Electronic three wave mixing spectra of transient species produced by UV laser photolysis of benzene

Abstract

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 collision‐free condition. The spectra consist of many features at anti‐Stokes frequency shifts of 900–3100 cm⁻¹ from a 532 nm ω₁ pump. 90° fluorescence studies of the photolysis zone show that C₂ is produced in various electronic states and energetic consideration require that dissociation of C₆H₆ must involve two or more photons at 266 nm. 3WM spectra of C₆D₆ are identical to those of C₆H₆ in the anti‐Stokes shift region near 3000 cm⁻¹ and hence the transients do not contain CH bonds. 3WM spectra of C₂H₂ fragments are quite similar to those of benzene in the 3000 cm⁻¹ region so that C₂ is 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 electronic–electronic and vibrational–electronic 3WM processes are discussed.