110 research outputs found
Attosecond spectroscopy of bio-chemically relevant molecules
Understanding the role of the electron dynamics in the photochemistry of bio-chemically relevant molecules is key to getting access to the fundamental physical processes leading to damage, mutation and, more generally, to the alteration of the final biological functions. Sudden ionization of a large molecule has been proven to activate a sub-femtosecond charge flow throughout the molecular backbone, purely guided by electronic coherences, which could ultimately affect the photochemical response of the molecule at later times. We can follow this ultrafast charge flow in real time by exploiting the extreme time resolution provided by attosecond light sources. In this work recent advances in attosecond molecular physics are presented with particular focus on the investigation of bio-relevant molecules
Scattering effects from neighboring atoms in core-level WSe2 photoemission
Methods of attosecond science originally developed to investigate systems in the gas phase are currently being adapted to obtain temporal information on the electron dynamics that takes place in condensed-matter systems. In particular, streaking measurements have recently been performed to determine photoemission time delays from the WSe2 dichalcogenide. In this work we present a fully atomistic description of the photoemission process in WSe2 and provide angularly resolved photoemission cross sections and time delays from the W 4f, Se 3d and Se 4s core states of the system. Since these states are spatially localized, we propose a cluster approach in which we build up from smaller to larger clusters, so that we can assess the importance of scattering effects by each new layer of neighboring atoms. We use a static-exchange density functional theory method with B-spline functions, where a one-center angular-momentum expansion is supplemented by off-center expansions with fewer partial waves. This enhances convergence in comparison with a one-center expansion, which would require very high angular momenta to characterize the localized fast oscillations near each off-center atomic core. We find that the photoemission delays and fully differential cross sections are strongly affected by scattering events that take place off the neighboring atoms, implying the need to consider their effects for quantitative descriptions of the photoemission proces
Complex Coacervates between a Lactose-Modified Chitosan and Hyaluronic Acid as Radical-Scavenging Drug Carriers
Complex coacervation of two oppositely charged polysaccharides, namely a lactose-modified chitosan (CTL) and hyaluronan (HA), was investigated in this study. Coacervates of the two polysaccharides were prepared by drop-by-drop injection of HA into CTL. Transmittance and dynamic light scattering (DLS) measurements in combination with TEM analyses demonstrated the formation of spheroidal colloids in the nano-/microsize range showing good homogeneity. Strikingly, the presence of 150 mM supporting NaCl did not hamper the colloid formation. Stability studies on selected formulations demonstrated that HA/CTL coacervates were stable up to 3 weeks at 37 \ub0C and behaved as pH-responsive colloids since transition from entangled to disentangled chains was attained for a proper pH range. The possibility of freeze-drying the coacervates for storage purposes and the ability of encapsulating selected payloads were investigated as well, for two values of the fraction of the lactitol side-chain substitution (FL). Finally, biological tests using human neutrophils were undertaken at acidic pH value (pH = 6.0): under such experimental conditions, akin to those frequently occurring in the inflammatory microenvironment, coacervates scavenged reactive oxygen species (ROS) generated by these cells in basal conditions. Given the well documented bioactivity of CTL with respect to chitosan toward cartilage regeneration, these findings point to a possible application of HA/CTL-based colloids as scavenging and bioactive carriers for the delivery of therapeutic molecules at confined inflamed sites such as knee joints
Diffraction effects in the Recoil-Frame Photoelectron Angular Distributions of Halomethanes
Citation: Bomme, C., Anielski, D., Savelyev, E., Boll, R., Erk, B., Bari, S., . . . Rolles, D. (2015). Diffraction effects in the Recoil-Frame Photoelectron Angular Distributions of Halomethanes. 635(11). doi:10.1088/1742-6596/635/11/112020We have measured the Recoil Frame-Photoelectron Angular Distributions (RF-PADs) for inner-shell photoionization of CH3F, CH3I and CF3I halomethane molecules for photoelectron energies up to 300 eV detected within a 4? solid angle in the gas-phase. For high kinetic energies, the RF-PADs are dominated by diffraction effects that encode information on the molecular geometry. © Published under licence by IOP Publishing Ltd
Attosecond timing of electron emission from a molecular shape resonance
Shape resonances in physics and chemistry arise from the spatial confinement
of a particle by a potential barrier. In molecular photoionization, these
barriers prevent the electron from escaping instantaneously, so that nuclei may
move and modify the potential, thereby affecting the ionization process. By
using an attosecond two-color interferometric approach in combination with high
spectral resolution, we have captured the changes induced by the nuclear motion
on the centrifugal barrier that sustains the well-known shape resonance in
valence-ionized N. We show that despite the nuclear motion altering the
bond length by only , which leads to tiny changes in the potential
barrier, the corresponding change in the ionization time can be as large as
attoseconds. This result poses limits to the concept of instantaneous
electronic transitions in molecules, which is at the basis of the Franck-Condon
principle of molecular spectroscopy.Comment: 24 pages, 5 figure
Photoelectron recoil in CO in the x-ray region up to 7 keV
Carbon 1s photoelectron spectra of CO molecules in gas phase were recorded in
the tender x-ray energy range, from 2.3 to 6.9 keV. The intensity ratios of
individual peaks from ν=0 to 3 within the vibrational progression of the C 1s
photoelectron spectrum were determined at the various photon energies and are
shown to be strongly affected by the photoelectron recoil effect. The
experimental vibrational intensity ratios are compared with theoretical
predictions at different levels of accuracy. Developments of the recoil model,
using generalized Franck-Condon factors, rovibrational coupling, Morse
potential energy curves, and accurate angular averaging are presented and
applied to the analysis of the experimental results
Imaging Molecular Structure through Femtosecond Photoelectron Diffraction on Aligned and Oriented Gas-Phase Molecules
This paper gives an account of our progress towards performing femtosecond
time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe
setup combining optical lasers and an X-ray Free-Electron Laser. We present
results of two experiments aimed at measuring photoelectron angular
distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and
dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss
them in the larger context of photoelectron diffraction on gas-phase molecules.
We also show how the strong nanosecond laser pulse used for adiabatically
laser-aligning the molecules influences the measured electron and ion spectra
and angular distributions, and discuss how this may affect the outcome of
future time-resolved photoelectron diffraction experiments.Comment: 24 pages, 10 figures, Faraday Discussions 17
Attosecond photoionisation time delays reveal the anisotropy of the molecular potential in the recoil frame
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