Experimental Evidence for Interatomic Coulombic Decay in Ne Clusters

Abstract

Electron spectra of photoexcited Ne clusters are shown to display a signal at low kinetic energies that is neither present in the Ne monomer nor at photon energies below the inner-valence 2s threshold. These findings are strong evidence for the existence of interatomic Coulombic decays (ICD), a mechanism that was recently predicted theoretically [Phys. Rev. Lett. 79, 4778 (1997)]. In ICD, an inner-valence hole state in a weakly bonded system can undergo ultrafast relaxation due to energy transfer to a neighboring atom, followed by electron emission from this neighboring site. DOI: 10.1103/PhysRevLett.90.203401 PACS numbers: 36.40.Mr, 33.80.Eh, 34.30.+h, 82.33.Fg The nonradiative decay of a core hole vacancy in an excited atom by electron emission is a well known process in spectroscopy, commonly denoted as Auger decay. Traditionally, it is assumed that in extended aggregates -molecules or bulk matter-electrons only at the excited site actively take part in the decay and that the environment of the initially excited atom only modifies the Auger energy spectrum via its influence on the energy levels [1][2][3]. In contrast to that, for weakly bound aggregates, such as van der Waals clusters and hydrogen bonded systems, a radiationless decay mechanism has been predicted, which is possible only by electron emission from neighboring sites of the vacancy [4][5][6]. The final states populated in this so-called ''interatomic Coulombic decay'' (ICD) thus have two positive charges distributed at two different atoms of the system [7]. This lowers their total energy by shielding the Coulomb repulsion of the final state holes. For a lot of systems, it is just this shift in final state energy which makes the decay energetically possible. Where present, ICD should be an ultrafast relaxation pathway, which proceeds on time scales of 1-100 fs. It is therefore expected that it dominates over competing channels, like radiative decay or relaxation involving the nuclear dynamics.Although recently an indirect indication of the relevance of interatomic transitions in the core vacancy decay of molecules has been found [8], an experimental verification of the effect is missing. The major reason for this is probably the low kinetic energy of the electrons emitted by ICD, which is typically a few eV. The unambiguous identification of these electrons would be the only distinct proof for the process. However, the region close to zero kinetic energy in the electron spectrum of a bulk material exposed to vacuum ultraviolet radiation is dominated by inelastically scattered photoelectrons from the valence band. The same is true for clusters, but due to the limited spatial dimensions of the system short range effects like ICD make a relatively higher contribution to the signal close to zero kinetic energy.In this Letter we present direct experimental evidence for ICD in the photoexcited electron spectra of small Ne clusters.In the following, we first introduce the ICD mechanism in some detail. Excited electronic states can decay in two ways, ...

On the nature and origin of dicationic, charge-separated species formed in liquid water on X-ray irradiation

“…[10][11] In recent years a set of novel non-local autoionization processes has been identified to play an important role in weakly bonded atomic and molecular systems. [12][13][14] One such relaxation process is Intermolecular Coulombic Decay (ICD), initially observed upon innervalence ionization of van der Waals-bonded clusters, 12,[15][16][17] and later also (hydrogen bonded) water clusters. 18 In the ICD process, the energy provided by an outer valence electron upon filling the vacancy is used to expel a second electron from an atom or molecule neighboring the initially ionized site.…”

Experimental Separation of Virtual Photon Exchange and Electron Transfer in Interatomic Coulombic Decay of Neon Dimers

“…As ICD occurs, the energy of an excited atom in van der Waals or hydrogen bound matter is used to release a low energy electron from a neighboring atom. During the last three years, the existence of ICD has been proven experimentally by means of electron spectroscopy [2] and multiparticle coincidence momentum spectroscopy techniques (COLTRIMS) [3]. Later experimental evidence of ICD in species other than a neon cluster was found, as well [4].…”

“…For the case of ICD after 2s photoionization in neon dimers, the decay rate is proportional to jV L2p;R2p;L2s;k ÿ V L2p;R2p;k;L2s j 2 , where the two electron-electron Coulomb matrix elements (2) are denoted as ''direct'' and ''exchange'' contribution, respectively [6,12]. These two matrix elements are schematically illustrated in Fig.…”

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters

“…Upon core level photoionization, the created hole state undergoes ultrafast relaxation due to energy transfer to a neighbouring atom, and emits an electron from that neighbouring site. [11][12][13] The first successful attempt to measure non-protonated (H 2 O) n + (2≤n≤10) clusters was by Shinohara et al 14 who used the argon emission lines at 11.62 and 11.83 eV to ionize a mixed argon/water molecular beam. Since protonated water clusters and mixed (H 2 O) n Ar m + clusters were also observed, it was suggested that electron emission and Ar evaporation from these clusters led to the formation of the non-protonated species.…”