Intense three wave mixing (3WM) spectra are reported for transient fragments produced by 266 nm laser photolysis of benzene and several substituted benzenes. Single pulse broadband 3WM spectra taken with an optical multichannel analyzer establish that the fragments are primary photoproducts obtained under collisionhyphen;free condition. The spectra consist of many features at antihyphen;Stokes frequency shifts of 900ndash;3100 cmminus;1from a 532 nm ohgr;1pump. 90deg; fluorescence studies of the photolysis zone show that C2is produced in various electronic states and energetic consideration require that dissociation of C6H6must involve two or more photons at 266 nm. 3WM spectra of C6D6are identical to those of C6H6in the antihyphen;Stokes shift region near 3000 cmminus;1and hence the transients do not contain CH bonds. 3WM spectra of C2H2fragments are quite similar to those of benzene in the 3000 cmminus;1region so that C2is believed to be responsible for both 3WM and fluorescence spectra. The 3WM spectra cannot be interpreted in terms of simple CARS vibrational resonances of C2. Intensity considerations suggest that enhancement due to multiple resonance is likely, and various electronicndash;electronic and vibrationalndash;electronic 3WM processes are discussed.
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