363 research outputs found
Mass-ratio dependent strong-field dissociation of artificial helium hydride isotopologues
We study the effect of the nuclear-mass ratio in a diatomic molecular ion on the dissociation dynamics in strong infrared laser pulses. A molecular ion is a charged system, in which the dipole moment depends on the reference point and therefore on the position of the nuclear center of mass, so that the laser-induced dynamics is expected to depend on the mass asymmetry. Whereas usually both the reduced mass and the mass ratio are varied when different isotopologues are compared, we fix the reduced mass and artificially vary the mass ratio in a model system. This allows us to separate effects related to changes in the resonance frequency, which is determined by the reduced mass, from those that arise due to the mass asymmetry. Numerical solutions of the time-dependent Schrödinger equation are compared with classical trajectory simulations. We find that at a certain mass ratio, vibrational excitation is strongly suppressed, which decreases the dissociation probability by many orders of magnitude
A toolset of functionalized porphyrins with different linker strategies for application in bioconjugation
The reaction of amines with pentafluorophenyl-substituted A3B-porphyrins has
been used to obtain different useful reactive groups for further
functionalization and/or conjugation of these porphyrins to other substrates
or materials. Porphyrins with alkenyl, alkynyl, amino, azido, epoxide,
hydroxyl, and maleimido groups have thus been synthesized. For the first time
such functionalized porphyrins have been conjugated to hyperbranched
polyglycerol (hPG) as a biocompatible carrier system for photodynamic therapy
(PDT) using the copper(I)-catalyzed 1,3-dipolar cycloaddition (CuAAC). The
photocytotoxicity of selected porphyrins as well as of the porphyrin-hPG-
conjugates has been assessed in cellular assays with human epidermoid
carcinoma A-253 and squamous carcinoma CAL-27 cells. For several biomedical
applications a release of the active drug and/or fluorescent dye is desired.
Therefore, additionally, the synthesis of A3B-porphyrins with cleavable linker
moieties is presented, namely disulfide, cleavable in a reductive environment,
and acetal linkers whose cleavage is pH triggered
Non-Adiabatic Electronic and Vibrational Ring-Opening Dynamics resolved with Attosecond Core-Level Spectroscopy
Non-adiabatic dynamics and conical intersections play a central role in the
chemistry of most polyatomic molecules, ranging from isomerization to
heterocyclic ring opening and avoided photo-damage of DNA. Studying the
underpinning correlated dynamics of electronic and nuclear wave packets is a
major challenge in real-time and, many times involves optically dark transient
states. We show that attosecond core-level spectroscopy reveals the pathway
dynamics of neutral furan across its conical intersections and dark states. Our
method measures electronic-nuclear correlations to detect the dephasing of
electronic coherence due to nuclear motion and identifies the ring-opened
isomer as the dominant product. These results demonstrate the efficacy of
attosecond core level spectroscopy as a potent method to investigate the
real-time dynamics of photochemical reaction pathways in complex molecular
systems
Mid-infrared observations of the transitional disks around DH Tau, DM Tau, and GM Aur
Aims: We present mid-infrared observations and photometry of the transitional
disks around the young stellar objects DH Tau, DM Tau, and GM Aur, obtained
with VISIR/VLT in N band. Our aim is to resolve the inner region and the
large-scale structures of these transitional disks, carrying potential
signatures of intermediate or later stages of disk evolution and ongoing planet
formation.
Methods: We use the simultaneously observed standard-stars as PSF reference
to constrain the radial flux profiles of our target objects. Subtracting the
obtained standard-star profile from the corresponding science object profile
yields the flux residuals produced by the star-disk system. A detection
threshold takes into account the background standard deviation and also the
seeing variations during the observations to evaluate the significance of these
flux residuals. On the basis of a simple model for the dust re-emission, we
derive constraints on the inner radius of the dust disk.
Results: We spatially resolve the transitional disk around GM Aur and
determine an inner-disk hole radius of 20.5(+1.0,-0.5) AU. The circumstellar
disks around DH Tau and DM Tau are not spatially resolved but we are able to
constrain the inner-disk hole radius to <15.5(+9.0,-2.0) AU and
<15.5(+0.5,-0.5) AU, respectively. The performed photometry yields fluxes of
178+-31 mJy for DH Tau, 56+-6 mJy for DM Tau, and 229+-14 mJy for GM Aur.Comment: Accepted for publication in Astronomy & Astrophysics. (6 pages,
including 7 figures and 5 tables
Molecular isomerization and fragmentation of polyatomic molecules controlled by inner-valence recollision-ionization
Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets
Terahertz Spin-to-Charge Current Conversion in Stacks of Ferromagnets and the Transition-Metal Dichalcogenide NbSe2
Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, terahertz emission spectroscopy is used to study spin-to-charge current conversion (S2C) in the TMDC NbSe2 in ultra-high-vacuum-grown F|NbSe2 thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe2 thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe2. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe2 with the opposite, negative sign of the spin Hall angle as compared to Pt. By a quantitative comparison of the emitted THz radiation from F|NbSe2 to F|Pt reference samples and the results of ab initio calculations, it is estimated that the spin Hall angle of NbSe2 for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers
Enhanced ionization of acetylene in intense laser pulses is due to energy upshift and field coupling of multiple orbitals
Synopsis We describe a new enhanced ionization mechanism for polyatomic molecules. It works via a significant energy up-shift of valence orbitals for stretched bonds and a strong concomitant increase in the coupling between multiple molecular orbitals
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