Abstract

Resonance hyper-Raman spectra (RHRS) of monohalobenzenes and naphthalene in solution are obtained by utilizing the intracavity power and 76 MHz repetition rate of a synchronously pumped stilbene 420 dye laser. The nature of one- and two-photon transitions to the ¹L_a electronic state are examined for various halogen sub­stituents. For benzene, vibronic coupling by mode ν₁₂ (b_1u) makes the ¹B_1u (¹L_a) transition weakly two-photon allowed, suggesting that ¹B_1u couples to the electronic ground state (¹A_1g) more effectively than to any other excited electronic state. Fluorine substitution makes the ¹L_a transition two-photon dipole allowed, while both “weakly” perturbing chlorine and strongly perturbing iodine cause the transition to become strongly one- and two-photon allowed. The spectra confirm that the ≈1600 cm^-1 is the totally symmetric ν_8a mode rather than the previously misassigned ν_8b mode. The naphthalene spectrum demonstrates the utility of RHRS for investigating overlapping one- and two-photon allowed transitions.

© 1992 Optical Society of America