Photo dissociation dynamics of acetic acid and trifluoroacetic acid at 193
mn have been investigated by measuring laser-induced fluorescence spectra o
f OH fragments. The OH fragments are produced exclusively in the ground ele
ctronic state. The measured energy distributions among the fragments are f(
r)(OH) = 0.05, f(t) = 0.42, f(int)(Ac) = 0.53 and f(r)(OH) = 0.05,f(t) = 0.
33, and f(int)(FAc) = 0.62, for acetic and trifluoroacetic acid, respective
ly, and negligible vibrational excitation in the OH fragments was observed.
The dissociation does not depend on the polarization of the dissociating l
ight. It was concluded from the measured energy distribution and no polariz
ation dependence that the electronic transition at 193 nm leads the parent
molecule to the singlet excited surface, and the dissociation takes place a
long the triplet surface with an exit channel barrier. From the estimated i
nternal. energies in the acetyl radicals,, the lifetimes of the acetyl radi
cals are estimated from the RRKM theory.