Ultrafast photodissociation dynamics of Ni(CO)(4)

By time-resolved nonresonant (800 nm) multiphoton ionization we found six c onsecutive 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 corr espond to relaxation and internal conversion between metal-to-ligand charge -transfer states in the intact molecule. Only the next step (600 fs) repres ents elimination of a CO group. This is an order of magnitude slower than i n most other metal carbonyls investigated so far. The dissociation product is Ni(CO)(3) in its S-1 state. It does not relax to the ground state but lu minesces (probably after intersystem crossing). This implies the absence of an easily accessible S-1/S-0 conical intersection. Such an intersection is induced by the Jahn-Teller effect in other carbonyls (which therefore do n ot 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(C O)(2), which has probably slightly higher energy. Although the case of tetr acarbonyl nickel superficially looks very different from that of other meta l carbonyls, there are many similarities of the detailed pathway.