Anisotropy in the transient absorption change of a molecular system with two-dimensionally degenerate transitions

Anisotropy in the photoinduced transient absorption change originating from two-dimensional transitions, i.e., transitions of twofold spatial degeneracy, is investigated. The polarization ratio of the transient absorption change is derived for randomly oriented molecules with two-dimensional transitions. It is shown that the ratio is either 4:3 or 8:1, depending on the nature of transitions involved, while in randomly oriented molecules with one-dimensional character it is always 3:1. This specific anisotropy is experimentally demonstrated with a solution of molecules with ${\mathrm{C}}_{4\mathrm{v}}$ symmetry, a vanadylphthalocyanine derivative, by means of subpicosecond pump-probe spectroscopy. A polarization ratio of 4:3 is observed both in bleaching and in excited-state absorption, reflecting the two-dimensional character of the molecule.