Nonlinear optical (NLO) absorption of a metal dithiolene complex, [(C2H5)(4)N][Ni(dmit)(2)] has been studied by open-aperture (OA) Z-scan technique under irradiation with 15 ns and 1 ns laser pulses at 1053 nm and 40 ps laser pulses at 1064 nm. Distinct excited-state absorption behaviors were observed: a reverse saturable absorption (RSA) for 15 ns while a saturable absorption (SA) for I ns and 40 ps laser pulses, and the SA for I ns laser pulses was stronger than that for the latter. The mechanism responsible for the results with different laser pulses is analyzed using a five energy-level diagram. (c) 2006 Elsevier B.V. All rights reserved.