Femtosecond Dynamics of Fe(CO)5 Photodissociation at 267 nm Studied by Transient Ionization

We found five consecutive processes with time constants 21, 15, 30, 47, and 3300 fs in Fe(CO)5 after excitation at 267 nm in the gas phase. The first four represent a continuous pathway of the molecule from the Franck−Condon region down to the lowest singlet state (S0) of Fe(CO)4 through a chain of Jahn−Teller-induced conical intersections. The motion before dissociation initially involves more than one of the equatorial ligands, but then eliminates only one CO. The product Fe(CO)4 is initially generated in its first excited singlet state S1, then it relaxes to S0 in 47 fs via a triply degenerate conical intersection at tetrahedral geometry. The pathway for this process involves pseudorotation of the ligands. The fifth step is assigned to thermal elimination of a second CO. Intersystem crossing to the triplet ground states of Fe(CO)4 and Fe(CO)3 takes more than 500 ps.