Intermediate vibronic coupling in sexithiophene single crystals. II. Three-particle contributions.

Validity of the approach recently proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal is tested by assessing the importance of the (previously neglected) three-particle excitations. The Hamiltonian, denoted in the Lang-Firsov representation and including the three-particle terms, is numerically diagonalized to yield the eigenstates and ultimately the absorption spectrum. In addition, the wave functions are analyzed to probe the size and nature of the phonon cloud surrounding the exciton. The results demonstrate a marginal role of three-particle excitations but highlight the importance of a sufficiently extended vibrational basis to adequately describe the high-energy levels