SPIN-ORBIT INDUCED RADIATIONLESS TRANSITIONS IN ORGANOMETALLICS - QUANTUM SIMULATION OF THE INTERSYSTEM CROSSING PROCESSES IN THE PHOTODISSOCIATION OF HCO(CO)(4)

A theoretical description of the ''fast'' (<50 ps) intersystem crossin g (ISC) processes occurring during the photodissociation of HCo(CO)(4) is presented. The radiationless transitions are simulated by wave pac ket propagations on spin-orbit coupled two-dimensional potential energ y surfaces (CASSCF/CCI) calculated along two reaction coordinates (q(a )=[Co-H] and q(b)=[Co-COax]). A mechanism of deactivation of the singl et excited state of HCo(CO)(4) initially populated on UV excitation ha s been proposed. This mechanism differs slightly from the one deduced from a one-dimensional simulation performed separately, either along t he Co-H bond or along the Co-COax bond: (i) in a very short time scale (<20 fs) 35% of the system dissociates toward the primary products HCo(CO)(4) ((1)E), whereas the (1)E-->(3)A(1) and (1)E-->(3)E intersyst em crossings occur within 50 ps; (ii) as soon as the lowest triplet st ates are populated, the system dissociates either to H+Co(CO)(4) ((3)A (1)) or to H+Co(CO)(4) ((3)E) on the corresponding potential energy su rfaces; (iii) the indirect photodissociation mechanism through ISC may be described as a succession of elementary transitions followed by th e homolysis of the metal-hydrogen bond. (C) 1997 American Institute of Physics.