Translational Diffusion of a Transient Charge‐Separated Species in Carbon Dioxide and Trifluoromethane Studied by the Transient Grating Method

By means of the transient grating method, we have measured the translational diffusion constant of a transient charge‐separated species created by the photo‐isomerization of 1′,3′,3′‐trimethyl‐6‐nitrospiro[2H‐1‐benzopyran‐2,2′‐indoline] in carbon dioxide and trifluoromethane from ρr = 0.8 to 2.0, where ρr is the reduced density by the critical density of the solvent. It has been revealed that the effect of the charge‐separation on the diffusion constant is small in these fluids due to the large molecular size of the transient molecule. The viscosity dependence of the diffusion constant is well simulated by an empirical equation which also works well for benzene and naphthalene.