Vibronic coupling and Raman intensity of naphthalene and anthracene

The intensities of Raman lines of naphthalene and anthracene have been measured with exciting light of various wavelengths in the pre-resonance region. Great Raman intensity enhancements are found for the b_3g Raman line at 1629 cm^-1 and two totally symmetric Raman lines at 1579 and 1381 cm^-1 of naphthalene and for the corresponding Raman lines of anthracene. It is found that the Raman intensities of the b_3g vibrations are mainly responsible for the vibronic coupling between two π,π^* excited states ¹B_1u (¹L_a) and ¹B_2u(¹B_b) of these molecules. In the longer wavelength region, the b_3g Raman line of naphthalene exhibits an "intensity de-enhancement" in contrast to the great intensity enhancement in the shorter wavelength region. The observed "de-enhancement" is explained as the result of destructive interference of two different vibronic coupling mechanisms. The intensity enhancements of the a_g Raman lines are found to be reflecting the geometrical structures of the molecules in their ¹B_1u(¹L_a) states.