The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (
1MLCT) has been characterized by femtosecond fluorescence up-conversionand transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad
1MLCTemission band at ~600 nm, which decays in
20 fs, and a weak emission at ~660 nm, which we attributeto the
3MLCT, populated by intersystem crossing (ISC) from the
1MLCT state. The TA studies show ashort-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm,along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the
3MLCTstate. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state
5T
2 lifetime. A singular value decomposition and a global analysis of the TA data, basedon a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. Thefirst is the decay of the
3MLCT, the second is identified as the population time of the
5T
2 state, while thethird is its decay time to the ground state. The anomalously high ISC rate is identical in [Ru
II(bpy)
3]
2+ andis therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with theregular quasi-harmonic vibrational progression of the
1MLCT absorption, we propose a simple model ofavoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction.The subsequent relaxation steps down to the
5T
2 state dissipate ~2000 cm
-1/100 fs. This rate is discussed,and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.