Controlling bimolecular reactions: Mode and bond selected reaction of water with hydrogen atoms

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.