480 research outputs found
Adaptive Covariance Estimation with model selection
We provide in this paper a fully adaptive penalized procedure to select a
covariance among a collection of models observing i.i.d replications of the
process at fixed observation points. For this we generalize previous results of
Bigot and al. and propose to use a data driven penalty to obtain an oracle
inequality for the estimator. We prove that this method is an extension to the
matricial regression model of the work by Baraud
Detailed analysis of shake structures in the KLL Auger spectrum of H2S
Shake processes of different origin are identified in the KLL Auger spectrum
of H2S with unprecedented detail. The KLL Auger spectrum is presented together
with the S 1sâ1 photoelectron spectrum including the S 1sâ1Vâ1nλ and S
1sâ12pâ1nλ shake-up satellites with Vâ1 and nλ indicating a hole in the
valence shell and an unoccupied molecular orbital, respectively. By using
different photon energies between 2476 and 4150 eV to record the KLL Auger
spectra two different shake-up processes responsible for the satellite lines
are identified. The first process is a shake-up during the Auger decay of the
S 1sâ1 core hole and can be described by S 1sâ1â2pâ2Vâ1nλ. The second process
is the Auger decay of the shake-up satellite in the ionization process leading
to S 1sâ1Vâ1nλâ2pâ2Vâ1nλ transitions. By combining the results of
photoelectron and Auger spectra the involved Vâ1nλ levels are assigned
Resonant Inelastic X-ray Scattering of Methyl Chloride at the Chlorine K Edge
We present a combined experimental and theoretical study of isolated CH3Cl molecules using resonant inelastic x-ray scattering (RIXS). The high-resolution spectra allow extraction of information about nuclear dynamics in the core-excited molecule. Polarization-resolved RIXS spectra exhibit linear dichroism in the spin-orbit intensities, a result interpreted as due to chemical environment and singlet-triplet exchange in the molecular core levels. From analysis of the polarization-resolved data, Cl 2px, y and 2pz electronic populations can be determined
Simulation of Auger decay dynamics in the hard X-ray regime: HCl as a showcase
Auger decay after photoexcitation or photoemission of an electron from a deep inner shell in the hard X-ray regime can be rather complex, implying a multitude of phenomena such as multiple-step cascades, post-collision interaction (PCI), and electronic state-lifetime interference. Furthermore, in a molecule nuclear motion can also be triggered. Here we discuss a comprehensive theoretical method which allows us to analyze in great detail Auger spectra measured around an inner-shell ionization threshold. HCl photoexcited or photoionized around the deep Cl 1s threshold is chosen as a showcase. Our method allows calculating Auger cross sections considering the nature of the ground, intermediate and final states (bound or dissociative), and the evolution of the relaxation process, including both electron and nuclear dynamics. In particular, we show that we can understand and reproduce a so-called experimental 2D-map, consisting of a series of resonant Auger spectra measured at different photon energies, therefore obtaining a detailed picture of all above-mentioned dynamical phenomena at once
Ultrafast nuclear dynamics in the doubly-core-ionized water molecule observed via Auger spectroscopy
We present a combined experimental and theoretical study of the Auger-emission spectrum following double core ionization and excitation of gas-phase water molecules with hard-x-ray synchrotron radiation above the O Kâ2 threshold. We observe an indication of ultrafast proton motion occurring within the 1.5 fs lifetime of the double-core-hole (DCH) states in water. Furthermore, we have identified symmetric and antisymmetric dissociation modes characteristic for particular DCH states. Our results serve as a fundamental reference for state-of-the-art studies of DCH dynamic processes in liquid water both at synchrotron and free-electron-laser facilities
Deep-core photoionization of krypton atoms below and above the 1s ionization threshold
Electronic relaxation of atomic Kr below and above the 1s ionization threshold is investigated experimentally using hard x-ray photoelectron spectroscopy. The experimental results are interpreted with the aid of relativistic Dirac-Fock calculations. The 1s orbital core-hole lifetime is extracted and the satellite's structures accompanying the photoelectron main line are assigned. Auger spectra recorded below and above the K edge are also investigated. In particular, the Auger cascade originating from vacancies of different origin in the L shell is analyzed in great detail. Competition between radiative KL versus nonradiative KLL Auger emission is emphasized
Postcollision interaction effects in KLL Auger spectra following argon 1s photoionization
Postcollision interaction effects on the Auger decay of a deep core hole are
studied both experimentally and theoretically. KL2,3L2,3 decay spectra of the
Ar 1s vacancy are measured with high-energy resolution with excess photon
energies ranging from 0 to 200 eV above the ionization threshold. Interaction
of the Auger electron with the photoelectron and the ion field manifests
itself in the Auger spectra as a distortion of the energy distribution of the
Auger electron close to threshold. Moreover, recapture of the photoelectron
due to energy exchange is dominating in the low-photon-energy range above
threshold. The experimental results are compared with calculations based on
the semiclassical approach to the postcollision interaction. The energies of
the discrete levels and individual recapture cross sections are computed in
the Hartree-Fock approximation. Good agreement is found between the calculated
and experimental spectra, validating the model used
Cationic double K-hole pre-edge states of CS2 and SF6
Recent advances in X-ray instrumentation have made it possible to measure the
spectra of an essentially unexplored class of electronic states associated
with double inner-shell vacancies. Using the technique of single electron
spectroscopy, spectra of states in CS2 and SF6 with a double hole in the
K-shell and one electron exited to a normally unoccupied orbital have been
obtained. The spectra are interpreted with the aid of a high-level theoretical
model giving excellent agreement with the experiment. The results shed new
light on the important distinction between direct and conjugate shake-up in a
molecular context. In particular, systematic similarities and differences
between pre-edge states near single core holes investigated in X-ray
absorption spectra and the corresponding states near double core holes studied
here are brought out
Experimental observation and theoretical calculations of rydberg series in hollow lithium atomic states
Several extended Rydberg series have been experimentally identified in triply excited states of hollow lithium, by use of electron spectrometry and synchrotron radiation at the Advanced Light Source. Energies, partial cross sections, and quantum defects have also been calculated using the R-matrix approximation. Our results show that the two inner electrons stay in a core-excited state of given symmetry while the behavior of the third electron is mostly governed by the nuclear potential screened by the two inner electrons
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