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Using electron and ion spectroscopy, we studied the electron and nuclear dynamics in similar to 50 000-atom large krypton clusters, following excitation with an intense hard x-ray pulse. Beyond the single pulse experiment, we also present the results of a time-resolved, x-ray pump-near-infrared probe measurement that allows one to learn about the time evolution of the system. After core ionization of the atoms by x-ray photons, trapped Auger and secondary electrons form a nanoplasma in which the krypton ions are embedded, according to the already published scenario. While the ion data show expected features, the electron emission spectra miss the expected pump-probe delay-dependent enhancement except for a slight enhancement in the energy range below 2 eV. Theoretical simulations help to reveal that, due to the deep trapping potential of the ions during the long time expansion accompanied by electron-ion recombination, thermal emission from the transient nanoplasma becomes quenched.

Part of Focus on High Energy Density Physics. In this paper, we present a novel theoretical approach, which allows the study of nonequilibrium dynamics of both electrons and atoms/ions within free-electron laser excited semiconductors at femtosecond time scales. The approach consists of the Monte-Carlo method treating photoabsorption, high-energy-electron and core-hole kinetics and relaxation processes. Low-energy electrons localized within the valence and conduction bands of the target are treated with a temperature equation, including source terms, defined by the exchange of energy and particles with high-energy electrons and atoms. We follow the atomic motion with the molecular dynamics method on the changing potential energy surface. The changes of the potential energy surface and of the electron band structure are calculated at each time step with the help of the tight-binding method. Such a combination of methods enables investigation of nonequilibrium structural changes within materials under extreme ultraviolet (XUV) femtosecond irradiation. Our analysis performed for diamond irradiated with an XUV femtosecond laser pulse predicts for the first time in this wavelength regime the nonthermal phase transition from diamond to graphite. Similar to the case of visible light irradiation, this transition takes place within a few tens of femtoseconds and is caused by changes of the interatomic potential induced by ultrafast electronic excitations. It thus occurs well before the heating stimulated by electron–phonon coupling starts to play a role. This allows us to conclude that this transition is nonthermal and represents a general mechanism of the response of solids to ultrafast electron excitations.

We apply Boltzmann equations for modelling the radiation damage in samples irradiated by photons from free electron laser (FEL). We test this method in a study case of a spherically symmetric xenon cluster irradiated with VUV FEL photons. The results obtained demonstrate the potential of the Boltzmann method for describing the complex and non-equilibrium dynamics of samples exposed to FEL radiation., Comment: 31 pages, 11 figures

Theoretical predictions show that at low values of Bjorken $x$ the spin structure function, $g_1$ is influenced by large logarithmic corrections, $ln^2(1/x)$, which may be predominant in this region. These corrections are also partially contained in the NLO part of the standard DGLAP evolution. Here we calculate the non-singlet component of the nucleon structure function, $g_1^{NS}=g_1^p-g_1^n$, and its first moment, using a unified evolution equation. This equation incorporates the terms describing the NLO DGLAP evolution and the terms contributing to the $ln^2(1/x)$ resummation. In order to avoid double counting in the overlapping regions of the phase-space, a unique way of including the NLO terms into the unified evolution equation is proposed. The scheme-independent results obtained from this unified evolution are compared to the NLO fit to experimental data, GRSV'2000. Analysis of the first moments of $g_1^{NS}$ shows that the unified evolution including the $ln^2(1/x)$ resummation goes beyond the NLO DGLAP analysis. Corrections generated by double logarithms at low $x$ influence the $Q^2$-dependence of the first moments strongly., Comment: 13 pages, latex, 2 figures; Appendix added

Theoretical predictions for polarized nucleon structure function $g_1(x,Q^2)$ at low $Q^2$ are obtained in the framework of the Generalized Vector Meson Dominance model. Contributions from both light and heavy vector mesons are evaluated. In the photoproduction limit the first moment of $g_1$ is related to the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto sum rule. This property is employed to fix the magnitude of the light vector meson contribution to $g_1$, using the recent measurements in the region of baryonic resonances. Results are compared to the data on $g_1(x,Q^2)$. Finally, the DHGHY moment function $I(Q^2)$ is calculated, and our theoretical predictions are confronted with the recent preliminary data obtained at the Jefferson Laboratory., Comment: 12 pages including 6 postscript figures, corrected ref.22, caption to fig.4 and a few typos

Predictions for g_1(x,Q^2) at low Q^2 are obtained in the framework of the GVMD model. Contributions from both light and heavy vector mesons are evaluated. The DHGHY sum rule is employed to fix the magnitude of the light vector meson contribution to g_1, using the recent measurements in the region of baryonic resonances. The DHGHY moment function is calculated. Predictions are compared to the data., Comment: 5 pages, including 3 postscript figures; presented by B. Bade{\l}ek at 10th International Workshop on Deep Inelastic Scattering (DIS2002), Cracow, 30 April - 4 May 2002

Secondary electron cascades are responsible for significant ionizations in macroscopic samples during irradiation with X-rays. A quantitative analysis of these cascades is needed, e.g. for assessing damage in optical components at X-ray free-electron lasers, and for understanding damage in samples exposed to the beam. Here we present results from Monte Carlo simulations, showing the space-time evolution of secondary electron cascades in diamond. These cascades follow the impact of a single primary electron at energies between 0.5-12 keV, representing the usual range for photoelectrons. The calculations describe the secondary ionizations caused by these electrons, the three-dimensional evolution of the electron cloud, and monitor the equivalent instantaneous temperature of the free-electron gas as the system cools during expansion. The dissipation of the impact energy proceeds predominantly through the production of secondary electrons whose energies are comparable to the binding energies of the valence (40-50 eV) and the core electrons (300 eV) in accordance with experiments and the models of interactions. The electron cloud generated by a 12 keV electron is strongly anisotropic in the early phases of the cascade (t <= 1 fs). At later times, the sample is dominated by low energy electrons, and these are scattered more isotropically by atoms in the sample. The results show that the emission of secondary electrons approaches saturation within about 100 fs, following the primary impact. At an impact energy of 12 keV, the total number of electrons liberated in the sample is <= 400 at 1000 fs. The results provide an understanding of ionizations by photoelectrons, and extend earlier models on low-energy electron cascades (E=0.25 keV, [ziaja,ziaja2]) to the higher energy regime of the photoelectrons., Comment: 17 pages, latex, 10 figures

Here we describe model calculations to follow the spatio-temporal evolution of secondary electron cascades in diamond. The band structure of the insulator has been explicitly incorporated into the calculations as it affects ionizations from the valence band. A Monte-Carlo model was constructed to describe the path of electrons following the impact of a single electron of energy E 250 eV. The results show the evolution of the secondary electron cascades in terms of the number of electrons liberated, the spatial distribution of these electrons, and the energy distribution among the electrons as a function of time. The predicted ionization rates (5-13 electrons in 100 fs) lie within the limits given by experiments and phenomenological models. Calculation of the local electron density and the corresponding Debye length shows that the latter is systematically larger than the radius of the electron cloud. This means that the electron gas generated does not represent a plasma in a single impact cascade triggered by an electron of E 250 eV energy. This is important as it justifies the independent-electron approximation used in the model. At 1 fs, the (average) spatial distribution of secondary electrons is anisotropic with the electron cloud elongated in the direction of the primary impact. The maximal radius of the cascade is about 50 A at this time. As the system cools, energy is distributed more equally, and the spatial distribution of the electron cloud becomes isotropic. At 90 fs maximal radius is about 150 A. The Monte-Carlo model described here could be adopted for the investigation of radiation damage in other insulators and has implications for planned experiments with intense femtosecond X-ray sources., Comment: 26 pages, latex, 13 figures

Predictions for g_1(x,Q^2) at low Q^2 are obtained in the framework of the GVMD model. Contributions from both light and heavy vector mesons are evaluated. The DHGHY sum rule is employed to fix the magnitude of the light vector meson contribution to g_1, using the recent measurements in the region of baryonic resonances. The DHGHY moment function is calculated. Predictions are compared to the data., Comment: 5 pages, including 3 postscript figures; presented by B. Bade{\l}ek at 10th International Workshop on Deep Inelastic Scattering (DIS2002), Cracow, 30 April - 4 May 2002

Theoretical predictions for polarized nucleon structure function $g_1(x,Q^2)$ at low $Q^2$ are obtained in the framework of the Generalized Vector Meson Dominance model. Contributions from both light and heavy vector mesons are evaluated. In the photoproduction limit the first moment of $g_1$ is related to the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto sum rule. This property is employed to fix the magnitude of the light vector meson contribution to $g_1$, using the recent measurements in the region of baryonic resonances. Results are compared to the data on $g_1(x,Q^2)$. Finally, the DHGHY moment function $I(Q^2)$ is calculated, and our theoretical predictions are confronted with the recent preliminary data obtained at the Jefferson Laboratory., Comment: 12 pages including 6 postscript figures, corrected ref.22, caption to fig.4 and a few typos

Theoretical predictions show that at low values of Bjorken $x$ the spin structure function, $g_1$ is influenced by large logarithmic corrections, $ln^2(1/x)$, which may be predominant in this region. These corrections are also partially contained in the NLO part of the standard DGLAP evolution. Here we calculate the non-singlet component of the nucleon structure function, $g_1^{NS}=g_1^p-g_1^n$, and its first moment, using a unified evolution equation. This equation incorporates the terms describing the NLO DGLAP evolution and the terms contributing to the $ln^2(1/x)$ resummation. In order to avoid double counting in the overlapping regions of the phase-space, a unique way of including the NLO terms into the unified evolution equation is proposed. The scheme-independent results obtained from this unified evolution are compared to the NLO fit to experimental data, GRSV'2000. Analysis of the first moments of $g_1^{NS}$ shows that the unified evolution including the $ln^2(1/x)$ resummation goes beyond the NLO DGLAP analysis. Corrections generated by double logarithms at low $x$ influence the $Q^2$-dependence of the first moments strongly., Comment: 13 pages, latex, 2 figures; Appendix added

We have analyzed the cascade of secondary electrons in diamond and amorphous carbon generated by the thermalisation of a single Auger electron. The elastic electron mean free path was calculated as a function of impact energy in the muffin-tin potential approximation. The inelastic scattering cross section and the energy loss of the electron (expressed in terms of differential inverse mean free path) were estimated from two "optical" models, that utilise the measured dielectric constants of the materials. Using these data, a Monte-Carlo model describing the time evolution of the cascade was constructed. The results show that at most around 20-40 secondary cascade electrons are released by a single Auger electron in a macroscopic sample of diamond or amorphous carbon. Consideration of the real band structure of diamond reduces this number further. The release of the cascade electrons happens within the first 100 femtoseconds after the emission of the primary Auger electron. The results have implications to planned experiments with femtosecond X-ray sources., Comment: 18 pages, latex, 9 figures

Photoionisation of an atom by X-rays usually removes an inner-shell electron from the atom, leaving behind a perturbed "hollow ion" whose relaxation may take different routes. In light elements, emission of an Auger electron is common. However, the energy and the total number of electrons released from the atom may be modulated by shake-up and shake-off effects. When the inner-shell electron leaves, the outer-shell electrons may find themselves in a state that is not an eigen-state of the atom in its surroundings. The resulting collective excitation is called shake-up. If this process also involves the release of low energy electrons from the outer shell, then the process is called shake-off. It is not clear how significant shake-up and shake-off contributions are to the overall ionisation of biological materials like proteins. In particular, the interaction between the out-going electron and the remaining system depends on the chemical environment of the atom, which can be studied by quantum chemical methods. Here we present calculations on model compounds to represent the most common chemical environments in proteins. The results show that the shake-up and shake-off processes affect about 20% of all emissions from nitrogen, 30% from carbon, 40% from oxygen, and 23% from sulphur. Triple and higher ionisations are rare for carbon, nitrogen and oxygen, but are frequent for sulphur. The findings are relevant to the design of biological experiments at emerging X-ray free-electron lasers., Comment: 14 pages, latex, 1 figure

Through irradiation with intense femtosecond soft X-ray pulses the structural damage originating from ultra-fast non-thermal phase transitions is observed. A computational model which combines electronic structure theory with molecular dynamics (MD) for the atomic motion is extended to include interactions relevant at soft X-ray wavelengths (6-13.5 nm). Dedicated Boltzmann transport equation is derived, with electron relaxation rates calculated from density functional theory and energy loss function from binary encounter theory. Preliminary results for diamond are presented and compared to the earlier predictions from MD simulations