He + + e − sat + e − ICD generates two electrons and two He + ions flying apart due to Coulomb repulsion. Here, He + * denotes a He ion in an excited state with principal quantum number n > 1.…”

“…By detecting either electrons or ions with the VMI detector in coincidence with the corresponding particles of opposite charge with the TOF detector, we obtain either ion mass-correlated electron spectra or mass-selected ion kinetic energy (KE) distributions by Abel inversion of the VMIs [20]. The XUV photon energy is tuned near the first excited level of He + , hν E(He + * , n = 2) = 65.4 eV [2].…”

“…The satellite photoelectron e − sat is emitted directly with kinetic energy E sat = hν − E(He + * ) upon simultaneous ionization and excitation of a He atom. The ICD electron e − ICD is created by energy transfer from He + * to the neighboring He atom resulting in E ICD = E(He + * ) − 2 × E i − KER ∼ 8.5 eV [2]. Here, E i = 24.6 eV denotes the ionization energy of He and KER is the KE release of the He + -He + fragments.…”