No abstract

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖12〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction H+H2O→OH+H2. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Relative rate measurements for the three vibrational states ‖03〉−, ‖02〉−‖2〉, and ‖12〉−, which have similar total energies but correspond to very different distributions of vibrational energy, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉− promotes the H+H2O reaction much more efficiently than either the state having part of its energy in bending excitation (‖02〉−‖2〉) or the stretching state with the excitation shared between the two O–H oscillators (‖12〉−). The localized character of the vibrational overtone excitation in water has permitted the first observation of a bond selected bimolecular reaction using this approach. The reaction of hydrogen atoms with HOD molecules excited in the region of the third overtone of the O–H stretching vibration, 4νOH, forms at least a 100-fold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.

Non-thermal plasma techniques are being developed for the treatment of many gas-phase pollutants. In these methods electrical energy from electron beams or electrical discharges is directed selectively into the production of electrons, ions and radicals, or into molecular excitations that will result in the efficient destruction of the unwanted species. The processes by which this can be achieved are described. Kinetic analysis of the deposition of energy into contaminated air is illustrated by studies of the decomposition of nitrogen oxides, methylene chloride, carbon tetrachloride and methanol.

The reaction of HOD containing four quanta of O–H bond stretching vibration with H atoms produces OD fragments almost exclusively. Vibrational overtone excitation prepares HOD(4νOH) that reacts with H atoms formed in a microwave discharge. The endothermic reaction of water with hydrogen atoms does not occur for ground vibrational state water but proceeds at roughly the gas kinetic collision rate for the vibrationally excited molecule. The production of OD fragments from HOD(4νOH) in the reaction is at least two orders of magnitude more efficient than the production of OH, indicating very selective reaction of the vibrationally excited bond.

Non-thermal plasma processing methods have been shown to be effective for treating dilute concentrations of pollutants in large-volume atmospheric-pressure air streams. This paper presents results from basic experimental and theoretical studies aimed at identifying the main reactions responsible for the decomposition of four representative compounds: carbon tetrachloride, methylene chloride, trichloroethylene and methanol. Each of these compounds is shown to be decomposed by a different plasma species: electrons, nitrogen atoms, oxygen radicals and positive ions, respectively. By understanding what plasma species is responsible for the decomposition of a pollutant molecule, it is possible to establish the electrical power requirements of the plasma reactor and help identify the initial reactions that lead to the subsequent process chemistry. These studies are essential for predicting the scaling of the process to commercial size units.