Index of content:
Volume 111, Issue 7, 15 August 1999
111(1999); http://dx.doi.org/10.1063/1.479566View Description Hide Description
We study the dynamics in optical collisions of Na with Ne, Ar, Kr, and Xe in a differential scattering experiment. We report the observation of nonadiabatic transitions in the excited collisional quasimolecule based on measurements of the population ratio of the and fine-structure levels. Comparison with theoretical results shows a generally very good agreement over the range of collision energies (0.01–0.3 eV) scanned in our experiment, using the best available potentials. For the heavier rare-gas systems a strong influence of the crossing on the population ratios is observed. We further extract a universal function for the nonadiabatic transition probability for these systems. In the thermal energy range, our results are in good qualitative agreement with data from gas phase optical collision experiments.
Crossed beam reaction of the cyano radical, with methylacetylene, Observation of cyanopropyne, and cyanoallene,111(1999); http://dx.doi.org/10.1063/1.479567View Description Hide Description
The chemical dynamics to form cyanopropyne, and cyanoallene, via the neutral–neutral reaction of the cyano radical, CN with methylacetylene, is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ The laboratory angular distribution and time-of-flight spectra of the products are recorded at m/e=65, 64, 63, and 62. The reaction of with CN radicals yields reactive scattering signal at and demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the π electron density of the β carbon atom of the methylacetylene molecule to form a long lived collision complex. The latter decomposes via two channels, i.e., H atom loss from the group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.
Giant enhancement of electron-induced dissociation of chlorofluorocarbons coadsorbed with water or ammonia ices: Implications for atmospheric ozone depletion111(1999); http://dx.doi.org/10.1063/1.479613View Description Hide Description
The yield produced by dissociative electron attachment of a submonolayer of is enhanced by factors of and when is coadsorbed with waterice and ammonia ice, respectively, on a surface at ∼25 K. Moreover, the magnitude of enhancement increases strongly with decreasing concentration. This enhancement is attributed to dissociation of by capture of electrons self-trapped in polar water or ammonia molecules. This process may be an unrecognized sink for chlorofluorocarbons in the atmosphere. ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.
111(1999); http://dx.doi.org/10.1063/1.479568View Description Hide Description
A structure-based tunneling mechanism developed to predict the ionization of molecules subjected to intense, ultrafast irradiation is tested on the series of hydrocarbons: acetylene, ethylene, and ethane. Relative ionization probabilities (1, 4.1, and 8.7 for ethane, acetylene, and ethylene, respectively) are measured upon excitation with 780 nm, 125 fs pulses of and compared to predictions of the model (1, 4.1, and 7.9 for ethane, acetylene, and ethylene, respectively). Ionization probabilities determined via the ADK (Ammosov, Delone, and Krainov) model for atomic ionization (1, 2.7, and 13.1 for ethane, acetylene, and ethylene, respectively) are shown to be near those of the structure-based model.