Auger Transition Rates Research Articles

REMARKS ON RELATIONS BETWEEN X-RAY EMISSION AND AUGER TRANSITION RATES AND MOLECULAR PROPERTIES: CHEMICAL EFFECT

Relations are pointed out that there exist between (1) the X-ray emission rate and the π bond order and (2) the Auger transition rate and the valence electron density (chemical effects) resulting from C ls vacancy in carbon-containing molecules. Recognition of these relations is very important for analysis of experimental data and for prediction of these quantities, for which no data are available.

Core level photoionization mass spectroscopy of the fluoromethylsilanes, Si(CH 3) xF 4- x ( x = 0–4), around the Si 2p ionization edges

Total ion yield spectra, total electron yield spectra, photoionized mass spectra and mass resolved partial photoion yield spectra of the five fluoromethylsilane compounds, Si(CH 3) x F 4- x ( x = 0–4), measured around the Si 2p core ionization edges using monoc synchrotron radiation are reported. Photoionized mass spectra of the five compounds were also measured around the Si 2s and F 1s ionization edges. For each of the fluoromethylsilane compounds, total ion yield and total electron yield spectra are very similar to each other and to the corresponding photoabsorption spectra with the exception of the decreased intensities of the discrete resonances. The photoion mass spectra show that almost all possible molecular fragments are generated by excitation and ionization of the Si 2p, Si 2s and F 1s core electrons. Branching ratios of the photoion mass spectra peak areas indicate that the methyl groups are much more labile than the fluorine atoms at all photon energies, both below and above the core ionization edges, in the mixed fluoromethylsilane compounds. Relative areas of the methyl peaks in the photoion mass spectra are enhanced at the discrete resonances below the ionization threshold with more pronounced enhancements above the ionization threshold. An explanation for the greater lability of the methyl ligands is proposed based on the effect of the electron withdrawing and donating properties of the fluorine atoms and methyl groups which also affect the relative Auger transition rates and core hole lifetimes. Partial ion yields of the SiF + and SiMe + fragment ions also exhibit some specific enhancements at Si 2p→σ* resonances below the ionization edge.

Band shape and vibrational structure in Auger spectra: Theory and application to carbon monoxide

A time-dependent approach to Auger spectra is presented and used to derive simple working equations for computing the vibrational broadening and the vibrationally induced shift of the peaks in the spectrum. The formulas give the explicit dependence of the vibrational envelope on the local details of the electronic potential energy surfaces of the intermediate and final states, providing interesting general insights which we discuss in detail. It is shown that, in polyatomic molecules, relevant interaction terms among different nuclear modes arise. The theory applies as well to other processes which involve a core ionized or core excited intermediate state like, for instance, x-ray emission or resonant Auger decay. As a test application, the double ionization spectrum of CO is computed by the Green’s function method, and the new equations, together with a two-hole population analysis of the pole strengths, are used to obtain theoretical Auger spectra. The experimental spectral profiles, characteristically shaped by the varying vibrational broadening and substantial energy shifts, are accurately reproduced, giving most peak positions to within a few tenths of eV. The results present very different vibrational effects for the carbon and oxygen spectra, showing the general inadequacy of interpretations based on vertical transition energies only. Hole localization in the dicationic states is discussed in the light of the population analysis results and used to estimate the electronic Auger transition rates.

Auger transition rates and fluorescence yields for the double-K-hole state.

Auger and radiative transition rates and fluorescence yields for the double-{ital K}-hole state have been calculated for five elements with 10{le}{ital Z}{le}36 using the multiconfiguration Dirac-Fock method. The Auger and x-ray energies are found to shift to higher energies by 68--269 and 100--388 eV, respectively, due to the presence of the {ital K}-shell spectator vacancy. For low-{ital Z} elements, the transition rates per {ital K} hole for the double-{ital K}-hole state are much larger than the corresponding rates for the single-{ital K}-hole state. As {ital Z} increases to higher values, the differences in transition rates diminish. {ital K}-shell fluorescence yields for the double-{ital K}-hole state compared to the results for the single-{ital K}-hole state are found to increase by 33% at {ital Z}=10 but by only 2.6% at {ital Z}=36.

Interchannel interactions in highly energetic radiationless transitions of neonlike ions.

Relativistic K-LL Auger transition rates are presented for nine ions in the neon isoelectronic sequence up to uranium. We employ the intermediate-coupling scheme including interchannel interactions. For neutral neon a comparison with experimental data is given. We demonstrate that intercontinuum interactions result in a remarkable redistribution of individual transition rates even in highly energetic transitions.

A least-squares variational method for calculating continuum orbitals for Auger electrons

A variational least-squares technique is developed for calculating general non-spherical continuum orbitals for Auger electrons. The use of the continuum orbitals in obtaining Auger transition rates and the special cases of one-center and two-center systems are discussed.

Many dicationic states and two-hole population analysis as a bridge to Auger spectra: Strong localization phenomena in BF3

The outer valence double ionization spectrum of BF3 is investigated by computing 100 double ionization transitions with the Green’s function method. The results show that one-site and two-site hole localization on the fluorine atoms takes place to full extent and the energy split between clusters of states with similar hole distribution exceeds 10 eV. Double hole localization as a symmetry breaking phenomenon is discussed, pointing out the inherent inability of uncorrelated methods to describe it. A two-hole population analysis of the dicationic states for the quantitative measure of their localized character is presented. The strong effects of hole localization on Auger transition rates are discussed, showing that the two-hole population analysis can be used to develop a new statistical approach to the computation of Auger line shapes. Application of this method to the fluorine and boron KLL Auger spectra of BF3 permits, for the first time, an accurate reproduction of the spectral profiles and a complete interpretation of their features.

An investigation of the KL2,3 to L2,3L2,3 V Auger satellites of Mg and Al

The KL2.3 to L2.3L2.3 V Auger vacancy satellite transitions of Mg and Al, which lie to high kinetic energy of the KL2.3 V Auger profile, are analysed using a combination of self-consistent densities of states (DOS) and atomic-like Auger transition rate calculations. The transition rate calculations demonstrate that previous work based on statistical weight arguments is incorrect. The DOS calculations show that the valence s contribution to the local DOS forms a bound state in the presence of a double core-hole state, though the major contribution to the core-hole screening is provided by the local p DOS. The calculation successfully reproduces the spectral profile for the Al transitions, but is unable to account for one of the features in the Mg spectrum.

Electron-correlation effects in double-electron-capture collisions of 60-keV C6+ with He.

The method of zero-degree Auger spectroscopy was used to measure cross sections for the production of K and L Auger electrons by double electron capture in 60-keV ${\mathrm{C}}^{6+}$+He collisions. Corresponding atomic-structure calculations were performed to determine the related Auger and x-ray transition rates. The resulting Auger yields are found to deviate significantly from unity. The Auger yields were applied to evaluate cross sections for the production of the configuration 3lnl' (n=3 to 5) and 2lnl' (n=3 to 8). These data are found to compare well with experimental results obtained previously for the isocharged system ${\mathrm{O}}^{6+}$+He. Comparison is also made with recent calculations including the electron-electron interaction. It is shown that electron-correlation effects play a significant role for double electron capture in the studied collision system.

On the probability of changing the Auger transition rates between the hyperfine states of a mesic atom in matter

Possibilities of using negative muons are discussed: (i) for understanding the behaviour of wave functions of outer electron shells in the vicinity of a nucleus and (ii) for investigation of the exchange effects in solid state physics. It is noticed that the effective life time of μ bound in light atoms (nuclear spin i≠O) depends on the state of the outer electron shell. This fact can be used for measurement ofK-capture rates from different components of hyperfine structure of a mesic atom and for estimation of the weak interaction constant.

Auger cascades in atoms and ions of astrophysically important elements

The Monte-Carlo method has been used to simulate Auger cascades in atoms and ions of Mg, Al, Si, S, Ar, Ca, Fe with account for the radiative and Auger transitions. The probabilities of ejection of various numbers of electrons after the initial creation of vacancy in different shells have been calculated. Corresponding probabilities for atoms and ions of C, N, O, Ne (where single Auger transitions are possible) are presented. The data on radiative and Auger transition rates available in the literature for all the above-mentioned atoms and ions are collected and discussed.

Auger Electron Spectra of Molecules: The First Row Hydrides

A semi-empirical molecular orbital approach primarily developed to predict the Auger energies and transition rates associated with complex polyatomic molecules has been evaluated by reference to the first row hydrides. For this suite of molecules, spanning a range of symmetry classes, high resolution experimental spectra and ab initio energy calculations are available for comparison. The method proposed provides a good first estimate for transition rates. Relative Auger energies are less satisfactory, especially when electron correlation effects are strong. Absolute kinetic energies are too low, principally due to the neglect of electronic relaxation. The usefulness of the approach is mainly for the interpretation of spectra for larger systems where ab initio calculations are not readily accomplished. A need for further experimental measurements obtained using photoelectron-Auger electron coincidence studies is identified to remove uncertainties associated with the extent of satellite contributions present in all the spectral profiles so far recorded. In the present work the satellite contribution to the methane Auger spectrum is estimated to be 16% of the total intensity compared with an earlier estimate in the literature of 49%.

Auger transition rates for systems with open subshells

McGuire (1975) has given a general expression for the evaluation of Auger transition rates in multiply ionised atoms. Simplified expressions are derived, in various coupling schemes appropriate to the particular cases in which the multiple ionisation is restricted to those subshells directly involved in the Auger transitions. The cases in which the final state holes are inequivalent or equivalent are treated separately. The analysis is used to calculate the Auger transition rates of the Mg KL2,3-L2,32 V Auger vacancy satellites.

Core-hole screening in metallic magnesium

An atomic-like formalism has been used to calculate the KL1V Auger transition rate for magnesium as a function of the initial-state valence configuration. The transition rate for KL1V processes involving valence s and p holes, KL1Vs and KL1Vp, have been calculated for several valence configurations. A comparison between the theoretical and the experimental values of the KL1Vs:KL1Vp ratio suggests that a 1s core hole in metallic magnesium is screened by a valence charge of p character.

Effects of relativity and configuration interaction on L-shell Auger and radiative decays of the doubly excited 3l3l' states of sodiumlike ions.

Auger and radiative transition energies and rates for the states from the 2${p}^{5}$3l3l' and 2s2${p}^{6}$3l3l' configurations of the Na-like ions have been calculated for 18 ions with atomic number 18\ensuremath{\le}Z\ensuremath{\le}92 using the multiconfiguration Dirac-Fock method. The Z dependence of the transition rates for a few selected states has been analyzed. Numerous irregularities are present in many transitions due to the level-crossing interaction. The effects of relativity and configuration interaction are simultaneously important for medium and heavy ions. These effects can sometimes change the rates for weak transitions by orders of magnitude. Strong configuration interaction between states from the 2${p}^{5}$3${p}^{2}$ and 2${p}^{5}$3s3d configurations has been found. Inclusion of the Breit interaction in calculations of the Auger matrix elements can change the Auger rates by as much as a factor of 2. The spin-orbit mixing and Breit interaction are responsible for the decay of most of the quartet states. In particular, the 2${p}^{5}$3s3${p}^{4}$${D}_{7/2}$ metastable state of low-Z ions decays predominantly by Auger-electron emission via the magnetic interaction.

Measurements ofL-subshell fluorescence yields for light and medium heavy elements (28?Z?47)

We report on the first determination ofL-subshell fluorescence yields, ωLi, for various elements with 28≤Z≤47. The method applied is based on the subshell ionization by relativistic electron impact with an electron energy ofE0=50 keV and the detection of characteristic X rays by means of a high resolution crystal spectrometer calibrated absolutely with respect to its transmission and reflectivity. The number of initialL vacancies and its subshell distribution as well as the normalized X-ray transition probabilities, the Coster-Kronig yields and theK-shell Auger transition rates are taken from theory. The results obtained for ωLi yield values that are for all three subshells in agreement with the predictions of a widely used semi-empirical formula and for ωL2 and ωL3 also with theoretical calculations. The values of ωL1, however, exceed the theory systematically.

A method of determining the overlap integrals used in calculations of Auger transition rates in semiconductors

Overlap integrals of Bloch functions are required for calculations of Auger transition rates in semiconductors. The usual approximations (such as those based on the effective mass sum rule or the simple four-band kp method) are known to be unreliable but the pseudopotential and 15-band kp methods (which produce accurate values) involve too much computation to be directly incorporated in numerical calculations of Auger transition rates. In the work reported here, the overlap integrals obtained from numerical diagonalisation of the kp Hamiltonian, with the effects of remote bands included by the Lowdin procedure, are shown to be in good agreement with those calculated from the empirical non-local pseudopotential band-structure method of Chelikowsky and Cohen (1976). Also it is shown how it is possible to obtain algebraic expressions for overlap integrals from the kp method by using perturbation theory. The perturbation theory is able to explain the general features of the numerical diagonalisation results and in many cases provides a good quantitative description.

The effects of non-parabolic band structure on Auger transition rates in bulk semiconductors, quantum wells and quantum well wires

A new approach to the problem of including non-parabolic band structure in calculations of Auger recombination rates is presented, giving results for quantum wells (QW) that are readily generalised to the cases of bulk semiconductors and quantum well wires (QWW). The band structure in the region of the excited Auger carrier is described using non-local pseudopotential band structure, whereas the other carriers, being close to their respective band edges, are described using parabolic bandstructure. The authors' approach differs from earlier work in that the energies and wavevectors of carriers close to the band edges are not neglected, but simple analytic results are obtained. Numerical results for the effects of non-parabolic band structure on bulk Auger transition rates in GaAs and GaSb, using the simple expressions derived in this paper, are found to be in good agreement with other calculations.

Charge-transfer and electron emission in the slow ( v < v0) multicharged-ion-surface interaction

Charge transfer and electron emission in the slow ( v < v 0) multicharged-ion surface interaction is examined. We discuss the form of the total electronic potential, the influence of image and screening interactions on the projectile state energies, and calculate Auger transition rates and Auger electron emission yields for KVV transitions for the oxygen-copper and oxygen-gold interactions. It is shown that efficient transfer of charge from the valence band of metals occurs within projectile-surface separations of order 2 q+7 a.u., where q is the projectile charge.

Auger emissions from the LiF molecule: Calculation of probabilities and energies.

A new ab initio method for computing Auger transition rates in molecules is proposed; it is based on the use of orthogonalized plane waves for the free electron and of Hartree-Fock many-center orbitals for the bound electrons. Transition rates and energies are calculated for the case of the LiF molecule ionized in its deepest shell, and the results are compared with the experimental spectrum. Various levels of approximation are tested for evaluating the partial transition rates of various decay channels, and a consistent improvement in the agreement with experiment is obtained when the plane waves are orthogonalized to the relaxed orbitals of the doubly ionized molecular ion.