653 research outputs found
Theoretical study of electronic damage in single particle imaging experiments at XFELs for pulse durations 0.1 - 10 fs
X-ray free-electron lasers (XFELs) may allow to employ the single particle
imaging (SPI) method to determine the structure of macromolecules that do not
form stable crystals. Ultrashort pulses of 10 fs and less allow to outrun
complete disintegration by Coulomb explosion and minimize radiation damage due
to nuclear motion, but electronic damage is still present. The major
contribution to the electronic damage comes from the plasma generated in the
sample that is strongly dependent on the amount of Auger ionization. Since the
Auger process has a characteristic time scale on the order of femtoseconds, one
may expect that its contribution will be significantly reduced for attosecond
pulses. Here, we study the effect of electronic damage on the SPI at pulse
durations from 0.1 fs to 10 fs and in a large range of XFEL fluences to
determine optimal conditions for imaging of biological samples. We analyzed the
contribution of different electronic excitation processes and found that at
fluences higher than - photons/m (depending on the
photon energy and pulse duration) the diffracted signal saturates and does not
increase further. A significant gain in the signal is obtained by reducing the
pulse duration from 10 fs to 1 fs. Pulses below 1 fs duration do not give a
significant gain in the scattering signal in comparison with 1 fs pulses. We
also study the limits imposed on SPI by Compton scattering.Comment: 35 pages, 9 figures, 1 table, 2 appendixes, 45 reference
Impact of ultrafast electronic damage in single particle x-ray imaging experiments
In single particle coherent x-ray diffraction imaging experiments, performed
at x-ray free-electron lasers (XFELs), samples are exposed to intense x-ray
pulses to obtain single-shot diffraction patterns. The high intensity induces
electronic dynamics on the femtosecond time scale in the system, which can
reduce the contrast of the obtained diffraction patterns and adds an isotropic
background. We quantify the degradation of the diffraction pattern from
ultrafast electronic damage by performing simulations on a biological sample
exposed to x-ray pulses with different parameters. We find that the contrast is
substantially reduced and the background is considerably strong only if almost
all electrons are removed from their parent atoms. This happens at fluences of
at least one order of magnitude larger than provided at currently available
XFEL sources.Comment: 15 pages, 3 figures submitted to PR
Reaction of Diphenyldiazomethane with Phosphorus Monothioacids
The mechanism of the reaction of phosphorus monothioacids
with diaryldiazomethanes was studied in different solvent systems
at 20 °c. The thiolo to thiono product ratios were determined by
1H NMR spectroscopy and g.l.c. of the reaction mixtures. The
results imply that the reaction involves two competing processes
leading to the corresponding S- and 0-isomeric esters
Angle resolved photoelectron spectroscopy of two-color XUV-NIR ionization with polarization control
Electron emission caused by extreme ultraviolet (XUV) radiation in the presence of a strong near infrared (NIR) field leads to multiphoton interactions that depend on several parameters. Here, a comprehensive study of the influence of the angle between the polarization directions of the NIR and XUV fields on the two-color angle-resolved photoelectron spectra of He and Ne is presented. The resulting photoelectron angular distribution strongly depends on the orientation of the NIR polarization plane with respect to that of the XUV field. The prevailing influence of the intense NIR field over the angular emission characteristics for He(1s) and Ne(2p) ionization lines is shown. The underlying processes are modeled in the frame of the strong field approximation (SFA) which shows very consistent agreement with the experiment reaffirming the power of the SFA for multicolor-multiphoton ionization in this regime
Gallium(III) chelates of mixed phosphonate-carboxylate triazamacrocyclic ligands relevant to nuclear medicine: structural, stability and in vivo studies
Three triaza macrocyclic ligands, H6NOTP (1,4,7-triazacyclononane-N,NâČ,Nâł-trimethylene phosphonic acid),
H4NO2AP (1,4,7-triazacyclononane-N-methylenephosphonic acid-NâČ,Nâł-dimethylenecarboxylic acid), and
H5NOA2P (1,4,7-triazacyclononane-N,NâČ-bis(methylenephosphonic acid)-Nâł-methylene carboxylic acid), and
their gallium(III) chelates were studied in view of their potential interest as scintigraphic and PET (Positron
Emission Tomography) imaging agents. A 1H, 31P and 71Ga multinuclear NMR study gave an insight on the
structure, internal dynamics and stability of the chelates in aqueous solution. In particular, the analysis of 71Ga
NMR spectra gave information on the symmetry of the Ga3+ coordination sphere and the stability of the chelates towards hydrolysis. The 31P NMR spectra afforded information on the protonation of the non-coordinated oxygen atoms from the pendant phosphonate groups and on the number of species in solution. The 1H NMR spectra allowed the analysis of the structure and the number of species in solution.
31P and 1H NMR titrations combined with potentiometry afforded the measurement of the protonation
constants (log KHi) and the microscopic protonation scheme of the triaza macrocyclic ligands. The remarkably
high thermodynamic stability constant (log KGaL =34.44 (0.04) and stepwise protonation constants of Ga
(NOA2P)2â were determined by potentiometry and 69Ga and 31P NMR titrations. Biodistribution and gamma
imaging studies have been performed on Wistar rats using the radiolabeled 67Ga(NO2AP)â and 67Ga
(NOA2P)2âchelates, having both demonstrated to have renal excretion. The correlation of the molecular
properties of the chelates with their pharmacokinetic properties has been analysed.The authors thank the financial support from the Fundação para a CiĂȘncia e Tecnologia
(F.C.T., Portugal, projects RREQ/481/QUI/2006 and RECI/QEQ-QFI/0168/2012), the Rede Nacional de RMN (RNRMN), the Hungarian Scientific Research Fund (OTKA grants K-109029 and K-120224), the JĂĄnos Bolyai Research Scholarship (Gy.T.) of the Hungarian Academy of Sciences and the EU COST Action TD1004 âTheragnostics Imaging and Therapyâ. The research was also supported by the EU and co-financed by the European Regional Development Fund (FEDER) under the projects CENTRO-07-CT62-FEDER) and
GINOP-2.3.2-15-2016-00008.info:eu-repo/semantics/publishedVersio
Clocking Auger electrons
Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. The relaxation occurs primarily via Auger emission, so excited-state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive in XFELs owing to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here we develop an approach termed âself-referenced attosecond streakingâ that provides subfemtosecond resolution in spite of jitter, enabling time-domain measurement of the delay between photoemission and Auger emission in atomic neon excited by intense, femtosecond pulses from an XFEL. Using a fully quantum-mechanical description that treats the ionization, core-hole formation and Auger emission as a single process, the observed delay yields an Auger decay lifetime of 2.2_â0.3^+0.2 fs for the KLL decay channel
Clocking Auger Electrons
Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving
it in inherently unstable states that decay on femtosecond timescales. As the
relaxation occurs primarily via Auger emission, excited state observations are
constrained by Auger decay. In situ measurement of this process is therefore
crucial, yet it has thus far remained elusive at XFELs due to inherent timing
and phase jitter, which can be orders of magnitude larger than the timescale of
Auger decay. Here, we develop a new approach termed self-referenced attosecond
streaking, based upon simultaneous measurements of streaked photo- and Auger
electrons. Our technique enables sub-femtosecond resolution in spite of jitter.
We exploit this method to make the first XFEL time-domain measurement of the
Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical
description, retrieve a lifetime of fs for the KLL
decay channel. Importantly, our technique can be generalised to permit the
extension of attosecond time-resolved experiments to all current and future FEL
facilities.Comment: Main text: 20 pages, 3 figures. Supplementary information: 17 pages,
6 figure
A biominĆsĂtĂ©s hatĂĄsa a fogyasztĂłk Ă©rzĂ©kelĂ©sĂ©re Ă©s attitƱdjĂ©re csokolĂĄdĂ©k esetĂ©n
The timeâenergy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modu- lations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science
Spectra of secondary electrons induced by channeled and nonchanneled ions in Si and Al
Prevenção de injĂșrias causadas por glyphosate em soja RR por meio do uso de aminoĂĄcido
- âŠ