Long time scale dynamics of vibrationally excited (HBr)n clusters.

We investigated the photodissociation dynamics of vibrationally excited HBr molecules and clusters. The species were generated in a molecular beam and excited with an IR laser to a v = 1 vibrational state. A subsequent ultraviolet (UV)-pulse with 243 nm radiation photolysed the molecules to yield H-fragments, which were resonantly ionized by the same UV-pulse (2 + 1 REMPI) and detected in a velocity map imaging (VMI) experiment. We performed action spectroscopy to distinguish between two expansion regimes: (i) expansion leading to isolated HBr molecules and (ii) generation of large (HBr)n clusters. Photodissociation of isolated HBr ( v = 1) molecules in particular J ro-vibrational states yielded faster H-fragments (by approximately 0.3 eV) with respect to the photodissociation of the ground state HBr ( v = 0). On the contrary, the IR excitation of molecules in (HBr) n clusters enhanced the yield of the H-fragments UV-photodissociated from the ground-state HBr ( v = 0) molecules. Our findings show that these molecules are photodissociated within clusters, and they are not free molecules evaporated from clusters after the IR excitation. Nanosecond IR-UV pump-probe experiments show that the IR-excitation enhances the H-fragment UV-photodissociation yield up to ∼100 ns after the IR excitation. After these long IR-UV delays, excitation of HBr molecules in clusters does not originate from the IR-excitation but from the UV-photodissociation and subsequent caging of HBr molecules in v > 0 states. We show that even after ∼100 ns the IR-excited larger (HBr) n clusters do not decay to individual molecules, and the excitation is still present in some form within these clusters enhancing their UV-photodissociation.

Imaging of rotational wave-function in photodissociation of rovibrationally excited HCl molecules.

We demonstrate a visualization of quantum mechanical phenomena with the velocity map imaging (VMI) technique, combining vibrationally mediated photodissociation (VMP) of a simple diatomic HCl with the VMI of its H-photofragments. Free HCl molecules were excited by a pump infrared (IR) laser pulse to particular rotational J levels of the v = 2 vibrational state, and subsequently a probe ultraviolet laser photodissociated the molecule at a fixed wavelength of 243.07 nm where also the H-fragments were ionized. The molecule was aligned by the IR excitation with respect to the IR laser polarization, and this alignment was reflected in the angular distribution of the H-photofragments. In particular, the highest degree of molecular alignment was achieved for the J=1←0 transition, which exclusively led to the population of a single rotational state with M = 0. The obtained images were analyzed for further details of the VMP dynamics, and different J states were studied as well. Additionally, we investigated the dynamic evolution of the excited states by changing the pump-probe laser pulse delay; the corresponding images reflected dephasing due to a coupling between the molecular angular momentum and nuclear spin. Our measurements confirmed previous observation using the time-of-flight technique by Sofikitis et al. [J. Chem. Phys. 127, 144307 (2007)]. We observed a partial recovery of the originally excited state after 60 ns in agreement with the previous observation.

A simple photoacoustic detector for highly corrosive gases.

In this work, we present a new design of a cantilever-type photoacoustic (PA) detector with high chemical resistance to be used for a broad range of gaseous samples including highly corrosive gasses. A thin mica cantilever used to sense the PA pressure is the only part that comes into direct contact with the sample gas as its deflection is sensed by a probe laser from outside of the gas cell. The design of the detector is simple, compact, and affordable. It can be constructed without any special fabrication procedure in laboratories equipped with a standard mechanical and electronic workshop. The detector has been tested and its performance evaluated in combination with commercially available pulsed IR tunable optical parametric oscillator and amplifier delivering 2-10 mJ of energy per pulse sampling highly corrosive HCl and HBr gasses. The ro-vibration PA spectrum of the first overtone (ν = 0 → ν = 2) of HCl molecules in the range from 5315 to 5855 cm-1 is presented.

Wavelength resolved UV photodesorption and photochemistry of CO2 ice.

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET--desorption induced by electronic transition--occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10-40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from "pre-irradiated" ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies.

Spread of multidrug-resistant Streptococcus pneumoniae among hospitalized children in Slovakia.

A multidrug-resistant serotype 14 strain of Streptococcus pneumoniae was isolated from sterile-site specimens and nasopharyngeal secretions from > 200 children in Slovakia between 1985 and 1990. Nasopharyngeal culture surveys were done to determine the extent of spread and means of transmission of this strain. The resistant strain was isolated from cultures of 8 (33.0%) of 24 children at hospital A and from 1 (0.8%) of 130 children attending outpatient clinics or day care centers (P < .001). One-quarter of the initially uncolonized children at hospital A acquired the resistant strain during hospitalization. Among hospitalized children, frequent antimicrobial drug use (P < .01), prior hospitalization (P < .005), and length of hospital stay (P < .001) were associated with infection with the resistant strain. These findings support limiting broad-spectrum antimicrobial drug use and nonessential hospitalizations in settings were drug-resistant pneumococci are prevalent. Development of a pneumococcal vaccine that is immunogenic in young children is urgently needed.

Multiple antimicrobial resistance of pneumococci in children with otitis media, bacteremia, and meningitis in Slovakia.

Penicillin-resistant pneumococci have been isolated from middle ear fluid, blood, cerebrospinal fluid, and nasopharyngeal secretions of several hundred children in Slovakia since 1985; 116 of these isolates were serotyped and tested for susceptibility to antimicrobial drugs at the Centers for Disease Control and Prevention. To define the prevalence of drug-resistant pneumococci and identify risk factors for infection, laboratory and medical records were reviewed. Nearly all (96%) of the resistant strains tested were serotype 14. Of these, all were resistant to penicillin (MIC, 4-16 micrograms/mL); most were resistant to cefaclor, erythromycin, tetracycline, and chloramphenicol; and many had decreased susceptibility to trimethoprim-sulfamethoxazole and ceftriaxone. Frequent antibiotic use, prior hospitalization, and length of hospital stay (P < .001 for all 3) were associated with infection with resistant strains. These findings suggest the need for routine screening of pneumococcal isolates for penicillin resistance and highlight the importance of controlling globally the spread of resistant pneumococci.