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Complementary to our recent report on the F+HD reaction, the reactive excitation functions for the other isotopomers are presented. Through analysis of the differential cross section data, the collisional energy dependencies of product vibrational branchings for F+HD are also reported here. Several important conclusions can be drawn from this work. First, the transition-state properties, in particular the barrier height, of this reaction are well-characterized by the SW PES, despite its neglect of spin-orbit couplings. Second, contrary to the theoretical conclusion in recent literatures, an experimental observation is presented which seems to suggest that a resonance may indeed exist for the F+H[sub 2] reaction in support of the original interpretation proposed by Lee and co-workers. Third, the vibrational branching for the F+HD→HF+D reaction elucidates another facet of resonance effects in the integral cross sections. Finally, the nonadiabatic reactivity of the spin-orbit excited F[sup *]([sup 2]P[sub 1/2]) atom is found to be small, which is in line with the conclusion inferred from a most recent, full quantum mechanical multisurface calculation. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2-5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark-Werner potential energy surface, which accurately reproduce the shape of the resonance feature. A detailed analysis of quantum dynamics using the spectral quantization method reveals that a quantum resonance exists near 0.52 kcal/mol, which is localized about the collinear FHD geometry. At collision energies below 1 kcal/mol, the reaction was found to proceed almost exclusively through resonant tunneling with very little contribution from direct, over the barrier, reaction. The properties of the quantum resonance, such as the position, lifetime, and partial widths were found to correlate well with the experimental results and the quantum scattering calculations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactions of C3H and C4H radicals with C6H2 were investigated for the first time. Reactants C3H, C4H, and C6H2 were synthesized in two beams of C2H2 diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C9H2 and C10H2 produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C3H (C4H) + C6H2 reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C9H2 (C10H2) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C9H2 (C10H2) is in good agreement with that of C9H2 (C10H2) produced from the C7H (C8H) + C2H2 reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C9H2 and 8.8 ± 0.1 eV for C10H2. The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of ³HC9H/c-¹HC3(C)C5H/c-¹HC7(C)CH + H and ¹HC10H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C9H2 remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C9H2 and C10H2) can be synthesized from reactions of CmH (e.g., m = 3 and 4) radicals with polyynes (e.g., HC6H) and gives some valuable implications to planetary, interstellar, and combustion chemistry. [ABSTRACT FROM AUTHOR]

The article presents a case study of a 68 year old man with a dual-chamber pacemaker who suffered with intermittent dizziness after participating in a Judo competition. On assessment, an electrocardiogram showed evidence of a high-grade atrioventricular block and a radiograph of the chest showed that the ventricular lead of the pacemaker was fractured into 3 parts. It is stated that the cause of fracture was likely excessive movement of the upper limbs and direct trauma.

Presents a letter to the editor about atrial fibrillation.

We present results of near-threshold photoionization of N3 photofragments produced by laser photodissociation of ClN3 at 248 nm. The time of flight of recoiling N3 is used to resolve two photochemical channels producing N3, which exhibit different translational energy release. The two forms of N3 resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear (X 2Πg) and cyclic N3. This result agrees with the existing theoretical calculations of excited and ionic states of N3 and strengthens previous experimental results which suggested that the ClN3 photolysis produces a cyclic form of N3. [ABSTRACT FROM AUTHOR]