We investigate the effect of the frequency dependence of friction on the rate of photochemical isomerization in solution. Recent experiments have shown a significantly smaller effect of viscosity on rates at high viscosities than that predicted by Kramersrsquo; theory. We show that this partial saturation of viscosity effects arises from the fact that the effective friction in the barrier region is much smaller than its zero frequency value at high viscosities. The fractional values of the exponent agr; (wherekiso=Athinsp;(eegr;s0)minus;agr;and eegr;s0is the zero frequency shear viscosity) observed in recent experiments naturally arise when the frequency dependence of the friction is taken into account. The physical significance of these results is discussed.
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