978 research outputs found
Vibrational spectroscopy of NO^+(H_2O)_n: Evidence for the intracluster reaction NO^+(H_2O)_nâH_3O^+(H_2O)_(n-2)(HONO) at nâ„4
Infrared spectra of massâselected clusters NO^+(H_2O)_n for n=1 to 5 were recorded from 2700 to 3800 cm^(â1) by vibrational predissociation spectroscopy. Vibrational frequencies and intensities were also calculated for n=1 and 2 at the secondâorder MĂžllerâPlesset (MP2) level, to aid in the interpretation of the spectra, and at the singles and doubles coupled cluster (CCSD) level energies of n=1 isomers were computed at the MP2 geometries. The smaller clusters (n=1 to 3) were complexes of H_2O ligands bound to a nitrosonium ion NO^+ core. They possessed perturbed H_2O stretch bands and dissociated by loss of H_2O. The H_2O antisymmetric stretch was absent in n=1 and gradually increased in intensity with n. In the n=4 clusters, we found evidence for the beginning of a second solvation shell as well as the onset of an intracluster reaction that formed HONO. These clusters exhibited additional weak, broad bands between 3200 and 3400 cm^(â1) and two new minor photodissociation channels, loss of HONO and loss of two H_2O molecules. The reaction appeared to go to completion within the n=5 clusters. The primary dissociation channel was loss of HONO, and seven vibrational bands were observed. From an analysis of the spectrum, we concluded that the n=5 cluster rearranged to form H_3O^+(H_2O)_3(HONO), i.e., an adduct of the reaction products
Dimensionality dependence of optical nonlinearity and relaxation dynamics in cuprates
Femtosecond pump-probe measurements find pronounced dimensionality dependence
of the optical nonlinearity in cuprates. Although the coherent two-photon
absorption (TPA) and linear absorption bands nearly overlap in both quasi-one
and two-dimensional (1D and 2D) cuprates, the TPA coefficient is one order of
magnitude smaller in 2D than in 1D. Furthermore, picosecond recovery of optical
transparency is observed in 1D cuprates, while the recovery in 2D involves
relaxation channels with a time scales of tens of picoseconds. The experimental
results are interpreted within the two-band extended Hubbard model.Comment: 10 pages, 4 figure
Signatures of the excitonic memory effects in four-wave mixing processes in cavity polaritons
We report the signatures of the exciton correlation effects with finite
memory time in frequency domain degenerate four-wave mixing (DFWM) in
semiconductor microcavity. By utilizing the polarization selection rules, we
discriminate instantaneous, mean field interactions between excitons with the
same spins, long-living correlation due to the formation of biexciton state by
excitons with opposite spins, and short-memory correlation effects in the
continuum of unbound two-exciton states. The DFWM spectra give us the relative
contributions of these effects and the upper limit for the time of the
exciton-exciton correlation in the unbound two-exciton continuum. The obtained
results reveal the basis of the cavity polariton scattering model for the DFWM
processes in high-Q GaAs microcavity.Comment: 11 pages, 1 figur
Diffuse Neutron Scattering Study of a Disordered Complex Perovskite Pb(Zn1/3Nb2/3)O3 Crystal
Diffuse scattering around the (110) reciprocal lattice point has been
investigated by elastic neutron scattering in the paraelectric and the relaxor
phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The
appearance of a diffuse intensity peak indicates the formation of polar
nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of
this diffuse scattering indicates that these regions are in the shape of
ellipsoids, more extended in the direction than in the direction.
The quantitative analysis provides an estimate of the correlation length, \xi,
or size of the regions and shows that \xi ~1.2\xi , consistent with
the primary or dominant displacement of Pb leading to the low temperature
rhombohedral phase. Both the appearance of the polar regions at T*and the
structural transition at Tc are marked by kinks in the \xi curve but not
in the \xi one, also indicating that the primary changes take place in a
direction at both temperatures.Comment: REVTeX file. 4 pages, 3 figures embedded, New version after referee
cond-mat/010605
Computational Studies of Intramolecular Hydrogen Atom Transfers in the Ă-Hydroxyethylperoxy and Ă -Hydroxyethoxy Radicals
The Ă-hydroxyethylperoxy (I) and Ă-hydroxyethoxy (III) radicals are prototypes of species that can undergo hydrogen atom transfer across their intramolecular hydrogen bonds. These reactions may play an important role in both the atmosphere and in combustion systems. We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2). Our multi-coefficient Gaussian-3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol. These predictions agree rather well with the MPW1K and BB1K density functional theory predictions but disagree with predictions based on B3LYP energies or geometries. Our RRKM/master equation simulations suggest that almost 50% of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain. For Reaction 1 at 298 K, the variational TST rate constant is ~30% lower than the conventional TST result, and the microcanonical optimized multidimensional tunneling (OMT) method predicts that tunneling accelerates the reaction by a factor of 3. TST calculations on Reaction 2 reveal no variational effect and a 298 K OMT transmission coefficient of 105. The Eckart method overestimates transmission coefficients for both reactions. [ACS abstract]http://pubs.acs.org/cgi-bin/abstract.cgi/jpcafh/asap/abs/jp0704113.htm
Endotoxin and cytokines in patients with gastrointestinal tract perforation
Plasma levels of endotoxin and various cytokines were assessed in 70 patients with gastrointestinal tract perforation. Sepsis developed in 29 of them, and eight of these (27.6%) had on admission endotoxin levels higher than 9.8 pg ml-1. The clinical outcome correlated with the level of tumour necrosis factor α (TNFα), rather than with the endotoxin level. The high interleukin 6 (IL-6) level was shown in septic patients and no correlation was observed between the IL-6 level and the clinical outcome. Plasma TNFα levels tended to change independently from endotoxin levels, suggesting that TNFα may have been locally produced in inflammatory lesions
Dynamical effects of the nanometer-sized polarized domains in Pb(Zn1/3Nb2/3)O3
Recent neutron scattering measurements performed on the relaxor ferroelectric
Pb[(Zn1/3Nb2/3)0.92Ti0.08]O3 (PZN-8%PT) in its cubic phase at 500 K, have
revealed an anomalous ridge of inelastic scattering centered ~0.2 A-1 from the
zone center (Gehring et al., Phys. Rev. Lett. 84, 5216 (2000)). This ridge of
scattering resembles a waterfall when plotted as a phonon dispersion diagram,
and extends vertically from the transverse acoustic (TA) branch near 4 meV to
the transverse optic (TO) branch near 9 meV. No zone center optic mode was
found. We report new results from an extensive neutron scattering study of pure
PZN that exhibits the same waterfall feature. We are able to model the dynamics
of the waterfall using a simple coupled-mode model that assumes a strongly
q-dependent optic mode linewidth Gamma1(q) that increases sharply near 0.2 A-1
as one approaches the zone center. This model was motivated by the results of
Burns and Dacol in 1983, who observed the formation of a randomly-oriented
local polarization in PZN at temperatures far above its ferroelectric phase
transition temperature. The dramatic increase in Gamma1 is believed to occur
when the wavelength of the optic mode becomes comparable to the size of the
small polarized micro-regions (PMR) associated with this randomly-oriented
local polarization, with the consequence that longer wavelength optic modes
cannot propagate and become overdamped. Below Tc=410 K, the intensity of the
waterfall diminishes. At lowest temperatures ~30 K the waterfall is absent, and
we observe the recovery of a zone center transverse optic mode near 10.5 meV.Comment: 8 pages, 9 figures (one color). Submitted to Physical Review
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