Femtosecond Real-Time Probing of Reactions. 6. A Joint Experimental and Theoretical Study of 8I_2 Dissociation

Abstract

The dynamics of the ultraviolet photofragmentation of bismuth dimer are studied experimentally and theoretically in the time domain. Employing the technique of femtosecond transition-state spectroscopy, the evolution of the dissociative process along two reaction channels leading to the 6p^3(^4S^0_(3/2) + 6p^3(^2D^0_(3/2) and 6p^3(^4S^0_(3/2)) + 6p^3(^D^0_(5/2)) levels of the atomic products is investigated following initial excitation of Bi_2 at λ. = 308 nm. The broad spectral width of the ultrashort probe laser pulse coupled with the closely spaced excited energy levels of Bi enables fluorescence via some 14 atomic transitions to be monitored in real time, rendering possible the detection of dissociating molecules at different internuclear separations on the controlling potential surfaces. Long-time detection of the ^2D^0_(3/2) spin-orbit level permits clocking of the reaction along the lower energy exit channel, for which we report a dissociation time τ_(1/2) of approximately 1 ps, at which time the product Bi atoms are separated by some 10.7 Å. Analogous measurements for the reaction giving rise to the higher-lying J = 5/2 level of the ^2D_j term yield a value of τ_(1/2) = 1.5 ps, corresponding to an interfragment distance of 7.5 Å. From the dissociation times so obtained, values for the length parameters that characterize noninteracting model potential curves V_1(r) and V_1(r) for dissociation via both exit channels may also be determined. Early time detection of [Biv•••Bi]•• reflects dynamical behavior over transition-state regions of the potential surfaces and allows various aspects of the nature of the force field governing fragmentation to be deduced. Finally, model quantum and classical calculations of the dissociation process are presented, which reproduce many of the salient features of the observed reaction dynamics.