265 research outputs found
Electron Photodetachment from Aqueous Anions. III. Dynamics of Geminate Pairs Derived from Photoexcitation of Mono- vs. Poly- atomic Anions
Photostimulated electron detachment from aqueous inorganic anions is the
simplest example of solvent-mediated electron transfer. Here we contrast the
behavior of halide anions with that of small polyatomic anions, such as
pseudohalide anions (e.g., HS-) and common polyvalent anions (e.g., SO32-).
Geminate recombination dynamics of hydrated electrons generated by 200 nm
photoexcitation of aqueous anions (I-, Br-, OH-, HS-, CNS-, CO32-, SO32-, and
Fe(CN)64-) have been studied. Prompt quantum yields for the formation of
solvated, thermalized electrons and quantum yields for free electrons were
determined. Pump-probe kinetics for 200 nm photoexcitation were compared with
kinetics obtained at lower photoexcitation energy (225 nm or 242 nm) for the
same anions, where possible. Free diffusion and mean force potential models of
geminate recombination dynamics were used to analyze these kinetics. These
analyses suggest that for polyatomic anions (including all polyvalent anions
studied) the initial electron distribution has a broad component, even at
relatively low photoexcitation energy. There seem to be no well-defined
threshold energy below which the broadening of the distribution does not occur,
as is the case for halide anions. Direct ionization to the conduction band of
water is the most likely photoprocess broadening the electron distribution. Our
study suggests that halide anions are in the class of their own; electron
photodetachment from polyatomic, especially polyvalent, anions follows a
different set of rules.Comment: to be submitted to J. Phys. Chem. A; 28 pages, 5 figs + Supplemen
Frequency-domain "single-shot" (FDSS) ultrafast transient absorption spectroscopy using compressed laser pulses. Part I. Basic treatment
Single-shot ultrafast absorbance spectroscopy based on the frequency encoding
of the kinetics is analyzed theoretically and implemented experimentally. The
kinetics are sampled in the frequency domain using linearly chirped, amplified
33 fs FWHM pulses derived from a Ti:sapphire laser. A variable length grating
pair compressor is used to achieve the time resolution of 500-1000 channels per
a 2-to-160 ps window with sensitivity > 5x10-4. In terms of the acquisition
time, FDSS has an advantage over the pump-probe spectroscopy in a situation
when the "noise" is dominated by amplitude variations of the signal, due to the
pump and flow instabilities. The possibilities of FDSS are illustrated with the
kinetics obtained in multiphoton ionization of water and aqueous iodide and
one-photon excitation of polycrystalline ZnSe and thin-film amorphous Si:H.
Unlike other "single-shot" techniques, FDSS can be implemented for fluid
samples flowing in a high-speed jet and for thin solid samples that exhibit
interference fringes; no a priori knowledge of the excitation profile of the
pump across the beam is needed. Another advantage is that due to the
interference of quasimonochromatic components of the chirped probe pulse, an
oscillation pattern near the origin of the FDSS kinetics emerges. This pattern
is unique and can be used to determine the complex dielectric function of the
photogenerated species.Comment: 29 pages, 10 figure
Geminate recombination of hydroxyl radicals generated in 200 nm photodissociation of aqueous hydrogen peroxide
The picosecond dynamics of hydroxyl radicals generated in 200 nm photoinduced
dissociation of aqueous hydrogen peroxide have been observed through their
transient absorbance at 266 nm. It is shown that these kinetics are nearly
exponential, with a decay time of ca. 30 ps. The prompt quantum yield for the
decomposition of H2O2 is 0.56, and the fraction of hydroxyl radicals escaping
from the solvent cage to the water bulk is 64-68%. These recombination kinetics
suggest strong caging of the geminate hydroxyl radicals by water.
Phenomenologically, these kinetics may be rationalized in terms of the
diffusion of hydroxide radicals out of a shallow potential well (a solvent
cage) with an Onsager radius of 0.24 nm.Comment: 14 pages, 1 figur
Investigating the retention of intermediate-mass black holes in star clusters using N-body simulations
Contrary to supermassive and stellar-mass black holes (SBHs), the existence
of intermediate-mass black holes (IMBHs) with masses ranging between 10^{2-5}
Msun has not yet been confirmed. The main problem in the detection is that the
innermost stellar kinematics of globular clusters (GCs) or small galaxies, the
possible natural loci to IMBHs, are very difficult to resolve. However, if
IMBHs reside in the centre of GCs, a possibility is that they interact
dynamically with their environment. A binary formed with the IMBH and a compact
object of the GC would naturally lead to a prominent source of gravitational
radiation, detectable with future observatories. We use N-body simulations to
study the evolution of GCs containing an IMBH and calculate the gravitational
radiation emitted from dynamically formed IMBH-SBH binaries and the possibility
that the IMBH escapes the GC after an IMBH-SBH merger. We run for the first
time direct-summation integrations of GCs with an IMBH including the dynamical
evolution of the IMBH with the stellar system and relativistic effects, such as
energy loss in gravitational waves (GWs) and periapsis shift, and gravitational
recoil. We find in one of our models an intermediate mass-ratio inspiral
(IMRI), which leads to a merger with a recoiling velocity higher than the
escape velocity of the GC. The GWs emitted fall in the range of frequencies
that a LISA-like observatory could detect, like the European eLISA or in
mission options considered in the recent preliminary mission study conducted in
China. The merger has an impact on the global dynamics of the cluster, as an
important heating source is removed when the merged system leaves the GC. The
detection of one IMRI would constitute a test of GR, as well as an irrefutable
proof of the existence of IMBHs.Comment: Accepted for publication by A&A, minor modification
Ultrafast Pulse Radiolysis Using a Terawatt Laser Wakefield Accelerator
We report the first ultrafast pulse radiolysis transient absorption
spectroscopy measurements from the Terawatt Ultrafast High Field Facility
(TUHFF) at Argonne National Laboratory. TUHFF houses a 20 TW Ti:sapphire laser
system that generates 2.5 nC sub-picosecond pulses of multi-MeV electrons at 10
Hz using laser wakefield acceleration. The system has been specifically
optimized for kinetic measurements in a pump-probe fashion. This requires
averaging over many shots which necessitates stable, reliable generation of
electron pulses. The latter were used to generate excess electrons in pulse
radiolysis of liquid water and concentrated solutions of perchloric acid. The
hydronium ions in the acidic solutions react with the hydrated electrons
resulting in the rapid decay of the transient absorbance at 800 nm on the
picosecond time scale. Time resolution of a few picoseconds has been
demonstrated. The current time resolution is determined primarily by the
physical dimensions of the sample and the detection sensitivity. Subpicosecond
time resolution can be achieved by using thinner samples, more sensitive
detection techniques and improved electron beam quality.Comment: submitted to J. Appl. Phys. 5 figures, 23 page
Geminate recombination of electrons generated by above-the-gap (12.4 eV) photoionization of liquid water
The picosecond geminate recombination kinetics for hydrated electrons
generated by 200 nm two photon absorption (12.4 eV total energy) has been
measured in both light and heavy water. The geminate kinetics are observed to
be almost identical in both H2O and D2O. Kinetic analysis based upon the
independent reaction time approximation indicates that the average separation
between the electron and its geminate partners in D2O is 13% narrower than in
H2O (2.1 nm vs. 2.4 nm). These observations suggest that, even at this high
ionization energy, autoionization of water competes with direct ionization.Comment: 10 pages + 2 figures, submitted to Chem. Phys. Letter
Electrode Strip Deposition for the CMS Barrel Drift Tube System
The full production ideation, design, set up and realization of the Electrode Strip Deposition for the entire construction of the CMS Barrel Drift Tube System are described in detail
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