246 research outputs found
Atomic and molecular ionization dynamics in strong IR and XUV fields probed by time-resolved coincidence spectroscopy
In the work for this thesis, a split-mirror-setup was designed and build, which was used to split the XUV laser-pulse of FELs (Free Electron Laser) into two identical pulses from which one can be delayed. With this setup the laser pulses of FLASH, Hamburg and SCSS, Harima(Japan) where characterized temporally, to determine the temporal pulse-structure for subsequent experiments. The intermolecular dynamics of the homonuclear diatomic molecules nitrogen and oxygen were examined and the experimental results were reproduced by classical simulations. In the measurement with oxygen for an energy band of the coincident singly charged ions, an ionization probability was found that depends on the delay between the two XUV-pulses. This can most probably be explained by the autoionization of an excited singly charged molecular state. Subsequently the investigation of the two photon double ionization (TPDI) of deuterium is presented. In the single pulse experiments simulations within the Born-Oppenheimer approximation made it possible to distinguish between the direct and sequential TPDI. In the pump-probe experiments light was shed onto the dynamics of the TPDI. In addition, experiments with strong few-cycle near-infrared (NIR) pulses are presented that examined the carrier envelope phase (CEP) dependence of the non-sequential double ionization of argon. Implementing single-shot CEP-tagging in conjunction with coincidence spectroscopy allowed to achieve unprecedented accuracy in measuring correlated electron dynamics
Attosecond control of electron dynamics in carbon monoxide
Laser pulses with stable electric field waveforms establish the opportunity
to achieve coherent control on attosecond timescales. We present experimental
and theoretical results on the steering of electronic motion in a
multi-electron system. A very high degree of light-waveform control over the
directional emission of C+ and O+ fragments from the dissociative ionization of
CO was observed. Ab initio based model calculations reveal contributions to the
control related to the ionization and laser-induced population transfer between
excited electronic states of CO+ during dissociation
Laser-assisted collision effect on nonsequential double ionization of helium in a few-cycle laser pulse
Nonsequential double ionization (NSDI) of helium in an intense few-cycle
laser pulse is investigated by applying the three-dimensional semi-classical
re-scattering method. It is found that the momentum distribution of He
shows a single-double-single peak structure as the pulse intensity increases.
According to the different mechanisms dominating the NSDI process, the laser
intensity can be classified into three regimes where the momentum distribution
of He exhibits different characteristics. In the relatively high
intensity regime, an NSDI mechanism named the "laser-assisted collision
ionization" is found to be dominating the NSDI process and causing the single
peak structure. This result can shed light on the study of non-sequential
ionization of a highly charged ion in a relatively intense laser pulse
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
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
Quantifying the Performance of Protein-Resisting Surfaces at Ultra-Low Protein Coverages using Kinesin Motor Proteins as Probes
No abstract.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57376/1/3171_ftp.pd
Label-free electrochemical impedance biosensor to detect human interleukin-8 in serum with sub-pg/ml sensitivity
Biosensors with high sensitivity and short time-to-result that are capable of detecting biomarkers in body fluids such as serum are an important prerequisite for early diagnostics in modern healthcare provision. Here, we report the development of an electrochemical impedance-based sensor for the detection in serum of human interleukin-8 (IL-8), a pro-angiogenic chemokine implicated in a wide range of inflammatory diseases. The sensor employs a small and robust synthetic non-antibody capture protein based on a cystatin scaffold that displays high affinity for human IL-8 with a KD of 35 ± 10 nM and excellent ligand specificity. The change in the phase of the electrochemical impedance from the serum baseline, ∆θ(ƒ), measured at 0.1 Hz, was used as the measure for quantifying IL-8 concentration in the fluid. Optimal sensor signal was observed after 15 min incubation, and the sensor exhibited a linear response versus logarithm of IL-8 concentration from 900 fg/ml to 900 ng/ml. A detection limit of around 90 fg/ml, which is significantly lower than the basal clinical levels of 5-10 pg/ml, was observed. Our results are significant for the development of point-of-care and early diagnostics where high sensitivity and short time-to-results are essential
Preparation and Characterization of Covalently Binding of Rat Anti-human IgG Monolayer on Thiol-Modified Gold Surface
The 16-mercaptohexadecanoic acid (MHA) film and rat anti-human IgG protein monolayer were fabricated on gold substrates using self-assembled monolayer (SAM) method. The surface properties of the bare gold substrate, the MHA film and the protein monolayer were characterized by contact angle measurements, atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) method and X-ray photoelectron spectroscopy, respectively. The contact angles of the MHA film and the protein monolayer were 18° and 12°, respectively, all being hydrophilic. AFM images show dissimilar topographic nanostructures between different surfaces, and the thickness of the MHA film and the protein monolayer was estimated to be 1.51 and 5.53 nm, respectively. The GIXRD 2θ degrees of the MHA film and the protein monolayer ranged from 0° to 15°, significantly smaller than that of the bare gold surface, but the MHA film and the protein monolayer displayed very different profiles and distributions of their diffraction peaks. Moreover, the spectra of binding energy measured from these different surfaces could be well fitted with either Au4f, S2p or N1s, respectively. Taken together, these results indicate that MHA film and protein monolayer were successfully formed with homogeneous surfaces, and thus demonstrate that the SAM method is a reliable technique for fabricating protein monolayer
Waveguide-Based Biosensors for Pathogen Detection
Optical phenomena such as fluorescence, phosphorescence, polarization, interference and non-linearity have been extensively used for biosensing applications. Optical waveguides (both planar and fiber-optic) are comprised of a material with high permittivity/high refractive index surrounded on all sides by materials with lower refractive indices, such as a substrate and the media to be sensed. This arrangement allows coupled light to propagate through the high refractive index waveguide by total internal reflection and generates an electromagnetic wave—the evanescent field—whose amplitude decreases exponentially as the distance from the surface increases. Excitation of fluorophores within the evanescent wave allows for sensitive detection while minimizing background fluorescence from complex, “dirty” biological samples. In this review, we will describe the basic principles, advantages and disadvantages of planar optical waveguide-based biodetection technologies. This discussion will include already commercialized technologies (e.g., Corning’s EPIC® Ô, SRU Biosystems’ BIND™, Zeptosense®, etc.) and new technologies that are under research and development. We will also review differing assay approaches for the detection of various biomolecules, as well as the thin-film coatings that are often required for waveguide functionalization and effective detection. Finally, we will discuss reverse-symmetry waveguides, resonant waveguide grating sensors and metal-clad leaky waveguides as alternative signal transducers in optical biosensing
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