Theoretical study of excitations in furan:: Spectra and molecular dynamics

The excitation spectra and molecular dynamics of furan associated with its low-lying excited singlet states (1)A(2)(3s), B-1(2)(V), (1)A(1)(V'), and B-1(1)(3p) are investigated using an ab initio quantum-dynamical approach. The ab initio results of our previous work [J. Chem. Phys. 119, 737 (2003)] on the potential energy surfaces (PES) of these states indicate that they are vibronically coupled with each other and subject to conical intersections. This should give rise to complex nonadiabatic nuclear dynamics. In the present work the dynamical problem is treated using adequate vibronic coupling models accounting for up to four coupled PES and thirteen vibrational degrees of freedom. The calculations were performed using the multiconfiguration time-dependent Hartree method for wave-packet propagation. It is found that in the low-energy region the nuclear dynamics of furan is governed mainly by vibronic coupling of the (1)A(2)(3s) and B-1(2)(V) states, involving also the (1)A(1)(V') state. These interactions are responsible for the ultrafast internal conversion from the B-1(2)(V) state, characterized by a transfer of the electronic population to the (1)A(2)(3s) state on a time scale of similar to25 fs. The calculated photoabsorption spectrum of furan is in good qualitative agreement with experimental data. Some assignments of the measured spectrum are proposed. (C) 2004 American Institute of Physics.