Measurement of the Excited-State Molecular Polarizability of C 60
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N. TANG,* R. W. HELLWARTH, AND J. P. PARTANEN Department of Physics and Electrical Engineering, University of Southern California, Los Angeles, CA 90089-0484. *Current address: 3005 P St., Ste.1, WL/MLPJ, Bldg. 651, Area B, WPAFB, OH 45433.
ABSTRACT We use -30 ps pulses at 532 nm to measure the complex excited-state molecular polarizability cae in a C60/benzene solution. We determine the imaginary part of 0e by measuring the excited-state absorption cross-section in a pump-probe experiment. In a degenerate-four-wave-mixing (DFWM) experiment, we find that in delayed probing of the complex index gratings formed by -30 ps pulses, the thermal and the excited-state polarizability changes both contribute to these transient gratings.
INTRODUCTION The most natural parameter characterizing the complete excited-state linear optical property is the complex excited-state linear molecular polarizability cXe, whose imaginary part is related to the absorption cross-section while the real part is responsible for the refractive behavior. It is believed that excited singlet C60 molecules go through intersystem crossing to the lowest triplet state in a fraction of a nanosecond (up to about I ns depending on the preparation), where they stay for tens of microseconds before decaying back to the ground state. 1 , 2 In an earlier paper, we did not observe difference in the absorption cross-section between the excited singlet and the lowest triplet state in a pump-probe experiment.3 We reported the excited-state absorption cross-section at 532nm to be ae = (12.3± 0.8)x 10" cm2 , which agrees well with the literature value. 2 We also determined
the ground state absorption cross-section at 532 nm to be ag = (3.5 + 0.3)x 10-18cm 2 . In this paper we try to determine the complete complex excited-state molecular polarizability ae by adding the information extracted from a degenerate-four-wave-mixing (DFWM) experiment. The linear polarizability of the excited-state contributes to the third order nonlinearity due to the fact that molecules must first be promoted to the excited-state. However promoting molecules to the excited-state by absorption inevitably dumps energy into the sample. In a typical DFWM experiment with parallel polarized laser pulses, there are usually contributions in the DFWM signal from thermal and acoustic origin due to the hydrodynamic process following the absorption. This process can be described by the Navier-Stokes equations, which have solutions in close form under certain circumstances. 4 At the same time, the spatially periodic population of the excited-state molecules modifies the dielectric constant, 511 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
which in turn contributes to the DFWM signal. In the following we incorporate both excitedstate and hydrodynamic contributions into our theory.
EXPERIMENT AND THEORY We obtain 99.5% pure C6 0 powder from MER Corporation (Tucson, Arizona) and use it as is. The benzene solvent is spectrophotometry grade from J. T. Baker Inc. (Phillipsburg, New
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