Experiments, employing crossed molecular beams, with vibrational state resolution have been performed on the simplest four-atom reaction, OH + D2 --> HOD + D. In good agreement with the most recent quantum scattering predictions, mode-specific reaction dynamics is observed, with vibration in the newly formed oxygen-deuterium bond preferentially excited to v = 2. This demonstrates that quantum theoretical calculations, which in the past decade have achieved remarkable accuracy for three-atom reactions involving three dimensions, have progressed to the point where it is now possible to accurately predict energy disposal in four-atom reactions involving six dimensions.

The reactions of transition metal (M) atoms Zr and Nb with ethylene (C 2 H 4 ) were studied using the technique of crossed molecular beams. Angular and velocity distributions of MC 2 H 2 products following H 2 elimination were measured at collision energies between 5 and 23 kcal/mol using electron impact and 157 nm photoionization mass spectrometry. Photodepletion studies identify that the atomic reactants are predominantly in their ground electronic states and that the observed MC 2 H 2 products result primarily from reactions of these ground-state atoms. Center-of-mass product angular distributions derived from the data indicate that reactions involve the formation of intermediate complexes having lifetimes longer than their rotational periods. Product translational energy distributions demonstrate that a large fraction of excess available energy is channeled into product internal excitation. Wide-angle nonreactive scattering of metal atom reactants following decay of long-lived MC 2 H 4 association complexes was also observed for both transition metal reactants at collision energies g 9 kcal/mol, with approximately 36% of the initial translational energy converted into C 2 H 4 internal excitation. At collision energies of e 6 kcal/mol, nonreactive scattering of Zr from ZrC 2 H 4 decay was found to be negligible, whereas this channel was clearly observed for Nb. RRKM modeling of the competition between decay of MC 2 H 4 complexes back to M + C 2 H 4 and C-H insertion forming HMC 2 H 3 indicates that there exists an adiabatic potential energy barrier for M + C 2 H 4 association in the case of Zr and that the transition state for this process is tighter than for the analogous process in Nb + C 2 H 4 . The barrier for Zr + C 2 H 4 association is attributed to the repulsive s 2 ground state configuration of Zr, whereas for Nb the s 1 ground state configuration results in no barrier for association. The absence of decay of ZrC 2 H 4 back to Zr + C 2 H 4 at low collision energies indicates that the barrier for C-H insertion forming HZrC 2 H 3 lies below the barrier for Zr + C 2 H 4 association. This opens up the possibility that direct C-H insertion without initial ZrC 2 H 4 formation may play an important role.

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A 2 A 2 ) vibrational states. For all levels studied, corresponding to OClO(A 2 A 2 ←X 2 B 1 ) excitation wavelengths between 350 and 475 nm, the dominant product ͑Ͼ96%͒ was ClO͑ 2 ⌸͒ϩO( 3 P). We also observed production of ClϩO 2 with a quantum yield of up to 3.9Ϯ0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A 2 A 2 ) excited state vibrational mode. The ClϩO 2 yield was enhanced slightly by exciting A 2 A 2 levels having symmetric stretchingϩbending, but diminished by as much as a factor of 10 for neighboring peaks associated with symmetric stretchingϩasymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO͑ 2 ⌸͒ϩO( 3 P) channel. The photochemical dynamics of OClO possesses two energy regimes with distinctly different dynamics observed for excitation energies above and below ϳ3.1 eV ͑ϳ400 nm͒. At excitation energies below 3.1 eV ͑Ͼ400 nm͒, nearly all energetically accessible ClO vibrational energy levels were populated, and the minor ClϩO 2 channel was observed. Although at least 20% of the O 2 product is formed in the ground (X 3 ⌺ g Ϫ ) state, most O 2 is electronically excited (a 1 ⌬ g ). At EϽ3.1 eV, both dissociation channels occur by an indirect mechanism involving two nearby excited states, 2 A 1 and 2 B 2 . Long dissociation time scales and significant parent bending before dissociation led to nearly isotropic polarization angular distributions ͑␤ϳ0͒. At excitation energies above 3.1 eV ͑Ͻ400 nm͒, the ClϩO 2 yield began to decrease sharply, with this channel becoming negligible at Ͻ370 nm. At these higher excitation energies, the ClO product was formed with relatively little vibrational energy and a large fraction of the excess energy was channeled into ClOϩO translational energy. The photofragment anisotropy parameter ͑␤͒ also increased, implying shorter dissociation time scales. The sharp change in the disposal of excess energy into the ClO products, the decrease of ClϩO 2 production, and more anisotropic product angular distributions at EϾ3.1 eV signify the opening of a new ClOϩO channel. From our experimental results and recent ab initio calculations, dissociation at wavelengths shorter than 380 nm to ClOϩO proceeds via a direct mechanism on the optically prepared A 2 A 2 surface over a large potential energy barrier. From the ClO͑ 2 ⌸͒ϩO( 3 P) translational energy distributions, D 0 ͑O-ClO͒ was found to be less than or equal to 59.0Ϯ0.2 kcal/mol.

This first human clinical investigation of this technology demonstrates that the EPC capture coronary stent is safe and feasible for the treatment of de novo coronary artery disease. Further developments in this technology are warranted to evaluate the efficacy of this device for the treatment of coronary artery disease.