Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence.

Recurrent fluorescence (RF) from isolated carbon clusters containing between 24 and 60 atoms is theoretically investigated as a function of internal energy, cluster size, and structural features. The vibrational relaxation kinetics and the associated IR emission spectra are determined by means of a Monte Carlo approach with vibrational density of states computed in the harmonic approximation. RF is generally found to be highly competitive with vibrational emission. The behaviors predicted for clusters of various sizes and archetypal structures indicate that the IR emission spectra are strongly influenced by RF, an energy gap law being obtained for the evolution of the RF rate constant depending on the electronic excitation state. The present results are relevant to the photophysics of the interstellar medium and could contribute to elucidating the carriers of the extended red emission bands and the continuum emission lying below the aromatic infrared bands believed to originate from mixed aromatic-aliphatic compounds.

Photoinduced electron transfer in a molecular dyad by nanosecond pump-pump-probe spectroscopy.

The design of robust and inexpensive molecular photocatalysts for the conversion of abundant stable molecules like H2O and CO2 into an energetic carrier is one of the major fundamental questions for scientists nowadays. The outstanding challenge is to couple single photoinduced charge separation events with the sequential accumulation of redox equivalents at the catalytic unit for performing multielectronic catalytic reactions. Herein, double excitation by nanosecond pump-pump-probe experiments was used to interrogate the photoinduced charge transfer and charge accumulation on a molecular dyad composed of a porphyrin chromophore and a ruthenium-based catalyst in the presence of a reversible electron acceptor. An accumulative charge transfer state is unattainable because of rapid reverse electron transfer to the photosensitizer upon the second excitation and the low driving force of the forward photodriven electron transfer reaction. Such a method allows the fundamental understanding of the relaxation mechanism after two sequential photon absorptions, deciphering the undesired electron transfer reactions that limit the charge accumulation efficiency. This study is a step toward the improvement of synthetic strategies of molecular photocatalysts for light-induced charge accumulation and more generally, for solar energy conversion.

Size Effect in the Ionization Energy of PAH Clusters.

We report the first experimental measurement of the near-threshold photoionization spectra of polycyclic aromatic hydrocarbon clusters made of pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron-photoion coincidence spectrometry with a VUV synchrotron beamline. The experimental results of the ionization energy are compared to calculated ones obtained from simulations using dedicated electronic structure treatment for large ionized molecular clusters. Experiment and theory consistently find a decrease of the ionization energy with cluster size. The inclusion of temperature effects in the simulations leads to a lowering of this energy and to quantitative agreement with the experiment. In the case of pyrene, both theory and experiment show a discontinuity in the IE trend for the hexamer. This work demonstrates the ability of the models to describe the electronic structure of PAH clusters and suggests that these species are ionized in astronomical environments where they are thought to be present.

Intramolecular Processes Revealed Using UV-Laser-Induced IR-Fluorescence: A New Perspective on the "Channel Three" of Benzene.

Radiative relaxation in the infrared (IR) is common following nonradiative electronic relaxation processes, but it is rarely measured. We present ultraviolet laser-induced infrared fluorescence (UV-LIIRF) excitation spectroscopy and dispersed UV-LIIRF spectroscopy of gas phase benzene vibronically excited around the onset of channel 3, using a homemade spectrometer. We found that the vibrational IR fluorescence yield is clearly higher when benzene is excited above the onset than when it is excited below. Significant changes in dispersed IR emission profiles resulting from excitations below and above the onset of channel 3 were also observed. These results suggest that isomerization of benzene toward fulvene occurs efficiently below the opening of channel 3 and confirm that channel 3 involves a photophysical relaxation pathway that efficiently competes with isomerization.

Detection of atomic and molecular mega-electron-volt projectiles using an x-ray charged coupled device camera.

We show that an x-ray charge coupled device (CCD) may be used as a particle detector for atomic and molecular mega-electron-volt (MeV) projectiles of around a few hundred keV per atomic mass unit. For atomic species, spectroscopic properties in kinetic energy measurements (i.e., linearity and energy resolution) are found to be close to those currently obtained with implanted or surface barrier silicon particle detectors. For molecular species, in order to increase the maximum kinetic energy detection limit, we propose to put a thin foil in front of the CCD. This foil breaks up the molecules into atoms and spreads the charges over many CCD pixels and therefore avoiding saturation effects. This opens new perspectives in high velocity molecular dissociation studies with accelerator facilities.

Temperature and anharmonic effects on the infrared absorption spectrum from a quantum statistical approach: application to naphthalene.

A method is developed to calculate the finite-temperature infrared absorption spectrum of polyatomic molecules with energy levels described by a second-order Dunham expansion. The anharmonic couplings are fully incorporated in the calculation of the quantum density of states, achieved using a Wang-Landau Monte Carlo procedure, as well as in the determination of transition energies. Additional multicanonical simulations provide the microcanonical absorption intensity as a function of both the absorption wavelength and the internal energy of the molecule. The finite-temperature spectrum is finally obtained by Laplace transformation of this microcanonical histogram. The present scheme is applied to the infrared spectrum of naphthalene, for which we quantify the shifting, broadening, and third-order effects as a continuous function of temperature. The influence of anharmonicity and couplings is manifested on the nontrivial variations of these features with increasing temperature.

Fragmentation branching ratios of highly excited hydrocarbon molecules CnH and their cations CnH+ (n

We have measured fragmentation branching ratios of neutral C(n)H and C(n)H(+) cations produced in high velocity (4.5 a.u) collisions between incident C(n)H(+) cations and helium atoms. Electron capture gives rise to excited neutral species C(n)H and electronic excitation to excited cations C(n)H(+). Thanks to a dedicated setup, based on coincident detection of all fragments, the dissociations of the neutral and cationic parents were recorded separately and in a complete way. For the fragmentation of C(n)H, the H-loss channel is found to be dominant, as already observed by other authors. By contrast, the H-loss and C-loss channels equally dominate the two-fragment break up of C(n)H(+) species. For these cations, we provide the first fragmentation data (n>2). Results are also discussed in the context of astrochemistry.

Gas-phase electronic spectra of two substituted benzene cations: phenylacetylene+ and 4-fluorostyrene+.

The visible spectra of phenylacetylene+ and 4-fluorostyrene+ have been measured by laser photodissociation spectroscopy. The observed vibronic systems were assigned to the B2A'' <-- X2A'' and C2B1 <-- X2B1 electronic transition in the 4-fluorostyrene+ and phenylacetylene+ cations, respectively. Two methods were employed and compared: a resonant multiphoton dissociation scheme of the bare cations and a resonant photodissociation technique applied to the chromophore+-argon n=1,2 ionic complexes. The latter approach allowed the intrinsic profile to be resolved, revealing different intramolecular dynamical behavior. Their electronic relaxation has been rationalized in terms of an apparent energy gap law for the benzene derivative cations.

Theoretical rates for the emission of atomic hydrogen from a naphthalene cation.

The statistical phase space theory (PST) in its orbital transition state version has been used to calculate the dissociation rate associated with the loss of atomic hydrogen from a polycyclic aromatic hydrocarbon molecule. The PST model has been applied to the naphthalene cation with input data obtained exclusively from first-principle calculations using density functional theory. Without any fitting parameters, the calculated dissociation rates are found to agree well with available measurements. The effects of vibrational anharmonicities are investigated and are shown to lower the dissociation rates by a factor of about five.

Laboratory spectra of cold gas phase polycyclic aromatic hydrocarbon cations, and their possible relation to the diffuse interstellar bands.

A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorption spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visible. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photon ionization, then to photodissociate this ionic complex by means of a delayed laser pulse in a time-of-flight mass spectrometer. The method is illustrated by presenting the visible spectra of the Naphthalene, Phenanthrene, Fluorene and Phenylacetylene cations. Such spectra can be unambiguously compared to the astronomical spectra of reddened stars, which exhibit the so-called diffuse interstellar bands (DIBs) in absorption. An interesting feature of the technique is its ability to measure the absolute absorption cross-sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consists in considering that the PAH cations could be carriers for some of the DIBs.

Peroxide sensors for the fission yeast stress-activated mitogen-activated protein kinase pathway.

The Schizosaccharomyces pombe stress-activated Sty1p/Spc1p mitogen-activated protein (MAP) kinase regulates gene expression through the Atf1p and Pap1p transcription factors, homologs of human ATF2 and c-Jun, respectively. Mcs4p, a response regulator protein, acts upstream of Sty1p by binding the Wak1p/Wis4p MAP kinase kinase kinase. We show that phosphorylation of Mcs4p on a conserved aspartic acid residue is required for activation of Sty1p only in response to peroxide stress. Mcs4p acts in a conserved phospho-relay system initiated by two PAS/PAC domain-containing histidine kinases, Mak2p and Mak3p. In the absence of Mak2p or Mak3p, Sty1p fails to phosphorylate the Atf1p transcription factor or induce Atf1p-dependent gene expression. As a consequence, cells lacking Mak2p and Mak3p are sensitive to peroxide attack in the absence of Prr1p, a distinct response regulator protein that functions in association with Pap1p. The Mak1p histidine kinase, which also contains PAS/PAC repeats, does not regulate Sty1p or Atf1p but is partially required for Pap1p- and Prr1p-dependent transcription. We conclude that the transcriptional response to free radical attack is initiated by at least two distinct phospho-relay pathways in fission yeast.

Sin1: an evolutionarily conserved component of the eukaryotic SAPK pathway.

The fission yeast Sty1/Spc1 mitogen-activated protein (MAP) kinase is a member of the eukaryotic stress-activated MAP kinase (SAPK) family. We have identified a protein, Sin1, that interacts with Sty1/Spc1 which is a member of a new evolutionarily conserved gene family. Cells lacking Sin1 display many, but not all, of the phenotypes of cells lacking the Sty1/Spc1 MAP kinase including sterility, multiple stress sensitivity and a cell-cycle delay. Sin1 is phosphorylated after stress but this is not Sty1/Spc1-dependent. Importantly, Sin1 is not required for activation of Sty1/Spc1 but is required for stress-dependent transcription via its substrate, Atf1. We find that in the absence of Sin1, Sty1/Spc1 appears to translocate to the nucleus but Atf1 is not fully phosphorylated and becomes unstable in response to environmental stress. Sin1 is also required for effective transcription via the AP-1 factor Pap1 but does not prevent its nuclear translocation. Remarkably chimaeric fusions of sin1 with chicken sin1 sequences rescue loss of sin1 function. We conclude that Sin1 is a novel component of the eukaryotic SAPK pathway.

Trisomy 12 and p53 deletion in chronic lymphocytic leukemia detected by fluorescence in situ hybridization: association with morphology and resistance to conventional chemotherapy.

The incidence of trisomy 12 and p53 deletion was studied in a group of chronic B-lymphocytic leukemia (B-CLL) patients, using fluorescence in situ hybridization (FISH). Trisomy 12 was detected in eight of 50 patients (16%) and p53 deletion in six of 38 cases analyzed (15.8%). A statistically significant difference was observed between the incidence of trisomy 12 in patients with typical and atypical morphology (3.03% versus 41.18%). No correlation was found between this alteration and the rest of the clinical and biological parameters studied (adenopathies, hepatomegaly, splenomegaly, lymphocyte count, staging, CD11c expression, and resistance to chemotherapy). The p53 deletion was correlated with the presence of hepatomegaly and splenomegaly, advanced stage of disease, and resistance to conventional chemotherapy. The application of FISH to whole blood cell nuclei, without prior manipulation or culture, showed a higher percentage of cells with trisomy 12 than when the method was used following culture. We conclude that 1) FISH is a simple and sensitive technique for the detection of numerical and structural chromosome abnormalities; 2) Its application to uncultured samples obviates the alteration of results originated by the probable growth advantage of the normal or neoplastic cell population in vitro; 3) Trisomy 12 appears to define a B-CLL subgroup of atypical morphology; and 4) The p53 deletion is correlated with advanced stage of disease and resistance to treatment.

[Intestinal malacoplakia in pediatrics: report of a case and review of the literature]. / Malacoplaquia intestinal en pediatría: a propósito de un caso y revisión de la literatura.

We are presenting the case of a 13 year-old male child who was hospitalized at INSN, with generalized edema, chronic diarrhea and hematoquezia. Biopsy studies of the intestine established the diagnosis of Malacoplakia. Despite of the treatment the clinical evolution didn't improve. A review of medical literature is discussed principally relate a Pediatrics.

Parto vaginal en embarazo gemelar. / Vaginal delivery in twin pregnancy

Se estudiaron restrospectivamente los partos gemelares atendidos por via vaginal en el Hospital "Guillermo Grant B." de Concepcion, entre 1980 y 1983. El mayor riesgo de depresion neonatal y de mortalidad perinatal aparecio en el segundo gemelo con presentacion distocica (transversa o podalica). Sin embargo, esta diferencia no fue estadisticamente significativa al compararla con la presentacion cefalica del segundo gemelo agrupados segun peso de nacimiento. Al aumentar el peso de nacimiento mejora el pronostico perinatal independiente de la presentacion