Electron-transfer-mediated decay and interatomic Coulombic decay from the triply ionized states in argon dimers

We report the first observation of electron-transfer-mediated decay (ETMD) and interatomic Coulombic decay (ICD) from the triply charged states with an inner-valence vacancy, using the Ar dimer as an example. These ETMD and ICD processes, which lead to fragmentation of Ar(3+)-Ar into Ar(2+)-Ar(2+) and Ar(3+)-Ar+, respectively, are unambiguously identified by electron-ion-ion coincidence spectroscopy in which the kinetic energy of the ETMD or ICD electron and the kinetic energy release between the two fragment ions are measured in coincidence.     

Experimental observation of interatomic coulombic decay in neon dimers

Recently Cederbaum et al. [Phys. Rev. Lett. 79, 4778 (1997)]] predicted a new decay channel of excited atoms and molecules termed interatomic Coulombic decay (ICD). In ICD the deexcitation energy is transferred via virtual photon exchange to a neighboring atom, which releases it by electron emission. We report on an experimental observation of ICD in 2s ionized neon dimers. The process is unambiguously identified by detecting the energy of two Ne1+ fragments and the ICD electron in coincidence, yielding a clean, background free experimental spectral distribution of the ICD electrons.     

Autoionization mediated by electron transfer

Electron-electron coincidence spectra of Ar-Kr clusters after photoionization have been measured. An electron with the kinetic energy range from 0 to approximately 1 eV is found in coincidence with the Ar 3s cluster photoelectron. The low kinetic energy electron can be attributed to an Ar + Kr+ + Kr+ final state which forms after electron transfer mediated decay. This autoionization mechanism results from a concerted transition involving three different atoms in a van der Waals cluster; it was predicted theoretically, but hitherto not observed.     

Dynamics and post-collision interaction effects in two electron decay from the Xenon 4d hole

Two Auger electrons, one very slow, one fast, have been detected in coincidence following near threshold 4d photoionization of the Xe atom. The distribution in the energy the two electrons share has been measured for the first time revealing the presence of post-collision interaction effects that provide unique information on the decay dynamics of the 4d hole. Analysis of the distorted line shapes indicates that the dominant process is decay of Xe+(4d(-1)) to Xe3+ through cascade emission of a zero kinetic energy Auger electron followed by a fast Auger electron. The widths of the intermediate Xe2+* states are estimated to be about 60 meV.     

Intermolecular coulomb decay at weakly coupled heterogeneous interfaces

Surface ejection of H(+)(H(2)O)(n=1-8) from low energy electron irradiated water clusters adsorbed on graphite and graphite with overlayers of Ar, Kr or Xe results from intermolecular Coulomb decay (ICD) at the mixed interface. Inner valence holes in water (2a(1)(-1)), Ar (3s(-1)), Kr (4s(-1)), and Xe (5s(-1)) correlate with the cluster appearance thresholds and initiate ICD. Proton transfer occurs during or immediately after ICD and the resultant Coulomb explosion leads to H(+)(H(2)O)(n=1-8) desorption with kinetic energies that vary with initiating state, final state, and interatomic or molecular distances.     

Multiple photoionization of atoms and small molecules by synchrotron radiation

Absorption of one VUV photon by an atom or a molecule can induce the ejection of several electrons through different processes. Such multiple ionization processes, studied by coincidence electron spectroscopy, provide a wealth of information on electron correlations. A magnetic bottle electron time of flight spectrometer implemented on synchrotron radiation centers has allowed the efficient detection in coincidence of two, three and up to five electrons with good energy resolution. The branching ratios of the different processes are easily extracted from the experimental spectra due to the constant transmission of the spectrometer. Multiple Auger decay was observed in rare gases atoms after inner-shell ionization, while core-valence and core-core initial double ionization followed by Auger decay are other pathways to multiple ionization. For molecules, Coulomb explosion with energy released in ionic fragments may occur after multiple ionization, nevertheless, coincidence electron spectroscopy can also provide a clear interpretation for peculiar decay channels in molecules.     

Properties of resonant interatomic Coulombic decay in Ne dimers

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.     

Energy correlation among three photoelectrons emitted in core-valence-valence triple photoionization of Ne

The direct observation of triple photoionization involving one inner shell and two valence electrons is reported. The energy distribution of the three photoelectrons emitted from Ne is obtained using a very efficient multielectron coincidence method using the magnetic bottle electron spectroscopic technique. A predominance of the direct path to triple photoionization for the formation of Ne3+ in the 1s 2s2 2p4 configuration is observed. It is demonstrated that the energy distribution evolves with photon energy and indicates a significant difference with triple photoionization involving only valence electrons.     

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.     

Lifetime of core two-hole states in atoms

The lifetime of core two-hole states in atoms is discussed using an atomic many-body theory. The lifetime widths of the two-hole states with the core holes in the Lx (X = l–3) shells of Ar atom are calculated by an ab initio atomic many-body theory (Green's function method using the extended random-phase approximation with exchange (RPAE)). The present theory gives good agreement with experiment. The changes not only in the Coster-Kronig electron kinetic energy but also in the polarization effect on the Coster-Kronig electron by the presence of extra two holes created by decay of the partner hole or by the presence of the partner hole, affects considerably the lifetime width of the two-hole state in Ar atom so that it deviates much from the sum of that of the individual hole.     

Auger electron emission from fixed-in-space CO

We have measured the angular distribution of carbon K-Auger electrons from fixed in space, core-ionized, CO molecules in coincidence with the kinetic energy release of the C+ and O+ fragments. We find a very narrow ejection of Auger electrons in the direction of the oxygen and an oscillatory diffraction pattern. Even for similar electron energies, the angular distribution strongly depends on the symmetry of the final state.     

Energy and angular distributions of electrons emitted by direct double auger decay

We have observed the direct L(2,3)MMM double Auger transition after photoionization of the 2p shell of argon by angle-resolved electron-electron coincidence spectroscopy. The process is responsible for about 20% of the observed Auger electron intensity. In contrast to the normal Auger lines, the spectra in double Auger decay show a continuous intensity distribution. The energy and angular distributions of the emitted electrons allow one to obtain information on the electron correlations giving rise to the double Auger process as well as the symmetry of the associated two-electron continuum state.     

Radiative relaxation in 2p-excited argon clusters

The emission of ultraviolet fluorescence radiation from variable size argon clusters is investigated with high spectral resolution in the Ar 2p-excitation regime. The fluorescence excitation spectra reveal strong fluorescence intensity in the Ar 2p-continuum, but no evidence for the occurrence of discrete low-lying core-exciton states in the near-edge regime. This finding is different from the absorption and photoionization cross section of argon clusters and the solid. The dispersed fluorescence shows a broad molecular band centered near 280 nm. The present work indicates that double and triple ionization via the LMM-Auger decay are required to initiate the fluorescence processes in the Ar 2p-continuum. The present results are consistent with the formation of singly charged, excited moieties within the clusters, which are assigned as sources of the radiative emission in the 280 nm regime. A fast energy transfer process (interatomic Coulombic decay (ICD)), which has been proposed by recent theoretical work, is assigned to be primarily the origin of these singly charged, excited cations besides intra-cluster electron impact ionization by the Auger electrons. Our findings give first possible experimental evidence for ICD in the core level regime.     

Argon nanobubbles in Al(111): a photoemission study

Two fundamental manifestations of Al conduction electron response to Ar atom core hole in the final state of photoemission have been studied in implanted Ar bubbles in Al(111). Ar 2p binding energy and the Doniach-Sunji? asymmetry of the core-level line shape vary systematically as functions of Ar+ implantation energy and number of ions bombarded (fluence). The observations are explained by relating the strength of Al conduction electron screening to the size of the Ar nanobubbles.     

Electrical property effect of oxygen vacancies in the heterojunction of LaGaO_3/SrTiO_3

Density functional theory within the local density approximation is used to investigate the effect of the oxygen vacancy on the LaGaO_3/SrTiO_3(001) heterojunction. It is found that the energy favorable configuration is the oxygen vacancy located at the 3~(rd) layer of the STO substrate, and the antiferrodistortive distortion is induced by the oxygen vacancy introduced on the SrTiO_3 side. Compared with the heterojunction without introducing oxygen vacancy, the heterojunction with introducing the oxygen vacancy does not change the origin of the two-dimensional electron gas(2DEG), that is, the 2DEG still originates from the d_(xy) electrons, which are split from the t~(2g) states of Ti atom at interface; however the oxygen vacancy is not beneficial to the confinement of the 2DEG. The extra electrons caused by the oxygen vacancy dominantly occupy the 3d_(x~2-y~2) orbitals of the Ti atom nearest to the oxygen vacancy, thus the density of carrier is enhanced by one order of magnitude due to the introduction of oxygen vacancy compared with the density of the ideal structure heterojunction.     

Measurement of the energy dependences of the photoelectron angular distribution parameters for Ar 3p and Kr 4p electrons using an electron-electron coincidence experiment

The energy dependence of the photoelectron angular distribution parameter (β) has been measured for Ar 3p and Kr 4p electrons in the energy ranges 4–54 eV and 4–40 eV above threshold respectively, using 3.5 keV electron impact with coincidence detection of the scattered and ejected electrons. This energy range is considerably greater than that covered in previous experiments. The data over the whole energy range are compared with the Hartree-Fock calculations of Kennedy and Manson and with those of Amusia et al., using the random phase approximation with exchange method. For Ar the calculations of Amusia et al., which take into account many-electron correlations, are shown to agree more closely with the present data. The data are also shown to be in very good agreement with earlier experimental data obtained over a more limited energy range using electron synchrotron radiation.     

Investigations of positronium formation and destruction using 3gamma/2gammaannihilation-ratio measurements

Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.     

Angular studies of potential electron emission in the interaction of slow ions with Al surfaces

We report energy distributions of electrons emitted from Al surfaces under impact by 1 keV Ar+ and 1-5 keV Ne+ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For Ar+ electron emission results mainly from Auger neutralization, while for Ne+ an important emission mechanism is the decay of plasmon excitations. We find a transition between surface and bulk plasmon excitations as the energy of the ion is increased.     

Ar+ excited aluminium Auger electron emission from Cu-Al alloys

The aluminium Auger electron emission from Cu-Al alloys in solid solution bombarded with 2–16 keV Ar⁺ ions is studied as a function of the Al concentration. A linear law is observed for the intensity of the high energy Auger peak at 76 eV which originates only from the primary asymmetric collisions Ar → Al where two vacancies are created in the 2p level of the Al atom. On the contrary, a parabolic law is found for the intensity of the principal Auger peak at 63.5 eV (one Al 2p vacancy) which originates from the asymmetric collisions Ar → Al and from the symmetric collisions Al → Al together. The proportion of asymmetric collisions among collisions effective for the principal Auger emission from pure aluminium can be deduced from these results. It appears as an increasing function of the bombardment energy: its value is nearly equal to 6% at 10 keV and 18% at 15 keV.     

Experimental separation of virtual photon exchange and electron transfer in interatomic coulombic decay of neon dimers

We investigate the interatomic Coulombic decay (ICD) of neon dimers following photoionization with simultaneous excitation of the ionized atom (shakeup) in a multiparticle coincidence experiment. We find that, depending on the parity of the excited state, which determines whether ICD takes place via virtual dipole photon emission or overlap of the wave functions, the decay happens at different internuclear distances, illustrating that nuclear dynamics heavily influence the electronic decay in the neon dimer.