Resonant Auger Electron Spectra Research Articles

Experimental and theoretical investigation of the Auger electron spectra of isothiocyanic acid, HNCS.

We investigate isothiocyanic acid, HNCS, by resonant and nonresonant Auger electron spectroscopy at the K-edge of carbon and nitrogen, and the L2,3-edge of sulfur, employing soft X-ray synchrotron radiation. The C1s and N1s ionization energies as well as the S2s and S2p ionization energies are determined and X-ray absorption spectra reveal the transitions from the core to the virtual orbitals. Final states for all normal Auger electron spectra and the resonant ones recorded at the carbon and nitrogen edge are assigned and rationalized with theoretical spectra obtained with a wave-function based protocol. The latter is based on ab initio configuration-interaction representations of the bound part of the electronic wave functions, the one-center approximation for Auger intensities, and a moment theory for band shapes. The computed spectra are in very good agreement with the experimental data and most of the relevant signals are assigned. The double ionization energy is derived from the S2p1/2 spectrum and in good agreement with a recently determined value. The Auger electron spectra are compared with those of the congener HNCO. A similar shape of the normal Auger electron spectra was found for the low binding energy final states, while intensities differed. Similarities are less pronounced in the resonant Auger electron spectra.

X-ray induced fragmentation of fulminic acid, HCNO.

The fragmentation of fulminic acid, HCNO, after excitation and ionization of core electrons was investigated using Auger-electron-photoion coincidence spectroscopy. A considerable degree of site-selectivity is observed. Ionization of the carbon and oxygen 1s electron leads to around 70% CH+ + NO+, while ionization at the central N-atom produces only 37% CH+ + NO+, but preferentially forms O+ + HCN+ and O+ + CN+. The mass-selected Auger-electron spectra show that these fragments are associated with higher binding energy final states. Furthermore, ionization of the C 1s electron leads to a higher propensity for C-H bond fission compared to O 1s ionization. Following resonant Auger-Meitner decay after 1s → 3π excitation, 12 different ionic products are formed. At the C 1s edge, the parent ion HCNO+ is significantly more stable compared to the other two edges, which we also attribute to the higher contribution of final states with low binding energies in the C 1s resonant Auger electron spectra.

The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission.

ConspectusThe liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle (NP) suspensions, since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on NPs dispersed in water, offering a unique opportunity to access the solid-electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid-water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode-water system and will be reviewed briefly. The situation is different for the NP-water system. Our experiments imply that the transition-metal oxide (TMO) NPs used in our studies reside close enough to the solution-vacuum interface that electrons emitted from the NP-solution interface (and from the NP interior) can be detected.We were specifically exploring aqueous-phase TMO NPs that have a high potential for (photo)electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquid-microjet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP(aq) system the TMO surface is in contact with a true extended bulk electrolyte solution rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP-water interactions can be uniquely investigated as a function of pH and provides an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface.The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signals via resonant photoemission, so-called partial electron yield X-ray absorption (PEY-XA) spectra, and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes and the associated ultrafast electronic relaxations for determining charge transfer or electron delocalization times, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.

Auger electron spectroscopy of fulminic acid, HCNO: an experimental and theoretical study.

HCNO is a molecule of considerable astrochemical interest as a precursor to prebiotic molecules. It is synthesized by preparative pyrolysis and is unstable at room temperature. Here, we investigate its spectroscopy in the soft X-ray regime at the C 1s, N 1s and O 1s edges. All 1s ionization energies are reported and X-ray absorption spectra reveal the transitions from the 1s to the π* state. Resonant and normal Auger electron spectra for the decay of the core hole states are recorded in a hemispherical analyzer. An assignment of the experimental spectra is provided with the aid of theoretical counterparts. The latter are using a valence configuration interaction representation of the intermediate and final state energies and wavefunctions, the one-center approximation for transition rates and band shapes according to the moment theory. The computed spectra are in very good agreement with the experimental data and most of the relevant bands are assigned. Additionally, we present a simple approach to estimate relative Auger transition rates on the basis of a minimal basis representation of the molecular orbitals. We demonstrate that this provides a qualitatively good and reliable estimate for several signals in the normal and resonant Auger electron spectra which have significantly different intensities in the decay of the three core holes.

Multi-reference protocol for (auto)ionization spectra: Application to molecules.

We present the application of the spherically averaged continuum model to the evaluation of molecular photoelectron and resonant Auger electron spectra. In this model, the continuum wave function is obtained in a numerically efficient way by solving the radial Schrödinger equation with a spherically averaged molecular potential. Different approximations to the Auger transition matrix element and, in particular, the one-center approximation are thoroughly tested against experimental data for the CH4, O2, NO2, and pyrimidine molecules. In general, this approach appears to estimate the shape of the photoelectron and autoionization spectra as well as the total Auger decay rates with reasonable accuracy, allowing for the interpretation of experimental results.

Normal and resonant Auger spectroscopy of isocyanic acid, HNCO.

In this paper, we investigate HNCO by resonant and nonresonant Auger electron spectroscopy at the K-edges of carbon, nitrogen, and oxygen, employing soft X-ray synchrotron radiation. In comparison with the isosteric but linear CO2 molecule, spectra of the bent HNCO molecule are similar but more complex due to its reduced symmetry, wherein the degeneracy of the π-orbitals is lifted. Resonant Auger electron spectra are presented at different photon energies over the first core-excited 1s → 10a' resonance. All Auger electron spectra are assigned based on ab initio configuration interaction computations combined with the one-center approximation for Auger intensities and moment theory to consider vibrational motion. The calculated spectra were scaled by a newly introduced energy scaling factor, and generally, good agreement is found between experiment and theory for normal as well as resonant Auger electron spectra. A comparison of resonant Auger spectra with nonresonant Auger structures shows a slight broadening as well as a shift of the former spectra between -8 and -9 eV due to the spectating electron. Since HNCO is a small molecule and contains the four most abundant atoms of organic molecules, the reported Auger electron decay spectra will provide a benchmark for further theoretical approaches in the computation of core electron spectra.

Site-dependent Si KL23L23 resonant Auger electron spectra following inner-shell excitation of Cl3SiSi(CH3)3

Spectator resonant Auger electron spectra with the Si 1s photoexcitation of Cl3SiSi(CH3)3 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a Si 1s electron on the Cl-side into a vacant valence orbital, generates the resonant Auger decay in which the excited electron remains in this valence orbital. Photoexcitation of 1s electrons into some Rydberg orbitals induces Auger shake-down transitions, because higher-lying Rydberg orbitals in the two Si atoms closely positioned hold spatially overlapping considerably. A broad TIY peak slightly above the 1s ionization thresholds appreciably yields resonant Auger decays in which a slow photoelectron is re-captured into a higher-lying Rydberg orbital. The normal Auger peak shape at this photon energy is distorted due to a post-collision interaction effect. These findings provide a clear understanding on properties of the excited orbitals which are ambiguous in the measurement of the TIY only.

A variety of characteristic behaviour of resonant KL23L23 Auger decays following Si K-shell photoexcitation of SiCl4

Spectator resonant Auger electron spectra with the Si 1s photoexcitation of SiCl4 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a 1s electron into the 9t2 valence orbital, generates the resonant Auger decay in which the excited electron remains in the 9t2 orbital. A TIY peak positioned slightly above the 1s ionization threshold induces Auger decay in which the slow photoelectron is re-captured into a higher lying Rydberg orbital or the normal Auger peak shape is distorted due to a post-collision interaction effect. Another structure above the threshold, originating from a doubly excited state, yields the normal Auger peak with the distortion of peak shape and a resonant Auger peak with a higher kinetic energy. These findings provide a clear understanding of the properties of the excited orbitals which were ambiguous previously.

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.

Resonant Auger decay of the core-excited C*O molecule in intense x-ray laser fields

The dynamics of the resonant Auger (RA) process of the core-excited C$^\ast$O(1s$^{-1}\pi^\ast,v_r=0$) molecule in an intense X-ray laser field is studied theoretically. The theoretical approach includes the analogue of the conical intersections of the complex potential energy surfaces of the ground and `dressed' resonant states due to intense X-ray pulses, taking into account the decay of the resonance and the direct photoionization of the ground state, both populating the same final ionic states coherently, as well as the direct photoionization of the resonance state itself. The light-induced non-adiabatic effect of the analogue of the conical intersections of the resulting complex potential energy surfaces gives rise to strong coupling between the electronic, vibrational and rotational degrees of freedom of the diatomic CO molecule. The interplay of the direct photoionization of the ground state and of the decay of the resonance increases dramatically with the field intensity. The coherent population of a final ionic state via both the direct photoionization and the resonant Auger decay channels induces strong interference effects with distinct patterns in the RA electron spectra. The individual impact of these physical processes on the total electron yield and on the CO$^+(A^2\Pi)$ electron spectrum are demonstrated.

Projection of excited orbitals into kinetic energies of emitted electrons in resonant SiKLLAuger decays of SiF4

Spectator resonant Auger-electron spectra have been measured in the Si $1s$ photoexcitation region of SiF${}_{4}$ using an electron spectroscopic technique combined with undulator radiation. A transition with the highest intensity in the total ion yield spectrum, which comes from excitation of a $1s$ electron into the $6{t}_{2}$ valence orbital, generates resonant Auger decays in which the excited electron remains predominantly in the valence orbital or is partly shaken up into a high-lying Rydberg orbital. The higher-lying peak generated through excitation into Rydberg orbitals induces resonant Auger decays in which the excited Rydberg electron is partly shaken up to a higher-lying Rydberg orbital or shaken down to a lower-lying valence molecular orbital. These findings exhibit a clear disentanglement effect among excited orbitals which are smeared out in the $1s$ electron excitation spectrum.

Strong interference effects in the angularly resolved Auger decay and fluorescence emission spectra of the core-excited NO molecule

Angular distributions of Auger electrons and subsequent fluorescence photons are studied theoretically and experimentally in the vicinity of the core excitations of NO. In the calculations, lifetime vibrational interference and electronic state interference were taken into account ab initio. The interference between excitation and deexcitation amplitudes for transitions via symmetry-different intermediate resonances 1s−12π2(2Δ, 2Σ±), which is forbidden in the solid-angle-averaged or magic-angle-recorded decay spectra, plays a crucial role in the formation of the angularly resolved decay spectra. Experimentally, angular distribution parameters for the NO+(A1Π → X1Σ+) fluorescence induced by linearly polarized synchrotron radiation are determined in the vicinity of the N*O resonance in the Raman regime for core excitation. Theoretical results are in good agreement with the present experimental fluorescence spectra and with the available vibrationally and angularly resolved resonant Auger electron spectra.

Large impact of the weak direct photoionization on the angularly resolved CO+(A 2Π) de-excitation spectra of the CO*(1σ−12π) resonance

Interference between the direct and resonant amplitudes for the population of the CO+(A 2Π) state in the vicinity of the O 1s → 2π excitation of CO* is studied by an ab initio theoretical approach. The weak direct photoionization induces Fano–Shore-type profiles resulting in long-range exciting-photon energy dependences of the computed angular distribution parameters for the CO+(A 2Π) photoelectrons, βeA(ω), and for the subsequent CO+(A 2Π → X 2Σ+) fluorescence, β2XA(ω). In the presence of direct photoionization, the lifetime vibrational interference causes substantial variations of the computed parameter βeA(ω) across the positions of the CO*(vr) vibronic states. Theoretical results are in qualitative agreement with the vibrationally and angularly resolved CO+(A 2Π) resonant Auger electron spectra recorded in the Raman regime at different exciting-photon energies across the CO* resonance.

Symmetry-resolved x-ray absorption fine structure and resonant Auger-spectator-electron decay study of O1s→Rydbergresonances inO2

A series of resonant Auger-electron spectra excited at selected photon energies across the O 1s -> Rydberg resonances in O-2 has been measured. Applying a spectator-electron shake relaxation model originally introduced for atomic resonant Auger-electron spectra, and using, to a large extent, the assignments for the core-excited resonances available from the literature, many of the observed spectator Auger-electron final states are assigned in terms of cationic Rydberg series. Vice versa the resonant Auger-electron spectra are used for consistency tests of the literature assignments for the core-excited intermediate Rydberg states. For some of the latter, alternative assignments are proposed for reasons discussed.

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s→σ* resonance: Experiment and theory

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.

Quenching and restoring of theAΠ2cationic state in resonant Auger electron spectra of CO in the vicinity of the O1s→2πresonance

The evolution of the vibrational intensity distribution of the singly ionized $A\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}$ state in CO is experimentally examined for photon energy detunings below the adiabatic 0-0 transition of the O $1s\ensuremath{\rightarrow}2\ensuremath{\pi}$ resonance. We have found a strong suppression of the entire vibrational fine structure of this state, leading to its almost complete quenching for certain excitation energies, followed by a partial restoring for larger values of negative photon energy detuning. Our observation, that cannot be rationalized by the known model of a vibrational collapse for energy detuning, may be explained in terms of a Fano interference between the direct and resonant photoionization channels in the presence of strong lifetime vibrational interference.

Isotope effects in H +(D +) desorption induced by 4a 1 ← O 1s resonant transition of condensed H 2O (D 2O)

Auger electron–photoion coincidence (AEPICO) experiments have been performed for condensed water (H 2O and D 2O) at the 4a 1 ← O 1s resonant transition. Several kinds of isotope effects were observed in H + (D +) desorption, they were, (1) the differences in ion kinetic energy for the selected Auger-final-states, (2) the relative intensity differences in AEPICO spectra, (3) peak shifts in AEPICO and resonant Auger electron spectra and (4) the intensity difference in prominent peak in ion yield spectra. Those isotope effects can be explained by a four-step ion desorption mechanism involving O–H (O–D) bond extension within the lifetime of the O 1s core-hole.

3D-ion-momentum/high-resolution-electron coincidence measurements

We have set up a coincidence apparatus consisting of a high-resolution electron spectrometer and a k-resolving ion spectrometer. As an example for measurements performed with this apparatus, we present ion mass spectra that are coincident with energy-resolved resonant Auger electrons emitted from F 1s excited SF 6 molecules and resonant Auger electron spectra that are coincident with mass-selected fragment ions, as a function of the detuning from the F(1s −1)a 1g ∗ resonance in SF 6. Significant changes of these spectra due to detuning suggest that a substantial fraction of the excess photon energy goes into fragmentation rather than into electron kinetic energy.

Application of an atomic relaxation model for the interpretation of O1s to Rydberg excited Auger electron spectra of molecular oxygen

Resonant and normal Auger electron spectra associated with an O1s initial vacancy of the O 2 molecule have been recorded using photon energies of 541.80, 541.97 and 650 eV. The resonant Auger spectra obtained at 541.80 and 541.97 eV are interpreted as composed essentially of two characteristic parts, one between 10 and 35 eV binding energy reminiscent to direct photoionisation, and another, above 35 eV, associated primarily with spectator decay of the intermediate core-hole state. The spectra above 35 eV owe some similarity to the ordinary Auger electron spectrum recorded at 650 eV, and an interpretation is made in terms of Rydberg series in the cation converging towards the dicationic ionisation limits. The interpretation is based on a relaxation model introduced earlier for atomic resonant Auger spectra. The assignments of the resonant spectra assume that the relevant intermediate neutral states at 541.80 and 541.97 eV governing the Auger decay are essentially the ν = 0 and 1 vibrational components of the (O1s) −1 ( 4 Σ u - ) 4p σ Rydberg state. Arguments for this interpretation are given.

Valence photoionization and resonant core excitation of ozone – experimental and theoretical study of the C̃-state of O 3+

Resonant Auger electron spectra of core excited O 3 ( O T 1s −12b 1 1) are presented for the first time. The photoionization valence spectrum with sample purity and resolution superior to those published is presented. The first direct experimental evidence for the C̃ 2 B 1 state of O 3 + is found. Ab initio calculations of the resonant Auger electron (RAE) spectrum have been performed supporting the assignment of the C̃-state.