Vibrational excitons, resonant energy transfer, and local structure in liquid benzene

Abstract

The presence of vibrational excitons in liquid benzene has been tested by the method of isotopic dilution. A C₆H₆/C₆D₆ concentration study on the infrared and Raman fundamental modes reveals that the umbrella (A_2u) vibrational exciton in solid benzene retains its characteristics upon melting and at room temperature. The total liquid exciton bandwidth is about 40 cm⁻¹, practically the same as in the solid. This indicates an instantaneous local liquid structure similar to that of the solid (the C_i crystal site symmetry is also nearly preserved), in general agreement with indications from other methods. The fastest nearest neighbor vibrational resonant transfer takes about 1 psec. The residual linewidth at isotopic dilution is 3–4 cm⁻¹, which is due to inhomogeneous and/or homogeneous broadening. The respective overall reorientational and/or translational relaxation takes about 2 psec or longer. The exciton linewidth is proportional to the square root of the isotopic concentration except for a sudden break at some critical concentration.