Determination of spin-orbit branching fractions in the photodissociation of halogenated hydrocarbons

Two methods based on vacuum ultraviolet (vuv) photoionization are presented for the determination of the spin-orbit branching fractions of the halogen atom produced in the photodissociation of halogenated hydrocarbons. Both methods make use of differences in the photoionization cross sections of the ²P_3/2 ground state and the ²P_1/2 excited-state of the neutral halogen atom. In the first approach, measurements of the total photoionization signal of the halogen atom are made at several vuv wavelengths, and the difference in the wavelength dependences for the ²P_3/2 and ²P_1/2 atoms allows the extraction of the branching fractions. In the second approach, the vuv wavelength is set close to the ionization threshold of the ²P _3/2 atom (well above that of the ²P_1/2 atom), and measurements are made at several electric field strengths, which shift the ionization threshold and thus vary the photoionization cross sections. In both methods, the relative cross sections of the ground- and excited-state atoms are determined by using the known branching fractions for the 266 nm photodissociation of methyl iodide. These methods are applied to the photodissociation of isopropyl iodide and allyl iodide, two systems for which standard ion-imaging techniques do not provide unique branching fractions.