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

Abstract

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 (S 0 ) 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 S 1, then it relaxes to S 0 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.