Reactive scattering of oxygen atoms: O+I2, ICl, Br2

Abstract

Angular and velocity distribution measurements of IO reactive scattering from crossed beams of O atoms and halogen molecules I2, ICl are reported. Angular distribution measurements are reported for BrO from O + Br2. The O atom beam was generated at ∼350 K from a microwave discharge source and the halogen molecule beam from a supersonic nozzle source at ∼380 K. The product time-of-flight distribution was recorded at each laboratory scattering angle by a mini-computer. The scattering data are found to be in excellent agreement with the RRKM-AM model of reactive scattering via a long-lived collision complex. The observation of IO product from O + ICl identifies the complex with a bound O-I-Cl triplet state, previously observed for O-Cl-Cl in matrix isolation studies, as proposed by Herschbach. The maximum centrifugal barrier B m′ for dissociation of the long-lived complex can be accurately determined, particularly for O + I2. The B m′ values indicate that both the entrance and exit valleys of the potential energy surface are governed by centrifugal barriers in the region of long-range van der Waals potentials. The comparatively small reaction cross section (e.g. Q ∼ 2 Å2 for O + Br2 from discharge flow measurements) is attributed primarily to an orientation requirement for reaction. The RRKM-AM model indicates a ‘tight linear’ transition state for dissociation of the O-I-I complex, corresponding to significant long-range IO orienting forces in the exit valley of the potential energy surface.