Determination of Spin−Orbit Branching Fractions in the Photodissociation of Halogenated Hydrocarbons

Abstract

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.

Near-UV photodissociation dynamics of CH2I2

“…At 118.2 nm, an accidental resonance with an autoionizing Rydberg state results in 19.2 times greater sensitivity for I over I*. 49,50 The I* quantum yield increases monotonically from zero at 355 nm to a maximum of around 0.46 at 248 nm. Consequently, I* atoms account for at most B2% of the total ion counts in the images acquired using the VUV probe.…”

Multiple product pathways in photodissociation of nitromethane at 213 nm

“…The   = 0.78 observed at 213 nm is considerably larger than the expected value. Although a small difference in the   values between the 215-nm band (0.78 at 213 nm) and the higher 201-nm band (0.61 at 193 nm) 30 is not fully explained, the large   in the 215-nm band at least indicates that the adiabatic correlation remains between the excited state involving the  * CC orbital in the Franck−Condon region and the product asymptotes.…”

“…However, as discussed by Fan and Pratt, 19,30 precise determination of the spinorbit state branching ratio from the multi-photon spectrum requires careful analysis. I adopted the procedure for determining the spin-orbit branching ratio…”

“…The experimental problems and the importance of the spinorbit branching ratio were discussed by Fan and Pratt in the photodissociation study of a series of halogenated hydrocarbons. 19,30 They applied two methods based on ionization detection by vacuum ultraviolet spectroscopy as complementary approaches to REMPI and translational spectroscopy. Their careful comparison and calibration yielded the fractions of I * ( 2 P 1/2 ) to be 0.61 and 0.90 with respect to the total I photofragment (I( 2 P 3/2 ) + I * ( 2 P 1/2 )) in the photodissociation of C 3 H 5 I at 193 nm and 266 nm, respectively.…”

“…Their careful comparison and calibration yielded the fractions of I * ( 2 P 1/2 ) to be 0.61 and 0.90 with respect to the total I photofragment (I( 2 P 3/2 ) + I * ( 2 P 1/2 )) in the photodissociation of C 3 H 5 I at 193 nm and 266 nm, respectively. 19,30 In this Chapter, I present the REMPI spectra and the spin-orbit stateresolved scattering distributions of the I fragment generated by the photodissociation of C 3 H 5 I at 266 nm and 213 nm. These two photolysis wavelengths are located in the lowest and next-lowest absorption bands.…”