Inner-shell Vacancy Production Research Articles

Calculations of electron shakeoff probabilities using pseudostates

SynopsisElectron shakeoff probabilities accompanying inner-shell vacancy production are calculated in the screened hydrogenic model using pseudostates. The results are compared with the values calculated with continuum wave functions.

Doubly-differential-cross-section calculations forK-shell-vacancy production in lithium by fastO8+-ion impact

Inner-shell vacancy production for the O$^{8+}$-Li collision system at 1.5 MeV/amu is studied theoretically. The theory combines single-electron amplitudes for each electron in the system to extract multielectron information about the collision process. Doubly-differential cross sections obtained in this way are then compared with the recent experimental data by LaForge et al. [J. Phys. B 46, 031001 (2013)] yielding good resemblance, especially for low outgoing electron energy. A careful analysis of the processes that contribute to inner-shell vacancy production shows that the improvement of the results as compared to single-active-electron calculations can be attributed to the leading role of two-electron excitation-ionization processes.

L-Shell X-Ray Yields and Production Cross Sections of Zr and Mo Bombarded by Slow Highly Charged Ar16+ Ions

The L-shell x-ray yields of Zr and Mo bombarded by slow Ar16+ions are measured. The energy of the Ar16+ions ranges from about 150 keV to 350 keV. The L-shell x-ray production cross sections of Zr and Mo are extracted from these yields data. The explanation of these experimental results is in the framework of the adiabatic direct-ionization and the binding energy modified BEA approximation. We consider, in the slow asymmetric collisions such as Ar and Mo/Zr, the transient united atoms (UA) are formed during the ion-surface interaction and the direct-ionization is the main mechanism for the inner-shell vacancy production. Generally, the theoretical results are in good agreement with the experimental data.

Postcollisional effects in multiple ionization of diatomic molecules by ion impact

Multiple electron ionization of diatomic molecular targets is studied for impact of proton beams. It is shown that for these molecular targets, as it was previously predicted for atomic ones, postcollision Auger type emission following inner-shell vacancy production dominates the reaction at high enough impact energies. Moreover, it is proven that this dominance increases as the ionization degree increases.

Search for short-time phase effects in the electronic damage evolution – A case study with silicon

This work focusses on the production and decay properties of inner-shell vacancies and valence-band excitations induced by swift highly charged ions interacting with amorphous and crystalline Si. High resolution electron spectra have been taken for fast heavy ions at 1.78–5MeV/u as well as for electrons of similar velocity incident on atomically clean Si targets of well defined phase. Various Auger-electron structures are analyzed concerning their width, their intensity and exact peak position. All measured peaks show a small shift towards lower energy when the charge of the projectile is increased. This finding is an indication for a nuclear-track potential inside the ion track. A detailed analysis of the Auger-electron spectra for amorphous Si and crystalline Si(111) 7×7 points to a small but significant phase effect in the short-time dynamics of ion tracks.

Inner-shell vacancy production and mean charge states of MeV/u Fe, Co, Ni and Cu ions in Au and Bi solid targets

The average charge states of 0.1–1.5 MeV/u Fe, Co, Ni and Cu ions in solid Au and Bi targets have been determined, by estimating the mean numbers of outer-shell spectator vacancies during the K-vacancy decay. The latter quantities were obtained from the yield and energy shifts of the K α, β X-rays, by comparing with calculations in the independent electron approximation. The reported equilibrium charges, mostly characteristic for the inside of the target, are in fairly agreement with Nikolaev and Dmitriev semi-empirical formula [Phys. Lett. 28A (1968) 277].

An efficient, selective collisional ejection mechanismfor inner-shell population inversionin laser-driven plasmas

A theoretical analysis of laser-driven collisionalejection of inner-shell electrons is presented to explain thepreviously observed anomalous kilovolt L-shell x-ray emissionspectra from atomic Xe cluster targets excited by intensesub-picosecond 248 nm ultraviolet radiation (McPherson Aet al 1994 Nature 370 631-4). Forincident ponderomotively-driven electrons photoionized bystrong above threshold ionization, the collisionalejection mechanism is shown to be highly l-state andsignificantly n-state (i.e. radially) selective fortime periods shorter than the collisional dephasing time of thephotoionized electronic wavefunction. The resulting preferencefor the collisional ejection of 2p electrons by anionized 4p state produces the measured anomalous Xe(L)emission which contains direct evidence for(i) the generation of Xe27+(2p53d10)and Xe28+(2p53d9) ions exhibiting inner-shellpopulation inversion and (ii) acoherent correlated electron state collision responsiblefor the production of double 2p vacancies. For longertime periods, the selectivity of this coherent impact ionizationmechanism is rapidly reduced by the combined effects ofintrinsic quantum mechanical spreading and dephasing - inagreement with the experimentally observed and extremely strong~λ-6 pump-laser wavelength dependence of theefficiency of inner-shell (2p) vacancy production in Xeclusters excited in underdense plasmas (Kondo Ket al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 2707-16).

Photoionization at Relativistic Energies

At MeV energies and beyond the inner-shell vacancy production cross section associated with the photoelectric and Compton effect decrease with increasing photon energy. However, when the photon energy exceeds twice the rest energy of the electron, ionization of a bound electron may be catalyzed by the creation of an electron-positron pair. Distinctly different from all other known mechanisms for inner-shell vacancy production by photons, we show that the cross section for this “vacuum-assisted photoionization” increases with increasing photon energy and then saturates. As a main result, we predict that vacuum-assisted photoionization will dominate the other known photoionization mechanisms in the highly relativistic energy regime.

Electron transition probability accompanying inner-shell vacancy production

Electron transition probability accompanying inner-shell vacancy production

Wavelength dependence of multiphoton-induced Xe(M) and Xe(L) emissions from Xe clusters

A direct comparative measurement of the dependence on the wavelength of irradiation of the kilovolt x-ray yields ( and ) multiphoton-induced from Xe clusters by excitation with intense femtosecond pulses at 248 and 800 nm has been made. The spectroscopic findings demonstrate that both the Xe(M) and Xe(L) emissions are strongly reduced with excitation at the longer wavelength (800 nm). The peak strengths of the Xe(M) and Xe(L) emissions are diminished by factors of and , respectively. Significant spectral differences are also observed. This sharp reduction in the amplitude of the excitation is in conflict with a thermal model for the production of kilovolt x-rays (Xe(M) and Xe(L)) from multiphoton 248 nm excited Xe clusters. These results are consistent with a dynamical mechanism of enhanced coupling which involves ordered many-electron motions in which a dephasing of the electrons can appreciably influence both the amplitude of excitation and the threshold intensity for inner-shell vacancy production. Within the framework of this picture, these experimental findings indicate an effective dephasing time for Xe clusters of - 2 fs, a range that is consistent with the measured k-space scattering dynamics of carriers in GaAs.

Dependence of inner-shell vacancy production upon distance in hard Li-Al collisions

We match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV $^{7}\mathrm{Li}^{+}$ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so we determine the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. We find that such an assumption is insufficient to fit our simulations to experimental observations, and that we must allow for a gradual turning on of the vacancy production probability as the distance of closest approach decreases. The resulting model may be useful in modeling atomic excitation effects in simulations of other ion-impact processes.

A Comparative Study of the Variable Screening and Hartree-Fock Models as Means for the Construction of Two-Center Correlation Diagrams

The aim of this work is to test the Variable Screening Model (VSM) [Wille U., Hippler R.: Phys. Rep. 132, 129 (1986)] as a method for computing correlation diagrams of neutral and ionized diatomic quasimolecules by comparing the results with corresponding Hartree-Fock calculations, and to elucidate the specific features of both aforementioned approaches in applications to given purpose. These calculations serve as the first step in the interpretation of inner-shell vacancy production in low-velocity ion-atom collisions inside bombarded solids.

Damage induced by monochromatic synchrotron X-rays at the resonance absorption energy of intracellular phosphorous in yeast cells

Two key experiments are analysed in terms of track structure parameters to obtain information on the excessive damage observed upon the production of inner shell vacancies at the resonance energy of bound phosphorus (Kobayashi et al., 1991) and for bromine incorporated into DNA (Usami et al., 1991). The relative damage enhancement factor due to resonance in phosphorus is 1.32. Additional Auger electron cascade events induced at resonance contribute about 35% to the total effect cross-section. In the bromine experiments, the intrinsic effectiveness is shown to be less than that for phosphorus. The effects can be quantified by the equilibrium electron fluences generated, combined with a quality specification determined from the net mean free path for ionization.

Multielectron transitions in X-ray absorption and inner-shell ionization phenomena

Multielectron transition processes accompanying deep inner-shell vacancy production are described. Shakeup and shakeoff processes in which atomic electrons are excited or ionized due to a sudden change of the central potential are considered. Experimental and theoretical studies of shake processes in various inner-shell ionization phenomena, such as radioactive decay, Auger effect, charged-particle impact, photoionization and X-ray absorption, are reviewed.

New trends in the study of alignment and orientation in ion-atom collisions

An overview of recent experimental and theoretical studies of the alignment of inner shell vacancies produced in fast ion-atom collisions is given. Information about impact parameter dependence of the alignment as obtained from coincidence measurements and from studies of the anisotropy of the X-ray and Auger satellite lines is discussed. The contribution of various mechanisms of inner-shell vacancy production (direct Coulomb ionization, electron capture, shake-off) is discussed. Recent experiments on the alignment in electron capture processes are analyzed. Finally, new types of experiments, namely, measurements of the alignment and orientation of a vacancy produced by proton impact ionization of laser excited and aligned atoms, is theoretically analyzed.

Charge-state distributions of scattered and recoil ions in C+-, F+-, and Ne+-Si-surface collisions.

The proportions of keV ions, incident on a clean silicon surface, which are scattered as doubly or triply charged ions are found to increase with atomic number from ${\mathrm{C}}^{+}$ to ${\mathrm{Ne}}^{+}$. In contrast, the recoil ${\mathrm{Si}}^{2+}$ and ${\mathrm{Si}}^{3+}$ ion yields decrease. A definite collision-energy threshold for the production of each of these multicharged ions is found. All the multicharged recoil silicon ion thresholds are consistent with inner-shell vacancy production at the 3d\ensuremath{\sigma}-3p\ensuremath{\pi} level crossing. However, the multicharged scattered ions seem to be produced by a different process. The surface-scattered ion yields may follow the same systematics as x-ray yields, which pass through maxima when the shell energies match.

Multiphoton-induced X-ray emission and amplification from clusters

The development of a unified picture of short-pulse high-intensity multiphoton processes, embracing atoms, molecules, and solids, appears possible through the study of clusters. Of particular significance are possible intra-cluster processes that can influence the mechanism of ionization and lead to the production of inner-shell vacancies. Inner-shell excitation leading to prompt X-ray emission is specifically considered and the treatment leads to the definition of a critical cluster size n c representing the achievement of maximal X-ray emission from the ensemble. These results suggest the possibility of designing a new class of molecular materials optimized for the efficient production and amplification of X-rays.

Production of Li subshell and M shell vacancies following inner-shell vacancy production

The probabilities for transfer of vacancies from the K shell to an L i Subshell ( η KL i ) and to the M shell (η KM), and from the L i Subshell to the M shell ( η L i M) are evaluated for elements with atomic numbers 18 ≤ Z ≤ 96 using the theoretical radiative transition rates of Scofield [Phys. Rev. A 9 (1974) 1041; At. Data Nucl. Data Tables 14 (1974) 121] and radiationless transition rates tabulated by Chen et al. [At Data Nucl. Data Tables 24 (1979) 13; Phys. Rev. A 21 (1980) 442]. The calculated vacancy transfer probabilities are least-squares fitted to polynomials to obtain analytical relations that represent these probabilities as a function of atomic number.

Evidence for production of inner-shell vacancies in slow multicharged F ions interacting with a Cu surface

Electron energy distributions have been measured for F q+ ions (∼ q × 10 keV, q = 2–8) interacting with a Cu surface at 5° grazing incidence. The measurements show evidence for K-shell excitation in F q+ ( q ⩽, 6) ions. The resultant inner-shell vacancy production is understood in terms of calculated molecular orbital diagrams which qualitatively describe the close collisions between the incident ion and the target atoms. When the K-shell vacancy is created in the collision, the mean KLL Auger electron energy shows a shift to higher energies relative to that observed for K-vacancies carried into the collision. This shift occurs since the KLL Auger decay for a vacancy created in the collision is constrained to take place late in the neutralization process, where the K- and L-shell energies differ from those in the less screened ion.

Evidence for production of inner-shell vacancies in slow multicharged F ions interacting with a Cu surface

Electron energy distributions have been measured for Fq+ ions (∽q×10 keV, q=2−8) interacting with a Cu surface at 5° grazing incidence. The measurements show evidence for K-shell excitation in Fq+ (q≤6) ions. The resultant inner-shell vacancy production is understood in terms of calculated molecular orbital diagrams which qualitatively describe the close collisions between the incident ion and the target atoms. When the K-shell vacancy is created in the collision, the mean KLL Auger electron energy shows a shift to higher energies relative to that observed for K-vacancies carried into the collision. This shift occurs since the KLL Auger decay for a vacancy created in the collision is constrained to take place late in the neutralization process, where the K- and L-shell energies differ from those in the less screened ion.