<title>Picosecond measurements of optical nonlinearities in King's complex and synthesized analogues</title>

Abstract

We have measured the photodynamics of reverse saturable absorption (RSA) in solutions of cyclopentadienyliron carbonyl tetramer (King's complex) using picosecond pump-probe techniques. Similar preliminary measurements in solutions of synthesized variations of the King's complex indicate that the excited state transition responsible for the observed RSA is most likely a second d-d transition within the metal core of the molecule. On time scales of hundreds of picoseconds, the observed RSA in the King's complex is well characterized by a three-level rate-equation, singlet-state absorption model, where the excited-state cross section is greater than that of the ground state. On nanosecond timescales and at fluences above 200 mJ.cm<SUP>-2</SUP>, however, we observe the onset of a response that is consistent with a thermally induced scattering process. Further evidence of this scattering is provided by angularly-resolved measurements of the transmitted and back-scattered signals for nanosecond excitation. When the King's complex is incorporated in a solid host negligible scatter was observed and the response is completely described by the singlet parameters extracted from the picosecond measurements. The observation of, scatter from solution, together with a time- resolved decay to the ground state that is rapid (approximately 120 ps) and largely nonradiative in this molecule, indicate that solutions of King's complex may provide a mechanism for efficiently generating thermal nonlinearities on a subnanosecond timescale.