Excited State Absorption in Cubane-Like Transition Metal Clusters
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W. JI*, H. J. DU*, AND S. SHI** * Department of Physics, National University of Singapore, Singapore 0511. ** Optical Crystal Laboratory and Department of Chemical Engineering, National University of Singapore, Singapore 0511.
ABSTRACT We report an investigation of excited-state absorption in cubane-like transition metal clusters. The fluence-dependent transmittances of the clusters have been measured with using 8ns laser pulses. Time-resolved transmission measurements show that the triplet-triplet absorption occurs within a few nanoseconds. We have also developed a five-level (two singlet states, two triplet states, and one ionized state) model to simulate the excited-state absorption in these cluster compounds. The comparison between the model and the experimental measurements indicates that the population of the triplet states is created mainly by an ionization-recombination process and the triplet-triplet transitions are responsible for the observed nonlinear absorption.
INTRODUCTION Recently materials that exhibit strong reverse saturable absorption have been investigated intensively. The interest in these materials has been stimulated by their potential applications to the optical limiter, a device that is highly transparent at low intensity of incident light but becomes nearly opaque when exposed to high-power light pulses. Such a device can be used to protect optical sensors from damage induced by intense light radiation. Reverse saturable absorption can be achieved by exploiting a material whose absorption cross-section of its excited state is greater than that of the ground state. Such excited-state absorption (ESA) has been observed in a number of molecules including fullerene C60 [1]-[3] and phthalocyanine derivatives [4][5]. In these materials the ESA mechanism has been interpreted by using a five-level model [2]-[4]. In this model, there are three singlet states and two triplet states involved. Molecules in the ground (lowest singlet) state are excited to the first excited singlet state by initial linear absorption. Then some of the excited molecules relax to the lower excited triplet state by inter-system crossing. Further absorption promotes the molecules from the first excited singlet state to the second excited singlet state or from the lower excited triplet state to the upper excited triplet state. Reverse saturable absorption occurs as a consequence of the absorption cross-section of these excited states being larger than that of the ground state. This model can explain the ESA at low incident energies and, however, fails in high energy regime [3]. As part of our search for optical limiting materials, we have synthesized a class of cubane-like transition metal clusters and observed large reverse saturable absorption in these clusters [6][7]. The origin of the observed nonlinear absorption has been tentatively attributed to 131
Mat. Res. Soc. Symp. Proc. Vol. 374 0 1 9 95 Materials Research Society
a ESA mechanism. In this report, we present a detailed study of the ESA phenomena in cubanelik
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