Multielectron Excitations Research Articles

Strong-field ionization of sputtered molecules for biomolecular imaging

Photoionization of molecules sputtered from molecular thin films has been achieved using high field 125 fs pulses in the mid-IR spectral range. Using several model systems, we show that it is possible to significantly reduce molecular fragmentation induced by the laser field by increasing the photoionization wavelength. By examining the photoionization spectra as a function of wavelength, it is apparent that the photoionization mechanism is changing from a non-adiabatic multi-electron excitation process to a process that involves tunnel ionization. The results of these observations are discussed in terms of their significance for bioimaging with focused ion beams and mass-spectrometry.

Ab initio theory and calculations of X-ray spectra

Abstract There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multi-electron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye–Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes. To cite this article: J.J. Rehr et al., C. R. Physique 10 (2009).

Measurement of x-ray multielectron photoexcitations at theI− Kedge

We report the direct measurement by x-ray-absorption spectroscopy of the complete spectrum of multielectron excitations at the ${\text{I}}^{\ensuremath{-}}\text{ }K$ edge. The photoabsorption cross section of crystalline tetramethylammonium iodide was measured at 28, 50, and 150 K, and room temperature. The latter spectrum has been found to be largely dominated by the opening of multielectron excitation channels, thus providing a very good model of the ${\text{I}}^{\ensuremath{-}}$ atomic background which can be used in the analysis of ionic iodine compounds. Features due to the simultaneous excitations $1s5p$, $1s5s$, $1s4d$, $1s4p$, $1s4s$, $1s3d$, and $1s3p$ have been recognized, and their identification is supported by previous experimental and theoretical determinations on Xe and HI.

X-ray spectra induced by slow highly charged Ar q+ ions in collision with Nb surface

The X-ray spectra of Nb surface induced by Arq+ (q = 16,17) ions with the energy range from 10 to 20 keV/q were studied by the optical spectrum technology. The experimental results indicate that the multi-electron excitation occurred as a highly charged Ar16+ ion was neutralized below the metal surface. The K shell electron of Ar16+ was excited and then de-excited cascadly to emit K X-ray. The intensity of the X-ray emitted from K shell of the hollow Ar atom decreased with the increase of projectile kinetic energy. The intensity of the X-ray emitted from L shell of the target atom Nb increased with the increase of projectile kinetic energy. The X-ray yield of Ar17+ is three magnitude orders larger than that of Ar16+.

Sequestration of Sr(II) by calcium oxalate—A batch uptake study and EXAFS analysis of model compounds and reaction products

Calcium oxalate monohydrate (CaC 2O 4·H 2O—abbreviated as CaOx) is produced by two-thirds of all plant families, comprising up to 80 wt.% of the plant tissue and found in many surface environments. It is unclear, however, how CaOx in plants and soils interacts with metal ions and possibly sequesters them. This study examines the speciation of Sr(II) aq following its reaction with CaOx. Batch uptake experiments were conducted over the pH range 4–10, with initial Sr solution concentrations, [Sr] aq, ranging from 1 × 10 −4 to 1 × 10 −3 M and ionic strengths ranging of 0.001–0.1 M, using NaCl as the background electrolyte. Experimental results indicate that Sr uptake is independent of pH and ionic strength over these ranges. After exposure of CaOx to Sr aq for two days, the solution Ca concentration, [Ca] aq, increased for all samples relative to the control CaOx suspension (with no Sr added). The amount of Sr aq removed from solution was nearly equal to the total [Ca] aq after exposure of CaOx to Sr. These results suggest that nearly 90% of the Sr is removed from solution to a solid phase as Ca is released into solution. We suggest that the other 10% is sequestered through surface adsorption on a solid phase, although we have no direct evidence for this. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the molecular-level speciation of Sr in the reaction products. Deconvolutions of the Sr K-edge EXAFS spectra were performed to identify multi-electron excitation (MEE) features. MEE effects were found to give rise to low-frequency peaks in the Fourier transform before the first shell of oxygen atoms and do not affect EXAFS fitting results. Because of potential problems caused by asymmetric distributions of Sr–O distances when fitting Sr K-edge EXAFS data using the standard harmonic model, we also employed a cumulant expansion model and an asymmetric analytical model to account for anharmonic effects in the EXAFS data. For Sr-bearing phases with low to moderate first-shell (Sr–O pair correlation) anharmonicity, the cumulant expansion model is sufficient for EXAFS fitting; however, for higher degrees of anharmonicity, an analytical model is required. Based on batch uptake results and EXAFS analyses of reaction products, we conclude that Sr is dominantly sequestered by a solid phase at the CaOx surface, likely the result of a dissolution–reprecipitation mechanism, to form SrC 2O 4 of mixed hydration state (i.e. SrOx· nH 2O, where n = 0, 1, or 2). Surprisingly, no spectroscopic or XRD evidence was found for a (Sr,Ca)Ox solid solution or for a separate SrCO 3 phase. In addition, we found no evidence for Sr(II) inner-sphere sorption complexes on CaOx surfaces based on lack of Sr–Ca second-neighbor pair correlations in the EXAFS spectra, although some type of Sr(II) surface complex (perhaps a type B Sr-oxalate ternary complex or an outer-sphere Sr(II) complex) or some as yet undetected Sr-bearing solid phases are needed to account for approximately 10% of Sr uptake by CaOx. The formation of a hydrated SrOx phase in environments under conditions similar to those of our experiments should retard Sr mobility and could be a significant factor in the biogeochemical cycling of Sr in soils and sediments or in plants and plant litter where CaOx is present.

Influence of multi-electron excitation on EXAFS spectroscopy of trivalent rare-earth ions and elucidation of change in hydration number through the series

We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L 3 edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La 3+ -Tb 3+ between 5-7 Å−1. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE 3+ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE 3+ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10 −3 Å 2 for La 3+ to 7.9, 2.31 Å, and 5.7 × 10 −3 Å 2 for Lu 3+ . These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE 3+ ions are inferred to change from the nonahydrated structure for La 3+ -Nd 3+ to the octahydrated structure for Tb 3+ -Lu 3+ through intermediate structures for Sm 3+ , Eu 3+ , and Gd 3+ . In addition, the hydration state of Y 3+ closely resembles that of Ho 3+ because the two have almost identical ionic radii.

X-ray absorption coefficient of iodine in the K edge region

X-ray absorption spectroscopy of iodine vapour is used for absolute determination of the absorption coefficient of iodine within a region of 4.3 keV around the K edge. The comparison with data in available compilations is given, together with the compressed form of the Victoreen asymptotics below and above the edge. The deviation from the asymptotics due to multielectron excitations and virtual processes immediately above the edge is discussed.

X-ray spectra produced by interaction of Ar16+ and Ar17+ with Zr

The 10—20qkeV Ar16+ and Ar17+ ions produced by SECRAL enter on metallic surface of Zr. In this interaction，the multi-electron excitation possibly occurred in the neutralization of the highly charged Ar16+ ions，which produced vacancy in the K shell. Electron of the high n state de-excited to K vacancy gives off X-ray. The experimental results show that X-ray intensities for the Ar hollow atom decrease with increase of incidence energy，and Lβ X-ray intensities of target atom Zr increase with increasing incidence energy. Kα X-ray yield per ion for Ar17+ was five orders of magnitude greater than that for Ar16+.

Many-pole model of inelastic losses in x-ray absorption spectra

Inelastic losses are crucial to a quantitative analysis of x-ray absorption spectra. However, current treatments are semi-phenomenological in nature. Here a first-principles, many-pole generalization of the plasmon-pole model is developed for improved calculations of inelastic losses. The method is based on the GW approximation for the self-energy and real space multiple scattering calculations of the dielectric function for a given system. The model retains the efficiency of the plasmonpole model and is applicable both to periodic and aperiodic materials over a wide energy range. The same many-pole model is applied to extended GW calculations of the quasiparticle spectral function. This yields estimates of multi-electron excitation effects, e.g., the many-body amplitude factor S 2 0 due to intrinsic losses. Illustrative calculations are compared with other GW calculations of the self-energy, the inelastic mean free path, and experimental x-ray absorption spectra.

Theory of the stopping of fast heavy highly charged structured ions in their collisions with complex atoms

We have developed a nonperturbative theory of the energy losses by fast heavy structured ions in their collisions with neutral complex atoms by taking into account all of the possible, including multiple, exci- tations and ionizations of both projectile and target. We have been able to achieve a significant simplification of the problem by considering multielectron targets and by restricting our analysis to highly charged structured ions with charges much larger than unity, when the characteristic size of the electron coat of the projectile ion is much smaller than that of the neutral target atom. The errors in the approximations used and in the calcula- tions of inelastic cross sections are estimated. We have derived formulas for the effective stopping similar to the well-known Bethe-Bloch formulas. To illustrate the contribution from multielectron excitations of the ion coat to the effective stopping, we make a comparison with the calculations based on the perturbation theory. The energy losses of U q + (10 ≤ q ≤ 70) ions in their collisions with argon atoms and the energy losses of Pb and Bi ions on several targets have been calculated. A comparison with experiment is made. PACS numbers: 34.50.Bw

Attosecond real-time observation of electron tunnelling in atoms

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

Control of Giant Breathing Motion inC60with Temporally Shaped Laser Pulses

Femtosecond laser pulses tailored with closed-loop, optimal control feedback were used to excite oscillations in C60 with large amplitude by coherent heating of nuclear motion. A characteristic pulse sequence results in significant enhancement of C2 evaporation, a typical energy loss channel of vibrationally hot C60. The separation between subsequent pulses in combination with complementary two-color pump-probe data and time-dependent density functional theory calculations give direct information on the multielectron excitation via the t(1g) resonance followed by efficient coupling to the radial symmetric a(g)(1) breathing mode.

Application of R matrix/MQDT method to valence and core excitations in NO

Both valence and core excitations of the NO molecule are studied using the R matrix/multi-channel quantum defect theory (MQDT) approach. In the case of valence excitation, the quantum defects and the coupling constant between pπ-Rydberg state and 2Π state of NO were evaluated. The theoretical quantum defects for 2Σ+, 2Π and 2Δ are in good agreement with the experimental values. The theoretical coupling constants not only for B(22Π) but also for L(32Π) are in good agreement with the experimental values. The same approach with some improvements to calculate the R matrix part is used to study the core excitation of NO. The potential curves above the N 1s ionization threshold of NO were calculated. Five potential energy curves responsible for the resonances around 412–414 eV are found. The peak at 412 eV in the absorption spectra of NO may possibly be assigned to the resonances whose character is σ⋆ excitation. On the other hand, the peak at 414 eV in the spectra may possibly be assigned to the resonance whose character is a mixture of multi-electron excitations and σ⋆ excitation.

X-ray emission produced by interaction of highly ionized Arq+ ions with metallic targets

This paper studies the X-ray spectra produced by the interaction of highly charged ions of Arq+(q=16,17,18) with metallic surface of Be, Al, Ni, Mo and Au respectively. The experimental results show that the Kα X-ray emerges from under the surface of solid in the interaction of ions with targets. The multi-electron excitation occurred in the process neutralization of the Ar16+ in electronic configuration of 1s2 in metallic surfaces, which produces vacancy in the K shell. Electron from high n state transition to K vacancy gives off X-ray. We find that there is no obvious relation between the shape of X-ray spectra and the different targets. The X-ray yield of incident ions are associated with initial electronic configuration. The X-ray yield of target is related to the kinetic energy of the incidentions.

X-ray absorption in atomic Cd in the K-edge region

The X-ray absorption in the K-edge region is measured on Cd vapor in a sealed high-temperature cell. The absorption spectrum, free of structural signal, represents atomic absorption of the element, with fingerprints of multielectron excitations visible after natural-width deconvolution. Absolute values of the atomic photoabsorption coefficient are obtained from renormalization with Cd foil data far from the edge. Possible sources of systematic error in synchrotron radiation measurement of absolute absorption are discussed.

Excitation dynamics of Rydberg states in C60

The electron and nuclear dynamics of C60 fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C60. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (hu) → LUMO+1 (t1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100 fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C60 to short-pulse laser radiation.

Study of Auger decay process following multielectron excitation accompanying F 1s photoionization of CF 4

Auger decay process following multielectron excitation accompanying F 1s photoionization of CF 4 molecule has been studied in the excitation energy range between 700 and 750 eV using angle-resolved Auger electron spectroscopy. The observed spectra have shown an enhancement in the specific band, at an electron kinetic energy of ∼ 644 eV, only in the spectrum recorded at 710 eV excitation energy of horizontally polarized light. This feature is discussed in terms of the resonant Auger emission originated from the doubly excited state. The evolution of spectral shape has also exhibited the excitation energy dependence above the threshold of multiple excitation. The origin of the spectral variation is considered due to gradual growth of the satellite Auger lines originated from the multielectron excitation.

Contribution of multielectron excitation to F 1s photoabsorption process in CaF 2 studied by soft X-ray absorption and emission spectroscopy

The photoexcited F K α X-ray intensity’s evolution was measured for CaF 2 with fine energy-step from the [1s2p] double excitation threshold to the saturation region, where square brackets indicate hole states. The observed evolution was directly compared with the X-ray absorption spectrum, and the contribution of multielectron excitation to the absorption spectrum was successfully resolved. The F-K fluorescence XANES spectra were calculated based on the coherent second-order optical processes for comparison.

Continuous Cauchy wavelet transform of XAFS spectra

The continuous Cauchy wavelet transform (CCWT) is applied to the analysis of XAFS spectra. Thanks to that method, XANES and EXAFS signals can be visualized in three-dimensions: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R′) and the CCWT modulus (corresponding to the continuous decomposition of the amplitude terms). Applied to EXAFS spectra, the CCWT analysis provides straightforward qualitative information related to the k-range of each “R′-EXAFS” contribution. Such information is particularly useful to perform next nearest-neighbors identification, despite the presence of spectral artifacts such as multiple-scattering features, multi-electronic excitations or noise. When applied to XANES spectra, the CCWT analysis helps highly to measure the “spectral limit” between XANES and EXAFS regions, as well as the energy range required to model properly next-nearest neighbors. To further illustrate the potential of CCWT analyses applied to XAFS spectra, we present examples related to: (1) a XANES spectrum collected at the Ti K-edge for titanite (CaTiSiO5); (2) an experimental Au LIII-edge EXAFS spectrum for gold sorbed on goethite (FeO(OH)).

Line shapes and satellites in high-resolution x-ray photoelectron spectra of large π-conjugated organic molecules

We present a high-resolution C1s and O1 s x-ray photoemission (XPS) study for condensed films of pi-conjugated organic molecules, namely, of the anhydrides 3,4,9,10-perylene-tetracarboxylic acid dianhydride, 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride, 1,8-naphthalene dicarboxylic acid anhydride, and benzoperylene-(1,8)-dicarboxylic acid anhydride as well as the quinoic acenaphthenequinone. Although the functional groups are identical for the anhydrides, the molecules show very different photoemission fine structure thus providing a detailed fingerprint. A simultaneous peak fit analysis of the XPS spectra of all molecules allows to consistently determine the ionization potentials of all chemically different carbon and oxygen atoms. Additional structures in the C1s and O1s spectra are interpreted as shakeup satellites and assigned with the help of singles and doubles configuration interaction calculations. These satellites provide further information on multielectron excitations and must be taken into account for quantitative investigations.