On the UV spectroscopy and photodynamics of octatetraene

The UV absorption spectrum of all-trans-octatetraene is reinvestigated theoretically, focussing on the strongly dipole-allowed 1A(g) - 1B(u) transition. The dynamical calculations rely on ab initio multistate CASPT2 (MS-CASPT2) computations of the underlying potential energy surfaces and coupling elements, and the multiconfiguration time-dependent Hartree (MCTDH) method for solving the time-dependent Schrodinger equation for the nuclear motion. The vibronic structure of the absorption band near 2700 angstrom is well reproduced, better than ever before by a purely ab initio approach. The C-C single bond and C = C double bond stretching modes dominate the vibrational structure of the transition. While the latter resembles nearly harmonic motion on weakly coupled potential energy surfaces, the time-dependent calculations reveal a subpicosecond electronic population transfer from the 1B(u) state to the lower-lying 2A(g) state proceeding on a time scale of similar to 50fs. The details of this internal conversion process and its relation to the conical intersection between the two excited states are discussed, paying particular attention to the impact of the 1B(u) - 2A(g) energy gap on the dynamics. [GRAPHICS] .