Ultrafast Photodissociation Dynamics of Ni(CO)4

By time-resolved nonresonant (800 nm) multiphoton ionization we found six consecutive exponential processes after excitation of Ni(CO)4 at 267 nm in the gas phase. Up to four steps (time constants 22 to 70 fs) probably correspond to relaxation and internal conversion between metal-to-ligand charge-transfer states in the intact molecule. Only the next step (600 fs) represents elimination of a CO group. This is an order of magnitude slower than in most other metal carbonyls investigated so far. The dissociation product is Ni(CO)3 in its S1 state. It does not relax to the ground state but luminesces (probably after intersystem crossing). This implies the absence of an easily accessible S1/S0 conical intersection. Such an intersection is induced by the Jahn−Teller effect in other carbonyls (which therefore do not luminesce), but not in Ni(CO)3. To explain a pump−wavelength-dependent time constant (42, 55, and 113 ps at 260, 267, and 276 nm, respectively), we assume that part of Ni(CO)3 dissociates to electronically excited Ni(CO)2, which has probably slightly higher energy. Although the case of tetracarbonyl nickel superficially looks very different from that of other metal carbonyls, there are many similarities of the detailed pathway.