title: Photodissociation and photoisomerization of alpha-fluorotoluene and 4-fluorotoluene in a molecular beam.
abstract: The photodissociation of jet-cooled alpha-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for alpha-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F-->C6H5CH2 (or C7H7)+F and C6H5CH2F-->C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F-->C6H5CHF+H and C6H5CH2F-->C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of alpha-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3-->C6H4FCH2+H and C6H4FCH3-->C6H4F+CH3 and two minor channels C6H4FCH3-->C6H5CH2 (or C7H7)+F and C6H4FCH3-->C6H5CH (or C7H6)+HF. The dissociation rates for alpha-fluorotoluene at 193 and 248 nm are 3.3 x 10(7) and 5.6 x 10(5) s(-1), respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0 x 10(6) s(-1). An ab initio calculation demonstrates that the barrier height for isomerization from alpha-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between alpha-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between alpha-fluorotoluene and 4-fluorotoluene.
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
authors.forename: Cheng-Liang
authors.initials: CL
authors.lastname: Huang
authors.forename: Jyh-Chiang
authors.initials: JC
authors.lastname: Jiang
authors.forename: Yuri A
authors.initials: YA
authors.lastname: Dyakov
authors.forename: Ming-Fu
authors.initials: MF
authors.lastname: Lin
authors.forename: Chien-Ming
authors.initials: CM
authors.lastname: Tseng
authors.forename: S H
authors.initials: SH
authors.lastname: Lin
authors.forename: Yuan T
authors.initials: YT
authors.lastname: Lee
authors.forename: Chi-Kung
authors.initials: CK
authors.lastname: Ni