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Cascade decay of atomic magnesium after photoionization with a photoelectron-photoion coincidence method
Abstract
For the K-shell decay, magnesium is the first element in the periodic table which shows cascading transitions. We investigated the whole cascade of magnesium by using the photoelectron-photoion coincidence technique on the 1s, 2s, and 2p decay. The experimentally determined and calculated decay probabilities for the 1s-1, the 2s-1, and the 2p-1 decay, i.e., the whole cascade, are in good agreement with each other. For the calculation of higher final ionic charge states, it was found that electron correlations have to be taken into account. The fluorescence yields for the K and L shell and the Coster-Kronig factor for the L shell were determined.
... In the boron atom having only three valence electrons, DA decay is expected to be small. As pointed out in [15], the probability of the DA processes is roughly proportional to the cube of the number of electrons that could be involved in the transitions. Then, keeping in mind that for neon with eight electrons in the L shell, the K-LLL DA probability is 6%, [16], in boron it should be about 0.3%. ...
The Monte Carlo technique is applied to simulate the processes of the cascade relaxation of gaseous boron at atomic density of 2.5 × 1022 m−3 ionized by photons with the energies of 0.7–25 Ryd passing through a cylindrical interaction zone along its axis. The trajectories of electrons are simulated based on photoionization and electron-impact ionization cross sections calculated in the one-electron configuration-average Pauli–Fock approximation. Numbers of electrons and photons leaving the interaction zone per one initial photoionization, their energy spectra, the energy transferred to the medium and the probabilities of final ion formations are shown to change noticeably as the incident photon energy is scanned through boron atom ionization thresholds. These variations can be explained only if secondary electron-impact-produced processes are considered. The density of secondary events decreases when going from the zone axis to its border, and the profiles of the density along the radial direction are found to be similar for all the initial exciting photon energies.
... The fluorescence yields are calculated by dividing those measurement results by the corresponding ionization cross sections, which again depend on other fundamental parameters. A direct experimental determination of this important parameter would be desirable [34]. ...
Fundamental parameter based quantification of X-ray fluorescence (XRF) measurement data requires an accurate knowledge of the spectrometer parameters, including the spectral distribution of the excitation radiation. In case of micro-XRF where a polycapillary optic is utilized in the excitation channel this distribution is changed due to the transmission properties of the lens. A new calibration procedure, based on fluorescence data of thin standard samples, was developed to determine the excitation spectrum, i.e., the product of the X-ray tube spectrum and the transmission of the used X-ray optic of a micro-XRF setup. The calibration result was validated by the quantitative analyses of certified multi-element reference standards and shows uncertainties in the order of 2% for main components, 10% for minor elements and 25% for trace elements. The influence of secondary order effects like Coster–Kronig transitions and cascade effects is analyzed and the accuracy of fundamental parameters in common databases is discussed.
A theoretical model to calculate the photoion yields registered in coincidence with fixed-energy Auger electrons is developed. The intermediate-coupling approximation is used to calculate the term structure of the two-hole states reached by Auger transitions, and the branching ratios of their further Auger, Coster–Kronig and super Coster–Kronig decays into the three-hole states. The remaining parts of the cascading decay de-excitation trees are simulated in configuration-average approximation for branching ratios and transition energies. Our calculation allowed us to explain the recent M45NN Auger-electron–photoion coincidence experiment upon M45 photoionization of Xe.
The triple-photoionization cross section of neon and argon near threshold has been investigated by ion time-of-flight spectrometry. We applied the Wannier power law to our data and confirmed the theoretical Wannier exponent in the cases of Ne and Ar. Our data also agree with previous findings regarding the Wannier exponent and its range of validity for Ne. However, the Wannier power law exhibits a much smaller range of validity of 2 eV for Ar compared to 5 eV for Ne. Also, in contrast to a previous experiment, we do not find a “second” power law but a gradual decrease of the exponent above the range of validity of the Wannier power law.
X-ray spectrometry is a wide spread technique for revealing reliable information concerning the elemental composition and binding state in various materials. Reference-free quantitation in X-ray spectrometry is based on the knowledge of both the instrumental and fundamental atomic parameters. Instrumental or experimental parameters involve the radiant power and spectral purity of the excitation radiation, the beam geometry, the solid angle of detection, and the response behavior and efficiency of the detector. The reliability of the quantitation equally depends on the relative uncertainty of the atomic fundamental parameters involved. Both the values and estimated uncertainty of atomic fundamental parameters given in the literature can be improved by dedicated experiments: By means of transmission and fluorescence experiments with tunable synchrotron radiation, the mass absorption coefficient and fluorescence yield of Al was determined. Furthermore, the transition probabilities of the fluorescence lines belonging to the Cu-Liii and Lii subshells were determined using a superconducting tunnel junction detector offering an energy resolution of about 10 eV in the soft X-ray range. Selected techniques and applications of reference-free X-ray spectrometry are presented. A particular advantage of reference-free quantitation modes is their capability to directly probe new materials without the need to wait for appropriate standard reference materials.
1
Rostov State University of Transport
Communication, Narodnogo Opolcheniya 2, Rostov-na-Donu 344038,
Russia
2
Observatoire de la Côte d'Azur, CNRS
Laboratoire Cassini, BP 4229, 06304 Nice Cedex 4, France
The decay channels of the Ar2s-1 and 2p-1 and Kr3p-1 and 3d-1 electronic hole states have been investigated by means of photoelectron-photoion coincidence measurements following innershell ionization using synchrotron radiation. With the method of final ion-charge resolving electron spectroscopy it has become possible to disentangle different contributions to the electron spectrum and to determine the decay probabilities P(nl-1→n+) of the above-mentioned hole states (nl-1) to the final ionic charge states n+. A high correlation with threefold or even fourfold charged ions has been found in all cases. Possible decay routes, via cascade or direct double Auger processes, are discussed on the basis of energy-level schemes calculated with the Hartree-Fock method. Special emphasis is laid on the examination of the Kr3p-1 decay process, where the two fine-structure components (j=1/2,3/2) exhibit noticeably different decay probabilities to Kr3+ and Kr4+ final ionic charge states.
The photoelectron-photoion coincidence method is shown to be very successful for the quantitative investigation of the ratio of radiative to nonradiative transitions and for the ratio of single to double Auger transitions. These decay probabilities are important parameters for analytical methods as well as for theoretical descriptions. The method is demonstrated for the neon 1s decay. Especially the fluorescence yield and the yield for the double Auger process have been determined both experimentally and theoretically.
The statistics of a specific class of two-particle coincidence experiments are discussed. Detection of the first particle provides for the start of a time-of-flight measurement in which the second particle is analyzed. Dead time of the electronics used for flight time measurements and registration of coincidence signals (time-to-amplitude converter or time-to-digital converter) may cause strong distortion of coincidence spectra. The problem is complicated by the different statistics of true and false coincidences. Without a convenient correction procedure, this effect limits the source strength to a value for which the distortion is small but the measurement duration is long. A calculation method is developed from which the measured coincidence spectra can be corrected for distortion at any source strength thereby separating true and false coincidences. This technique has been successfully applied in an electron-ion coincidence experiment where free atoms were photoionized by synchrotron radiation. © 1997 American Institute of Physics.
The K, L, and higher atomic shell x-ray fluorescence yield measured data, covering the period 1978 to 1993, following the major previous compilations by Bambynek etal. (1972) and Krause (1979), are reviewed. An annotated bibliography of x-ray fluorescence yield measurements, analyses, fits and tables 1978–1993 is presented. Comparisons of the fluorescence yields ωk, ω¯L, and ω¯M, based on measurements, and on theoretical models, are presented. Values of ωK, ω¯L, and ω¯M, fitted to standard empirical parametric formulations, are presented. In addition, selected well-characterized measured ωK, ω¯L, and ω¯M results restricted to the period 1978–1993 are listed. These selected measured values are fitted by least squares to polynomials in Z of the form ∑nanZn and compared with theoretical and with earlier fitted values. A section on application of fluorescence yield data to computations of x-ray energy-absorption coefficients is included.
The available body of information on (a) fluorescence, Auger, and Coster-Kronig yields, (b) radiative and radiationless transition rates, (c) level widths, (d) x-ray and Auger line widths, (e) x-ray and Auger spectra, and (f) Coster-Kronig energies has been used to generate an internally consistent set of values of atomic radiative and radiationless yields for the K shell (5 ?Z?110) and the L subshells (12 ?Z?110). Values of fluorescence yields ωk, ω1, ω2, ω3, Coster-Kronig yields F1, F1.2, F1.3, F1.3, F2.3. Auger yields ak, a1, a2, a3, and effective fluorescence yields ν1 and ν2 are presented in tables and graphs. Estimates of uncertainties are given. Updated and expanded graphs of partial and total widths of K, L1, L2, and L3 levels are presented as well as a reference list of papers published since about 1972.
Electron-ion coincidence spectroscopy of energy-analyzed electrons and charge-separated ions is used to investigate complex decay processes of atoms after inner-shell photoionization with monochromatized synchrotron radiation. Improvements in the experimental technique allow a complete electron spectrum resolved into the different contributions of the final ion-charge states to be measured. This method of final ion-charge resolving electron spectroscopy is demonstrated by the example of xenon for different photoionization processes in the region of the 4d-->εf resonance.
The charge spectra of multiple ions resulting from vacancy cascades in neon, argon, krypton and xenon are calculated by straightforward construction of de-excitation trees originating from various initial inner-shell vacancies. Included in the calculation are all possible radiative and non-radiative decay channels for each vacancy, and shake-off processes caused by diagram Auger, Coster-Kronig, and radiative transitions. The single-configurational Hartree-Fock approximation is employed to calculate the decay channel branching ratios and the probabilities of shake processes. The energies of levels and transition energies are calculated in the Hartree-Fock-Pauli (HFP) approximation. The dependences of level positions, level splittings, branching ratios, and shake probabilities on a configuration of a decaying ion are accounted for. Good agreement with experiment and some of the earlier calculations is obtained. The role of double electron ejection processes, other than monopole, is discussed.
The yields of multiple Xei+ ions produced by the cascade de-excitations of the L2 to N45 inner shell vacancies are calculated with inclusion of every possible pathway of deexcitation (Auger, Coster-Kronig and radiative) at each branching point of cascade deexcitation trees. Ejections of additional electrons through shake processes are included at each step of de-excitation. Good agreement with the experiment is obtained. Simplified de-excitation schemes suggested earlier by other authors for the initial M-shell vacancies are discussed.
The photoelectron–photoion method presented is shown to be very successful for the quantitative investigation of decay probabilities after innershell photoionization. Especially for the determination of K-shell fluorescence yields of light elements, where only scarce data exist up to now, this method opens up new ways. For all elements we present a general approach to determine fluorescence yields. As an example the investigation of the 1s decay of free sodium atoms is presented.
A new type of electron spectroscopy for the investigation of gas-phase targets has been developed. Based on electron-ion coincidence measurements using synchrotron radiation the method for the first time allows an electron spectrum to be split into the different final charge states. These spectra give deep insight into different, in part spin-dependent, multiple photoionization mechanisms, which is demonstrated for atomic samarium and europium in the photon energy range of the 4d giant resonances.
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