Localized versus delocalized excitations just above the 3d threshold in krypton clusters studied by Auger electron spectroscopy

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(52) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or "internal" ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation.     

Statistical analysis of photoelectron and Auger energy shifts in ionic solutions

A previously devised statistical model for solvation shifts of ionization energies of atomic ions is extended to Auger transition energies. The model expresses the ionization shift, the Auger energy shift and the sum of these shifts in terms of expectation values taken over radial and angular distribution functions for the solvent. Numerical evaluation is performed for some monovalent anions and cations and for Mg²⁺ in aqueous solutions. Results from the model are compared with experimental data for absolute photoelectron and Auger shifts as well as for the individual contribution to salvation energies that can be assessed from a combined use of ESCA data for photoelectron and Auger energy shifts.     

High resolution studies of deep core level threshold auger processes

The emission of Auger electrons associated with the photon induced threshold creation of an atomic inner-shell hole is a complex dynamical process. Detailed studies of this resonance phenomenon become possible through the combination of high-flux synchrotron radiation and state-of-the-art experimental techniques. We present experimental results for the resonantly excited L₃-M_4,5M_4,5 transition in Pd and Ag. While far above threshold the Auger lines are largely independent of the exciting photons, the emitted electron distribution undergoes drastic changes in the threshold region. The observations are discussed in terms of radiationless resonant inelastic scattering.     

Highly efficient electron stimulated desorption of O+ from gadolinia-doped ceria surfaces

Highly efficient electron stimulated desorption of O+ from gadolinia-doped ceria (GDC) surfaces annealed at 850 K in ultrahigh vacuum is observed and investigated. O+ desorption has a major threshold of approximately 40 eV and an intrinsic kinetic energy of approximately 5.6 eV. Since the threshold energy is close to Ce 5s and Gd 5s core levels, Auger decay of core holes is likely associated with O+ desorption from sites related to oxygen vacancies. The interactions of water and molecular oxygen with GDC surfaces result in a decrease in O+ desorption, suggesting that water and oxygen molecules adsorb mainly to oxygen vacancies. The dependence of O+ kinetic energies on the incident electron energy and temperature reveals surface charging as a result of electron trapping, hole trapping, and electron-hole recombination. The activation energy for surface charge dissipation is found to be 0.43 eV, close to the activation energy for ionic conduction (0.47 to 0.6 eV) in the same material.     

Dissociative double photoionization of N2 using synchrotron radiation: appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N2+ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2+/-0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N2+(2P)+N(4S) at 53.9 eV. The onset of a second threshold at 59.9+/-0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N2+ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N(2)2+ dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies.     

Resonant Auger spectroscopy at the carbon and nitrogen K-edges of pyrimidine

The resonant Auger electron spectra obtained after photoexcitation below the C and N 1s ionization thresholds in the pyrimidine molecule have been measured at several photon energies. The results show the relevance of the localization of the inner hole and of the matching between the symmetries of the intermediate and final states in the decay spectra via participator transitions. The comparison with the Auger electron spectra suggests some assignment for the two-hole-one-particle states reached via spectator transitions. The analysis of the participator decay is supported by state-of-the art density functional theory calculations.     

The photodissociation of NO(2) by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.     

Dissociation of core-valence doubly excited states in NO followed by atomic Auger decay

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined.     

Exciton multiplication and relaxation dynamics in quantum dots: applications to ultrahigh-efficiency solar photon conversion

Huge amounts of carbon-free energy will be required during the coming decades in order to stabilize atmospheric CO2 to acceptable levels. Solar energy is the largest source of non-carbonaceous energy and can be used to produce both electricity and fuel. However, the ratio of the areal cost to the conversion efficiency for devices converting solar photons to electricity or fuel must be reduced by at least 1 order of magnitude from the present values; this requires large increases in the cell efficiency and large reductions in the cost per unit area. We have shown how semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon. This is possible because quantization of energy levels in quantum dots slows the cooling of hot excitons, promotes multiple exciton generation, and lowers the photon energy threshold for this process. Quantum yields of 300% for exciton formation in PbSe quantum dots have been reported at photon energies 3.8 times the HOMO-LUMO transition energy, indicating the formation of three excitons/photon for all photoexcited quantum dots. Similar high quantum yields have also been reported for PbS quantum dots. A new model for this effect that is based on a coherent superposition of multiple excitonic states has been proposed.     

Dielectric dispersion in optical electron transfer in solution

Dielectric dispersion causes the outer-sphere reorganization free energy to vary with photon energy in optical electron transfer. The resulting distortion of the spectral response for photoelectron emission may yield misleading evidence about the emission yield versus photon energy relationship. Dispersion is taken into account in the calculation of emission threshold energies and the correlation between optical and thermal electron transfer. Results are given for V²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Fe(CN)₆^4?.     

Partial photoionization cross sections of large mercury clusters and liquid mercury in the vacuum ultraviolet photon energy region

Photoelectron spectra for neutral mercury clusters (up to a size of 109 atoms) and liquid mercury have been recorded for several different photon energies between 7.1 eV and 10.6 eV. For both large mercury clusters (Hg _x ,x≥60) and liquid mercury a strong increase of the partial photoionization cross sections near threshold with decreasing photon energy is observed. This shows clearly that the local electronic structure of large mercury clusters is very similar to the electronic structure of the metallic bulk material.     

Radiationless decay in the region of the 2t2g and 4eg resonances in SF6

The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.     

Theoretical investigation of the sulphur K-LL auger energies and chemical shifts in a series of sulphur compounds

Two series of investigations are reported on LL double-hole states of molecules containing sulphur. First the results on the LL double ionization potentials and K-LL Auger energies of H₂S and SO₂ show that the use of a frozen-orbital approximation is preferable to SCF methods for calculation of chemical shifts in K-LL Auger energies and LL ionization potentials. Secondly chemical shifts of K-LL Auger energies of a series of model molecules H₂S, H₂SO₄, H₄SO₂ and H₂SO₂ are correlated with the formal oxidation state of sulphur. This correlation gives a shift of 12 eV in Auger energy per formal charge on sulphur.     

Single and multiple photoionisation of H2S by 40-250 eV photons

Multi-electron coincidence measurements on photoionisation of H(2)S have been carried out at photon energies from 40 to 250 eV. They quantify molecular field effects on the Auger process in detail and are in good agreement with the existing theory. Spectra of core-valence double ionisation of H(2)S are presented and partially analysed. Auger decays from the core-valence states produce triply charged product spectra with unexplained and surprising intensity distributions. Triple ionisation by the double Auger process from 2p hole states shows little effect of the molecular field splitting, but includes a substantial contribution from cascade processes, some involving dissociation in intermediate states. The onset of triple ionisation at the molecular geometry is determined as 61 ± 0.5 eV.     

A study of selected fragmentation paths of the ethyne dication: theory and experiment

The fragmentation of the C(2)H(2)(2+) dication, formed upon inner shell ionization and the subsequent Auger decay, has been studied by means of Auger electron-ion and Auger electron-ion-ion coincidence spectroscopy at four different kinetic energies of the Auger electron. The experimental investigation of three fragmentation paths leading to the C(2)H(+)/H(+), C(2)(+)/H(+) and C(+)/H(+) pairs has been complemented by theoretical calculations of the Potential Energy Surfaces (PES). It is found that when the amount of internal energy of the dication increases this is mainly transferred into the kinetic energy of the fragments of the second step of the dissociation.     

Interatomic Coulombic decay in mixed NeKr clusters

We report the occurrence of interatomic Coulombic decay (ICD) in mixed NeKr clusters. A well-defined feature ranging from 9 to 12 eV in kinetic energy is observed in coincidence with the Ne 2s photoelectrons. It derives from an ICD process, in which an initial Ne 2s vacancy is filled by a Ne 2p electron and an electron is emitted from a 4p level on a neighboring Kr atom. We have studied the dependence of the effect on photon energy, cluster composition, and cluster size. Interestingly, the ICD electron energy increases slightly and grows a shoulder on going from 2% to 5% Kr in the coexpansion process, which we interpret in terms of surface versus bulk effects.     

Molecular effects on the angular distribution parameter from the photoelectron spectra of the 3d orbitals of Kr,HBr, Br2 and CH3Br using synchrotron radiation

The angular distribution parameter, β, has been measured as a function of photon energy from 3 to 40 cV above the ionization threshold for the 3d orbital in Kr, HBr, Br₂ and CH₃Br. A chemical effect was observed over the range of photoelectron energies from ≈ 5 to 15 cV.     

Application of positron annihilation induced Auger Electron Spectroscopy to the study of surface chemistry

Positron annihilation induced Auger Electron Spectroscopy (PAES), makes use a beam of low energy positrons to excite Auger transitions by annihilating core electrons. This novel mechanism provides PAES with a number of unique features which distinguishes it from other methods of surface analysis. In PAES the very large collisionally induced secondary electron background which is present under the low energy Auger peaks using conventional tecniques can be eliminated by using a positron beam whose energy is below the range of Auger electron energies. In addition, PAES is more surface selective than conventional Auger Spectroscopy because the PAES signal originates almost exclusively from the topmost atomic layer due to the fact that the positrons annihilating with the core electrons are trapped in an image correlation well just outside the surface. In this paper, recent applications of Positron Annihilation Induced Auger Electron Spectroscopy (PAES) to the study of surface structure and surface chemistry will be discussed including studies of the growth, alloying and inter-diffusion of ultrathin layers of metals, metals on semiconductors, and semiconductors on semiconductors. In addition, the possibilities for future application of PAES to the study of catalysis and surface chemistry will be outlined.     

Detailed analysis of the L3M4,5M4,5; 1G4 Auger transition in atomic zinc: Improved Auger energy calculations

A hitherto neglected relaxation term is introduced in the calculation of Auger electron energies. The agreement between the theoretical L₃M_4,5M_4,5; ¹G₄ Auger energy in atomic zinc and the experimental value of Aksela and Aksela is excellent. Relaxation energies are determined by means of OHFS hole state calculations.     

VUV photon induced fluorescence study of SF5CF3

The interaction of SF(5)CF(3) with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SF(5)CF(3) were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CF(3)X (X = H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF(3) and CF(2) fragments. The threshold for the CF(3) emission is 10.2 +/- 0.2 eV, giving an upper limit estimate for the SF(5)-CF(3) bond dissociation energy of 3.9 +/- 0.3 eV. The excitation functions of the CF(3) and CF(2) emissions were measured in the photon energy range 13.6-27.0 eV. The resonant structures observed in SF(5)CF(3) are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CF(3)X molecules. The photoabsorption spectrum of SF(5)CF(3) shows features at the same energies, indicating a strong contribution from Rydberg excitations.