SPIN-ORBIT INDUCED RADIATIONLESS TRANSITIONS IN ORGANOMETALLICS - QUANTUM SIMULATION OF THE (1)E-](3)A(1) INTERSYSTEM CROSSING PROCESS IN HCO(CO)(4)

A theoretical description of the "fast" (<50 ps) intersystem crossing processes occurring at critical geometries during the photodissociation of HCo(CO)4 is presented. The radiationless transitions are simulated by wave packet propagations along one-dimensional reaction coordinate on the spin-orbit coupled potential energy surfaces. The propagation are performed separately, either along the Co-H bond or along the Co-COax bond. This original approach has enabled us to understand the mechanism of desactivation of the initially populated singlet excited state in this molecule which should be considered as a model for other organometallics. We propose the following mechanism: (i) in a very short time scale (<20 fs) 40% of the system dissociates towards the primary products H+Co(CO)4 ( 1E), whereas the 1E→3A1 intersystem crossing along the Co-H bond elongation occurs within 50 ps; (ii) the dissociation of an axial carbonyl ligand occurs in a larger time scale (200 fs) and only 2% of the system dissociates along the Co-COax elongation. The dominant process is the 1E→3A 1 intersystem crossing leading to HCo(CO)4( 3A1); (iii) as soon as the lowest triplet state is populated, the system dissociates either to H+Co(CO)4 or to HCo(CO)3+COax on the 3A1 potential energy surface; (iv) the intersystem crossing process may be described as a succession of elementary transitions occurring at critical geometries or crossing points between the singlet and the triplet potential energy surfaces; (v) the efficiency of the radiationless transition is governed by the overlap of the propagated wave packet with the critical region of the coupled potential energy surfaces. © 1995 American Institute of Physics.