Molecular Auger Spectra Research Articles

Ab initio treatment of molecular Coster-Kronig decay using complex-scaled equation-of-motion coupled-cluster theory.

Vacancies in the L1 shell of atoms and molecules can decay non-radiatively via Coster-Kronig decay whereby the vacancy is filled by an electron from the L2,3 shell while a second electron is emitted into the ionization continuum. This process is akin to Auger decay, but in contrast to Auger electrons, Coster-Kronig electrons have rather low kinetic energies of less than 50 eV. In the present work, we extend recently introduced methods for the construction of molecular Auger spectra that are based on complex-scaled equation-of-motion coupled-cluster theory to Coster-Kronig decay. We compute ionization energies as well as total and partial decay widths for the 2s-1 states of argon and hydrogen sulfide and construct the L1L2,3M Coster-Kronig and L1MM Auger spectra of these species. Whereas our final spectra are in good agreement with the available experimental and theoretical data, substantial disagreements are found for various branching ratios suggesting that spin-orbit coupling makes a major impact on Coster-Kronig decay already in the third period of the periodic table.

The Auger spectrum of benzene.

We present an ab initio computational study of the Auger electron spectrum of benzene. Auger electron spectroscopy exploits the Auger-Meitner effect, and although it is established as an analytic technique, the theoretical modeling of molecular Auger spectra from first principles remains challenging. Here, we use coupled-cluster theory and equation-of-motion coupled-cluster theory combined with two approaches to describe the decaying nature of core-ionized states: (i) Feshbach-Fano resonance theory and (ii) the method of complex basis functions. The spectra computed with these two approaches are in excellent agreement with each other and also agree well with experimental Auger spectra of benzene. The Auger spectrum of benzene features two well-resolved peaks at Auger electron energies above 260eV, which correspond to final states with two electrons removed from the 1e1g and 3e2g highest occupied molecular orbitals. At lower Auger electron energies, the spectrum is less well resolved, and the peaks comprise multiple final states of the benzene dication. In line with theoretical considerations, singlet decay channels contribute more to the total Auger intensity than the corresponding triplet decay channels.

Auger electron angular distributions following excitation or ionization of the I 3d level in methyl iodide.

Auger electron spectra following excitation or ionization of the I 3d level in CH3I have been recorded with horizontally or vertically plane polarized synchrotron radiation. These spectra have enabled the Auger electron angular distributions, as characterized by the β parameter, to be determined. The I 3d photoionization partial cross section of CH3I has been calculated with the continuum multiple scattering approach, and the results show that in the photon energy range over which Auger spectra were measured, the I 3d cross section exhibits an atomic-like behavior and is dominated by transitions into the εf continuum channel. In this limit, the theoretical value of the alignment parameter (A20) characterizing the core ionized state in an atom becomes constant, independent of photon energy. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the β parameters extracted from our normal (non-resonant) molecular Auger spectra. The resulting anisotropy parameters for the M45N45N45 transitions in CH3I have been compared to those calculated for the corresponding transitions in xenon, and the experimental and theoretical results are in good agreement. Anisotropy parameters have also been measured for the M45N1N45, M45N23N45, and M45N45O23 transitions. For the M45N1N45 and M45N23N45 Auger decays in CH3I, the experimentally derived angular distributions do not exhibit the strong dependence on the final ionic state that is predicted for these transitions in xenon. Resonantly excited Auger spectra have been recorded at 620.4 and 632.0 eV, coinciding with the I 3d5/2 → σ* and 3d3/2 → σ* transitions, respectively. The resulting Auger electron angular distributions for the M4N45N45 and M5N45N45 decays were found to exhibit a higher anisotropy than those for the normal process. This is due to the larger photo-induced alignment in the neutral core excited state. For a particular Auger transition, the Auger electron kinetic energy measured in the resonantly excited spectrum is higher than that in the normal spectrum. This shift, due to the screening provided by the electron excited into the σ* orbital, has been rationalized by calculating orbital ionization energies of I 3d excited and I 3d ionized states in CH3I.

Conformational and nuclear dynamics effects in molecular Auger spectra: fluorine core-hole decay in CF4

In a molecular Auger spectrum information on the decaying state is implicitly ensemble-averaged. For a repulsive core-ionized state, for example, contributions from all parts of its potential curve are superimposed in the Auger spectrum. Using carbon tetrafluoride (CF4, tetrafluoromethane), we demonstrate for the first time that these contributions can be disentangled by recording photoelectron–Auger electron coincidence spectra with high energy resolution. For the F K-VV spectrum of CF4, there are significant differences in the Auger decay at different intermediate state (single core hole) geometries. With the help of calculations, we show that these differences result primarily from zero-point fluctuations in the neutral molecular ground state, but are amplified by the nuclear dynamics during Auger decay.

Theoretical molecular Auger spectra with electron population analysis

An approximation method is proposed to simulate the molecular Auger spectra. Auger transition rates are estimated with atomic populations of valence orbitals on an excited atom by Mulliken and Löwdin population analyses. Normal Auger energies and relative rates for H 2O and NH 3 molecules are evaluated using a full CI wave functions among the valence orbitals constructed from SCF orbitals for the neutral ground state and for the core-hole excited state. Theoretical spectra show fairly good correspondence with the experimental spectra. It is demonstrated that the present approach is applicable for qualitative assignment and analysis of Auger spectra for larger molecules.

Double-ionization satellites in the K-VV Auger spectra of neon-like molecules

The processes of monopole double ionization are investigated in the K-VV Auger spectra of the neon-like molecules (HF, H2O, NH3 , CH4 ). The spectra are calculated with simultaneous inclusion of multiplet splitting, final-state mixing and contributions of the ligands atomic orbitals to the molecular Auger transition amplitudes. The calculated relative integral intensities of the shake-off and shake-up spectator satellites are 18.2-20.5% of the total intensities of molecular Auger spectra. Taking the shake satellites into account allows us to assign some features of molecular Auger spectra.

Theoretical investigations of molecular triple ionization spectra

Triple ionization of molecular systems is investigated theoretically by means of the three-particle propagator. This enables us to efficiently calculate the very dense triple ionization spectra. To be able to interpret these spectra an atomic three-hole population analysis is developed which provides information about the charge distribution in the molecular trication. In exemplary applications on CO and a series of fluorides the use of the approach is demonstrated. A large number of triply ionized electronic states are energetically accessible in the available particle impact ionization experiments and it is shown that many of these states contribute to the observations. Triply ionized states are also produced by Auger decay. In particular, shake-off satellite lines of molecular Auger spectra can be reproduced using the triple ionization energies from the propagator calculation and an estimate of the transition rates based on the three-hole population analysis. In general a dramatically growing complexity of the triple ionization spectra with increasing molecular size is demonstrated. In spite of this complexity the three-hole population analysis is of valuable help for the interpretation of the spectra and often a simple picture in terms of various hole-localization patterns arises.

On the calculation of shake-off satellite contributions to molecular Auger spectra

An efficient approach to calculate shake-off satellite contributions to molecular Auger spectra is presented. The vertical ab initio energies of the transition to the triply ionized final states of the shake-off process are calculated by a Green's function method. An estimate of the Auger transition rates based on an analysis of the hole localization in the final states is used for a theoretical simulation of the spectrum. The method is applied to the Auger spectrum of LiF.

Ab initio calculation of transition rates for autoionization: the Auger spectra of HF and F −

A method for the quantitative computation angular integrated molecular Auger spectra is proposed. It combines quantum chemistry procedures for the description of the bound states involved with scattering theory for the representation of the scattering states. The bound states are described by configuration-interaction (CI) wavefunctions; the scattering problem is solved by a Green operator formalism. The method is illustrated by computing the Auger spectra of hydrogen fluoride and because we want to investigate the molecular effects, the isoelectronic fluoride ion.

Auger, zero-energy photoelectron, coincidence spectroscopy: Chemical-site-selective Auger electron spectroscopy

The Auger electron spectrum associated with decay of a core-hole on the terminal nitrogen and that associated with the central nitrogen of nitrous oxide, N 2O, are obtained individually through the use of a coincidence technique. Specifically, each of the two Auger electron spectra is obtained by detection of Auger electrons in coincidence with near zero energy (threshold) photoelectrons at the respective ionization thresholds. These zero energy electrons serve to identify the core-ionization continuum associated with the different Auger electrons. The salient features of the experimental spectra are in good agreement with theoretical calculations. The low counting rate generally associated with coincidence experiments, especially in the gas phase, is not encountered because the low energy electrons are collected over a 4π solid angle. Also, velocity discrimination is accomplished by a spatial filter rather than by time-of-flight to utilize the maximum duty cycle of the synchrotron source. These data are believed to be the first examples of chemical-site-selective molecular Auger spectra.

X-ray and Auger de-excitation processes in resonantly-excited and core-ionized molecules

Some recent developments in the understanding of molecular Auger and X-ray spectra are reviewed. Factors leading to complex molecular Auger spectra are discussed. The importance of correlation satellites in some spectra, such as for the SiF 4 molecule, are reported. The prediction of inner-shell photoelectron linewidths and the discrepancies with experiment for silicon-containing molecules are reported. The strong dependence of resonant X-ray emission spectra on photon excitation energy is discussed in the context of the CO 2 and N 2O molecules.

Changes in molecular KLL Auger spectrum of fluorine on going from HF to LiF, NaF, and KF

The molecular KLL Auger spectra of fluorine have been measured from HF, LiF, Naf and KF. The change in the ionic character of the chemical bond is clearly seen as narrowing of the KL 2,3L 2,3 structure on going from HF to KF. Theoretical calculations using the multiconfigurational CASSCF method are also found to predict well these changes. Comparison with the calculated KLL Auger spectrum of the fluorine ion F − verifies the conclusions.

Theoretical simulation of molecular Auger spectra: The carbon and oxygen KLL Auger spectra of formaldehyde

A theoretical ab initio simulation of the carbon and oxygen KLL Auger spectra of formaldehyde is presented and discussed. The effects of nuclear vibrational motion on the energy position and broadening of the Auger peaks in the two very different spectra are explicitly accounted for using a method derived from the time-dependent theory of the nuclear dynamics of decaying states. The underlying vertical double ionization spectrum, comprising hundreds of relevant electronic states, is computed using Green’s function methods, while charge distribution effects and Auger intensities are estimated via a two-hole population analysis of the eigenstates. The resulting theoretical spectra reproduce accurately the experimental band shapes and positions, showing that the observed spectra are complex convolutions of a very large number of transitions, with strong correlation and intensity redistribution effects. The nuclear motion analysis is found to be particularly important for the correct reproduction of the spectra, accounting very well for the substantial differences in energy shifts and band widths in the two spectra.

Method of calculation of molecular Auger spectra based on one-centre decomposition of MO LCAO

A method of calculating molecular Auger spectra is developed. The method is based on the use of a basis of non-coupling momenta for Auger transition final states, one-centre decomposition of the valence MOs, and one-centre calculation of Auger electron wavefunctions. The fluorine KVV spectrum of HF and the oxygen KVV spectrum of CO are calculated. Good agreement both with experiment and with previous calculations is obtained. The role of ligand electron density in calculation of Auger probabilities is discussed.

Double ionization satellites in KVV Auger spectra of the HF and CO molecules

Within the framework of molecular Auger spectra calculation based on a non-coupling momenta basis and on one-centre expansion of MO LCAOs, the energies and the absolute values of double ionization satellite intensities are calculated for KVV spectra of the HF and CO molecules.

Spin-orbit coupling in molecular Auger spectra: propensity rules tested for water

Electronic spectra of dicationic molecules comprise a high density of singlet and triplet states. By virtue of simple propensity rules only the singlet states are selected by core-valence (CVV) Auger spectroscopy. The authors use second-order response theory to explore the role of spin-orbit interaction in the manifold of low-lying dicationic states and its implications on the intensity analysis of the Auger spectrum of water.

A qualitative model for hybridization and bonding effects in molecular auger spectra

A four-level model for describing the molecular Auger spectra of first-row atoms in an appropriate surrounding is suggested. The model is used to reproduce the hybridization effects visible in the carbon Auger spectra of CH 4, C 2H 4 and C 2H 2. Furthermore, hybridization and bonding effects in the sequence CH 4, (CH 3) 2O, Si(CH 3) 4, Cl and NH 3 are interpreted.

Theoretical interpretation of molecular Auger spectra

Some recent developments in understanding molecular Auger spectra are presented. Methods for the calculation of transition energies and intensities are considered with particular emphasis on semiempirical approaches for complex polyatomic molecules. Participator and spectator resonant Auger effects, localisation effects, especially for the HF and CF 4 molecules and Auger spectra from two like atoms in different chemical environments, notably CH 3CN and CH 3NC, are addressed.

ChemInform Abstract: Qualitative Propagator Theory of CH3CN, CH3NC, and CH3CCH Auger Spectra.

A nonempirical two‐electron propagator is employed in the characterization of molecular Auger spectra. Based on the Mulliken approximation for many‐center Coulomb integrals, the model Hamiltonian requires three parameters for each valence atomic orbital: an exponent for a Slater function, an orbital energy, and an electron–electron repulsion integral. All of these quantities are taken from results of atomic calculations. Certain adjustments of resonance integrals are made to improve agreement with ab initio calculations of orbital energies. Forms of approximate two‐electron propagators are discussed, with an emphasis on qualitative factors that assist in interpreting the results. Important orbital energy differences and electron repulsion integrals that govern final state configuration mixing are identified. Intensities for various final states are separated according to the atoms on which the Auger process is initiated for CH3CN, CH3NC, and CH3CCH. Information about local bonding environments is gathered...

Qualitative propagator theory of CH3CN, CH3NC, and CH3CCH Auger spectra

A nonempirical two-electron propagator is employed in the characterization of molecular Auger spectra. Based on the Mulliken approximation for many-center Coulomb integrals, the model Hamiltonian requires three parameters for each valence atomic orbital: an exponent for a Slater function, an orbital energy, and an electron–electron repulsion integral. All of these quantities are taken from results of atomic calculations. Certain adjustments of resonance integrals are made to improve agreement with ab initio calculations of orbital energies. Forms of approximate two-electron propagators are discussed, with an emphasis on qualitative factors that assist in interpreting the results. Important orbital energy differences and electron repulsion integrals that govern final state configuration mixing are identified. Intensities for various final states are separated according to the atoms on which the Auger process is initiated for CH3CN, CH3NC, and CH3CCH. Information about local bonding environments is gathered from the calculations, especially spectral features that correspond to sp3 and sp hybridization schemes. Comparisons to the experimental CH3CN spectra are made; the theory helps to interpret the major spectral features. Regularities in the theoretical results are explained in terms of functional group molecular orbitals and electron repulsions.