Energy and Pressure Dependence of the CO_sub.2 Laser Induced Dissociation of Sulfur Hexafluoride

Low pressure SF6 with its isotopes in natural abundance was irradiated by a pulsed CO2 laser operated on theP20 line (10.6 μm band). Dissociation yields of32SF6 and34SF6 were measured separately. If the radiation is focussed into the cell, the dissociation yield is proportional to the 3/2 power of the laser energy, as was derived under general conditions and confirmed experimentally. The reaction probabilityP(Φ), the fraction of molecules dissociated by an energy flux Φ, was measured using parallel light. For both isotopes,P(Φ) saturates at high energy flux close toP=1. At a lower flux (2 J cm−2), the dissociation probability of32SF6 displays a threshold, whereas the dissociation probability of34SF6 is a very steep function of Φ over the whole range of fluxes.P(Φ) at the higher energy flux was measured in a cavity absorption cell, in which up to 80% of the molecules were dissociated by a single pulse. Below 0.2 mbar SF6 the dissociation yields for both isotopes are pressure independent. Above 2 mbar the isotopic selectivity is completely lost. Addition of hydrogen always decreases the dissociation yields.