M. Pajek

Communication: The electronic structure of matter probed with a single femtosecond hard x-ray pulse

Physical, biological, and chemical transformations are initiated by changes in the electronic configuration of the species involved. These electronic changes occur on the timescales of attoseconds (10−18 s) to femtoseconds (10−15 s) and drive all subsequent electronic reorganization as the system moves to a new equilibrium or quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is crucial for understanding the potential energy surfaces upon which chemical and biological reactions take place. Here, we report on the determination of the electronic structure of matter using a single self-seeded femtosecond x-ray pulse from the Linac Coherent Light Source hard x-ray free electron laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we were able to obtain information about the electronic density of states with a single femtosecond x-ray pulse. We show that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis and that the measured spectral shape is independent of the large intensity fluctuations of the incoming x-ray beam. Moreover, we demonstrate the chemical sensitivity and single-shot capability and limitations of HEROS, which enables the technique to track the electronic structural dynamics in matter on femtosecond time scales, making it an ideal probe technique for time-resolved X-ray experiments.

Application of the high-resolution grazing-emission x-ray fluorescence method for impurities control in semiconductor nanotechnology

We report on the application of synchrotron radiation based high-resolution grazing-emission x-ray fluorescence (GEXRF) method to measure low-level impurities on silicon wafers. The presented high-resolution GEXRF technique leads to direct detection limits of about 1012 atoms/cm2. The latter can be presumably further improved down to 107 atoms/cm2 by combining the synchrotron radiation-based GEXRF method with the vapor phase decomposition preconcentration technique. The capability of the high-resolution GEXRF method to perform surface-sensitive elemental mappings with a lateral resolution of several tens of micrometers was probed.

Multiple ionization effects in low-resolution X-ray spectra induced by energetic heavy ions

Abstract The method of analysis of the multiple ionization effects in low-resolution X-ray spectra induced by heavy ions is described here. It is shown that, by fitting the X-ray spectra measured by a semiconductor detector with the proposed model, which accounts for the multiple ionization effects, the ionization probabilities can be determined as free fitting parameters. This approach is based on the assumption that the intensity distribution of emitted X-ray satellites is approximately described by the binomial distribution parameterized by the ionization probability at the moment of X-ray emission. In particular, we demonstrate that the excited X-ray satellites, when measured by a low-resolution semiconductor detector, appear as the Gaussian profile, which is shifted and broadened with respect to the diagram line. Moreover, we find that both X-ray line shift and width are expressed in terms of the multiple ionization probabilities, as well as the X-ray energy shifts per vacancy. These observations allowed to develop a novel method of X-ray spectra fitting. Detailed discussion of the approximations used in the present approach is given. The method developed, using the calculated Dirac–Fock X-ray energy shifts per vacancy, was applied to determine the multiple ionization probabilities in M- and N-shell by fitting the measured X-ray spectra for L γ (L→N,O) transitions induced by heavy ions in selected high-Z atoms.

Establishing nonlinearity thresholds with ultraintense X-ray pulses

X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state.

Analysis of Copper Concentration in Human Serum by Application of Total Reflection X-ray Fluorescence Method

The chemotherapy and photon radiotherapy are the most often applied methods in treatment of the cancer diseases because of their effectiveness and high cure rates. Apart from eligible destruction of the tumour, one of the side effects of these treatment methods is possible modification of main and trace element concentration in different human tissues and fluids. In this paper, the copper (Cu) level in human serum was determined by total reflection X-ray fluorescence method in 142 chemotherapy patients and in 44 healthy persons being a control group. The Cu concentration in the chemotherapy group was found to be on the level 1.78 ± 0.909 mg/L, while in the control group, it was 1.08 ± 0.551 mg/L. Performed measurements allowed for calculation of the parameters of copper concentration distribution (mean value, standard deviation, median) for both analysed groups. The theoretical nature of the concentration distribution was tested and found as a log-normal distribution (control group) and a log-stable distribution (chemotherapy group). The copper concentration distributions for both studied group were statistically compared using Kolmogorov-Smirnov test, and the conclusion was that the distributions are statistically different. Serum Cu levels were significantly higher in the chemotherapy group than in the control group. Taking into account the results for the control group, the copper concentration reference quantile ranges in human serum were obtained. The values of the mean, median and other quantiles determined in this case can be applied in two-group comparison studies. The obtained results can be used as a diagnostic tool for chemotherapy patients.

Observation of L-X-ray satellites and hypersatellites in collisions of O and Ne ions with Mo and Pd

Abstract The satellite structure of Lα 1,2 and Lβ 1 X-ray transitions in 42 Mo and 46 Pd induced by an impact of O and Ne ions with energies 178–376 MeV have been studied with a high-resolution von Hamos crystal spectrometer. The observed M-shell satellites of the diagram Lα 1,2 and Lβ 1 lines are affected mostly by one- and two-vacancy configurations for oxygen ions and much more complex multi-vacancy configurations for neon impact. The L-shell hypersatellites are clearly observed in the measured spectra, both for Lα 1,2 and Lβ 1 X-ray transitions. The spectra are compared with the predictions of the multi-configuration Dirac–Fock (MCDF) calculations performed for up to two additional vacancies. Such MCDF calculations reproduce the main features of the M-shell satellite structure observed for oxygen ions, but they fail to describe the X-ray spectra measured for neon ions. Clearly, more complex multi-vacancy configurations have to be included in the MCDF calculations to achieve quantitative agreement in this case. The measured X-ray satellite structure for neon ions is thus interpreted by means of a simplified binomial model based on the average MCDF X-ray shifts and independent electron picture of multiple ionization of atoms by ion impact.

Depth profiling of low energy ion implantations in Si and Ge by means of micro-focused grazing emission X-ray fluorescence and grazing incidence X-ray fluorescence

Depth-profiling measurements by means of synchrotron radiation based grazing XRF techniques, i.e., grazing emission X-ray fluorescence (GEXRF) and grazing incidence X-ray fluorescence (GIXRF), present a promising approach for the non-destructive, sub-nanometer scale precision characterization of ultra shallow ion-implantations. The nanometer resolution is of importance with respect to actual semiconductor applications where the down-scaling of the device dimensions requires the doping of shallower depth ranges. The depth distributions of implanted ions can be deduced from the intensity dependence of the detected X-ray fluorescence (XRF) signal from the dopant atoms on either the grazing emission angle of the emitted X-rays (GEXRF), or the grazing incidence angle of the incident X-rays (GIXRF). The investigated sample depth depends on the grazing angle and can be varied from a few to several hundred nanometers. The GEXRF setup was equipped with a focusing polycapillary half-lens to allow for laterally resolved studies. The dopant depth distribution of the investigated low-energy (energy range from 1 keV up to 8 keV) P, In and Sb ion-implantations in Si or Ge wafers were reconstructed from the GEXRF data by using two different approaches, one with and one without a priori knowledge about the bell-shaped dopant depth distribution function. The results were compared to simulations and the trends predicted by theory were found to be well reproduced. The experimental GEXRF findings were moreover verified for selected samples by GIXRF.

Concentration distribution of trace elements: from normal distribution to Lévy flights☆

The paper discusses a nature of concentration distributions of trace elements in biomedical samples, which were measured by using the X-ray fluorescence techniques (XRF, TXRF). Our earlier observation, that the lognormal distribution well describes the measured concentration distribution is explained here on a more general ground. Particularly, the role of random multiplicative process, which models the concentration distributions of trace elements in biomedical samples, is discussed in detail. It is demonstrated that the lognormal distribution, appearing when the multiplicative process is driven by normal distribution, can be generalized to the so-called log-stable distribution. Such distribution describes the random multiplicative process, which is driven, instead of normal distribution, by more general stable distribution, being known as the Levy flights. The presented ideas are exemplified by the results ´ of the study of trace element concentration distributions in selected biomedical samples, obtained by using the conventional (XRF) and (TXRF) X-ray fluorescence methods. Particularly, the first observation of log-stable concentration distribution of trace elements is reported and discussed here in detail. 2003 Elsevier Science B.V. All rights reserved.

Determination of concentration distribution of trace elements near the detection limit

Abstract The results of a numerical simulation, performed to check the validity of a method developed for reconstruction of concentration distributions truncated by the detection limit, are reported in the context of trace element analysis in biomedical samples by total-reflection X-ray fluorescence. This method, by correcting a distribution over the whole range of concentrations in a population of samples, restores a number of measurements reporting results below the detection limit. We show by Monte Carlo simulations, assuming lognormal distributions to describe both the concentrations measured as well as the detection limits in the biomedical samples, that the method developed is accurate to within 5% for most typical situations. Moreover, we demonstrate that the factor limiting the accuracy of the correction is the number of measurements, not the correction procedure itself. We have found in simulations that the reconstruction of a concentration distribution, for a typical population size of N =100, is possible when the concentrations are measured only in 20–30% of samples. On the other hand, we show that by ignoring the truncation of a concentration distribution by the detection limit, the results can be systematically biased by as much as 50%. The method developed is applied to the analysis of trace elements in human breast tissue samples by total-reflection X-ray fluorescence (TXRF). The results are also discussed in terms of the size of the population studied.

Closing of Coster–Kronig transitions in multiply ionised gold atoms

Abstract The paper discusses the effect of closing of L- and M-shell Coster–Kronig (CK) transitions in multiply ionised Au atoms, for which the selected CK transitions become energetically forbidden. This effect plays an important role when the Coster–Kronig energy for single-hole configuration is relatively low, being comparable with a change of the electronic binding energies in multiply ionised atom. We show, by using a simplified model, that for gold the effect of closing of CK transitions occurs for strong L1–L3M4,5 transition for the L1-subshell as well as the M3–M5N6,7 and M4–M5O3,4 CK transitions for the M3- and M4-subshell, respectively. We demonstrate that the discussed effect of closing CK transitions substantially changes the X-ray fluorescence and Coster–Kronig yields and thus has to be considered in interpretation of X-rays excited by heavy ion impact.