FLUORINE SUBSTITUTION EFFECTS ON THE PHOTODISSOCIATION DYNAMICS OF IODOBENZENE AT 304 NM

The photodissociation dynamics of pentafluoroiodobenzene are investiga ted by state-selective one-dimensional translation spectroscopy at 304 nm. We have determined the one-dimensional recoil distribution and th e spatial distribution in the form of the anisotropy parameter, beta, as well as the photodissociation relative yields of both ground-state I(P-3(3/2)) and excited-state I(P-2(1/2)) iodine photofragments. The results are compared to those observed for iodobenzene at 304 nm. As i n iodobenzene, two velocity distributions were observed for the dissoc iation channel which gives ground-state iodine: a sharp, high recoil v elocity peak assigned previously to n,sigma excitation and a slow rec oil velocity distribution peak assigned previously to pi,pi excitatio n. Unlike in C6H5I, the I distribution is relatively strong and its s patial anisotropy can be measured. The fluorine perturbation has led t o a number of different observations that can be summarized as follows : (1) The high velocity distribution has a lower average value and muc h broader width, suggesting more rapid energy redistribution to the fl uorinated phenyl ring prior to and during the dissociation process, re sulting from stronger coupling between the n,sigma and pi,pi* states and/or a longer excited-state lifetime; (2) the slow distribution is w eaker and has an almost isotropic spatial distribution (the anisotropy parameter beta approximate to 1.0), while in the iodobenzene spectrum beta is correlated with the recoil velocity; (3) the I quantum yield for C6F5I is 14 times larger than that for iodobenzene; and (4) beta is correlated with the velocity in the I spectrum found for C6F5I whi ch is not observed for iodobenzene. These observed fluorine perturbati ons are attributed to an increased mixing between the charge-transfer state (resulting from electron transfer from the iodine nonbonding ele ctrons to pi orbitals of the fluorinated benzene ring) and both the n ,sigma and the ring pi,pi* states. This leads to two effects: (1) a d ecrease in the nonbonding electron density on the iodine, which decrea ses the spin-orbit interaction between the n,sigma states themselves, resulting in a decrease in the curve-crossing probability (thus incre asing the I yield) and (2) an increase in the coupling between the re pulsive n,sigma states and the fluorinated phenyl pi,pi* states, lead ing to an increase in the rate of energy redistribution.