309 research outputs found
Total partial and electron-capture cross sections for ionization of water vapor by 20-150 keV protons
IP
Experimental Observation of Resonance Effects in Intensely Irradiated Atomic Clusters
We have resolved the expansion of intensely irradiated atomic clusters on a femtosecond time scale. These data show evidence for resonant heating, similar to resonance absorption, in spherical cluster plasmas
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Interference in strong-field ionization of a two-centre atomic system
Strong-field photoionization of argon dimers by a few-cycle laser pulse is investigated using electron-ion coincidence momentum spectroscopy. The momentum distribution of the photoelectrons exhibits interference due to the emission from the two atomic argon centres, in analogy with a Young's doubleslit experiment. However, a simulation of the dimer photoelectron momentum spectrum based on the atomic spectrum supplemented with a theoretically derived interference term leads to distinct deviations from the experimental result. The deviations may have their origin in a complex electron dynamics during strong-field ionization of the Ar2 dimer. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Explosions of water clusters in intense laser fields
Energetic, highly-charged oxygen ions, (), are copiously
produced upon laser field-induced disassembly of highly-charged water clusters,
and , 60, that are formed by seeding high-pressure
helium or argon with water vapor. clusters (n40000) formed under
similar experimental conditions are found undergo disassembly in the Coulomb
explosion regime, with the energies of ions showing a
dependence. Water clusters, which are argued to be considerably smaller in
size, should also disassemble in the same regime, but the energies of fragment
O ions are found to depend linearly on which, according to
prevailing wisdom, ought to be a signature of hydrodynamic expansion that is
expected of much larger clusters. The implication of these observations on our
understanding of the two cluster explosion regimes, Coulomb explosion and
hydrodynamic expansion, is discussed. Our results indicate that charge state
dependences of ion energy do not constitute an unambiguous experimental
signature of cluster explosion regime.Comment: Submitted to Phys. Rev.
Enhanced ionization in small rare gas clusters
A detailed theoretical investigation of rare gas atom clusters under intense
short laser pulses reveals that the mechanism of energy absorption is akin to
{\it enhanced ionization} first discovered for diatomic molecules. The
phenomenon is robust under changes of the atomic element (neon, argon, krypton,
xenon), the number of atoms in the cluster (16 to 30 atoms have been studied)
and the fluency of the laser pulse. In contrast to molecules it does not
dissappear for circular polarization. We develop an analytical model relating
the pulse length for maximum ionization to characteristic parameters of the
cluster
Evaporative attachment of slow electrons to alkali nanoclusters
The abundance spectrum of Na^-_{n~7-140} anions formed by low energy electron
attachment to free nanoclusters is measured to be strongly and nontrivially
restructured with respect to the neutral precursor beam. This restructuring is
explained in quantitative detail by a general framework of evaporative
attachment: an electron is captured by the long-range polarization potential,
its energy is transferred into thermal vibrations, and dissipated by
evaporative cooling. The data also affirm a formulated relation between the
binding energies of cationic, neutral, and anionic clusters, and an adjustment
to the prior values of dimer evaporation energies.Comment: 9 pages, 3 figures, revise
Members of the fatty acid binding protein family are differentiation factors for the mammary gland
Mammary gland development is controlled by systemic hormones and by growth factors that might complement or mediate hormonal action. Peptides that locally signal growth cessation and stimulate differentiation of the developing epithelium have not been described. Here, we report that recombinant and wild-type forms of mammary-derived growth inhibitor (MDGI) and heart-fatty acid binding protein (FABP), which belong to the FABP family, specifically inhibit growth of normal mouse mammary epithelial cells (MEC), while growth of stromal cells is not suppressed. In mammary gland organ culture, inhibition of ductal growth is associated with the appearance of bulbous alveolar end buds and formation of fully developed lobuloalveolar structures. In parallel, MDGI stimulates its own expression and promotes milk protein synthesis. Selective inhibition of endogenous MDGI expression in MEC by antisense phosphorothioate oligonucleotides suppresses appearance of alveolar end buds and lowers the beta-casein level in organ cultures. Furthermore, MDGI suppresses the mitogenic effects of epidermal growth factor, and epidermal growth factor antagonizes the activities of MDGI. Finally, the regulatory properties of MDGI can be fully mimicked by an 11-amino acid sequence, represented in the COOH terminus of MDGI and a subfamily of structurally related FABPs. This peptide does not bind fatty acids. To our knowledge, this is the first report about a growth inhibitor promoting mammary gland differentiation
Dynamical ionization ignition of clusters in intense and short laser pulses
The electron dynamics of rare gas clusters in laser fields is investigated
quantum mechanically by means of time-dependent density functional theory. The
mechanism of early inner and outer ionization is revealed. The formation of an
electron wave packet inside the cluster shortly after the first removal of a
small amount of electron density is observed. By collisions with the cluster
boundary the wave packet oscillation is driven into resonance with the laser
field, hence leading to higher absorption of laser energy. Inner ionization is
increased because the electric field of the bouncing electron wave packet adds
up constructively to the laser field. The fastest electrons in the wave packet
escape from the cluster as a whole so that outer ionization is increased as
well.Comment: 8 pages, revtex4, PDF-file with high resolution figures is available
from http://mitarbeiter.mbi-berlin.de/bauer/publist.html, publication no. 24.
Accepted for publication in Phys. Rev.
Watching the acetylene vinylidene intramolecular reaction in real time
It is a long-standing dream of scientists to capture the ultra-fast dynamics
of molecular or chemical reactions in real time and to make a molecular movie.
With free-electron lasers delivering extreme ultraviolet (XUV) light at
unprecedented intensities, in combination with pump-probe schemes, it is now
possible to visualize structural changes on the femtosecond time scale in
photo-excited molecules. In hydrocarbons the absorption of a single photon may
trigger the migration of a hydrogen atom within the molecule. Here, such a
reaction was filmed in acetylene molecules (C2H2) showing a partial migration
of one of the protons along the carbon backbone which is consistent with
dynamics calculations on ab initio potential energy surfaces. Our approach
opens attractive perspectives and potential applications for a large variety of
XUV-induced ultra-fast phenomena in molecules relevant to physics, chemistry,
and biology.Comment: 21 pages, 3 figures, submitte
Multielectron effects in strong-field dissociative ionization of molecules
We study triple-ionization-induced, spatially asymmetric dissociation of N[subscript 2] using angular streaking in an elliptically polarized laser pulse in conjunction with few-cycle pump-probe experiments. The kinetic-energy-release dependent directional asymmetry in the ion sum-momentum distribution reflects the internuclear distance dependence of the fragmentation mechanism. Our results show that for 5–35-fs near-infrared laser pulses with intensities reaching 10[superscript 15] W/cm², charge exchange between nuclei plays a minor role in the triple ionization of N[subscript 2]. We demonstrate that angular streaking provides a powerful tool for probing multielectron effects in strong-field dissociative ionization of small molecules
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