FEMTOSECOND REAL-TIME PROBING OF REACTIONS .12. VECTORIAL DYNAMICS OFTRANSITION-STATES

Femtosecond time-resolved techniques with KETOF (kinetic energy time-o f-flight) detection in a molecular beam are developed for studies of t he vectorial dynamics of transition states. Application to the dissoci ation reaction of IHgI is presented. For this system, the complex [I . .. Hg ... I]double dagger is unstable and, through the symmetric and asymmetric stretch motions, yields different product fragments: [I ... Hg ... I]double dagger --> HgI(X2SIGMA+) + (I(P3/2))-P-2 [or I*(P-2( 1/2))] (1a); [I ... Hg ... I]double dagger --> (Hg(S0))-S-1 + (I(P3/2 ))-P-2 + (I(P3/2)))-P-2 [or I(P-2(1/2))] (1b). These two channels, (1 a) and (1b), lead to different kinetic energy distributions in the pro ducts. It is shown that the motion of the wave packet in the transitio n-state region can be observed by MPI mass detection; the transient ti me ranges from 120 to 300 fs depending on the available energy. With p olarized pulses, the vectorial properties (transition moments alignmen t relative to recoil direction) are studied for fragment separations o n the femtosecond time scale. The results indicate the nature of the s tructure (symmetry properties) and the correlation to final products. For 311-nm excitation, no evidence of crossing between the I and I po tentials is found at the internuclear separations studied. (Results fo r 287-nm excitation are also presented.) Molecular dynamics simulation s and studies by laser-induced fluorescence support these findings.