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/C 6D6 concentration study on the infrared and Raman fundamental modes reveals that the umbrella (A2u) vibrational exciton in solid benzene retains its basic identity 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 Ci 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. Copyright © 1977 American Institute of Physics.