Photochemistry and reaction dynamics of HBr in xenon matrices: Photodissociation of HBr and production of Br atoms

Translationally excited and mobile H atoms are produced by the 193 and 248 nm photodissociation of HBr in xenon matrices. In concentrated matrices the loss of HBr and production of Br atoms follows second order kinetics due to reactive depletion of the remaining HBr by photolytically produced H atoms. The second order rate constant for this process can be directly related to the laser fluence, photodissociation cross-section for HBr, and initial HBr concentration. At higher dilutions the average distance between HBr molecules becomes large compared to the mean displacement of an H atom and reactions between H atoms and HBr molecules are unlikely; the loss of HBr follows first order kinetics as does the production of Br atoms. The mean displacement of H atoms, as a result of 193 nm photolysis, is on the order of 100 Å. The Br atom co-fragment is thermally stable and does not exhibit significant photoinduced mobility at temperatures below 40 K. Formation of Xe₂H⁺ ions is observed but does not measurably effect the HBr loss or Br formation kinetics.