title: The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer.
abstract: We observe the weak S0 → S2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d6)2 about 250 cm(-1) and 220 cm(-1) above their respective S0 → S1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S0 → S2 electronic oscillator strength fel(S2) is ∼10 times smaller than fel(S1) and the S2 state lies ∼240 cm(-1) above S1, in excellent agreement with experiment. The S0 → S1 (ππ(∗)) transition is mainly localized on the "stem" benzene, with a minor stem → cap charge-transfer contribution; the S0 → S2 transition is mainly localized on the "cap" benzene. The orbitals, electronic oscillator strengths fel(S1) and fel(S2), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S1 and S2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S1 and S2 potential energy surfaces and reveal their relation to the "excimer" states at the stacked-parallel geometry. The fel(S1) and fel(S2) transition dipole moments at the C2v-symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S0 → S1 and S0 → S2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S0 → S1/S2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2, which has a rigid triangular structure with a tilted (Bz)2 subunit. The S0 → S1/ S2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2, confirming our assignment of the (Bz)2 S0 → S2 transition.
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Franziska A
authors.initials: FA
authors.lastname: Balmer
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Maria A
authors.initials: MA
authors.lastname: Trachsel
authors.affiliation: Theoretical Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
authors.forename: Ad
authors.initials: A
authors.lastname: van der Avoird
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Samuel
authors.initials: S
authors.lastname: Leutwyler