Dynamics of fragment recoil in the femtosecond photodissociation of triiodide ions in liquid solution

Novel femtosecond, multiple pulse experiments including polarization contro l are performed to elucidate the dynamics of fragment recoil in the 400-nm photolysis of triiodide ions in liquid ethanol solution. The instantaneous resonance Raman response of the dynamic system, induced at well defined del ays after impulsive bond fission of the parent ion, displays a time-depende nt vibrational frequency of the diatomic fragment. This time dependence is interpreted through interactions between the fragments which decay with inc reasing interfragment separation. Simultaneously, the instantaneous anisotr opic response of the reactive system exhibits electronically coherent contr ibutions at very early times after bond breakage and provides direct eviden ce for the existence of electronic degeneracies in the vicinity of the Fran ck-Condon region. The decay of this coherent anisotropic component reflects a dynamic lifting of these electronic degeneracies upon recoil of the prod uct species and decay of residual interactions between the fragments. From both experiments it can be concluded that in liquid solution it takes about 2 ps until the fragments arrive in the asymptotic limit of the reaction wi th negligible product interactions. (C) 1999 American Institute of Physics. [S0021-9606(99)51035-0].