Excited state dynamics of the s-trans-1, 3-butadiene cation: An ab initio quantum dynamical analysis

The excited state dynamics of the s-trans-1,3-butadiene cation, focusing on the second and third bands of the photoelectron spectrum, have been investigated using a fully quantal approach, for the first time. The five lowest electronic states X²B_g, A²A_u, B²A_g, C²B_u, and D²A_g considering the six vibrational modes S₃, S₅, S₁₃, S₁₇, S₁₉, and S₂₀ were taken into account in the nuclear quantum dynamical investigation. The potential energy curves have been calculated along these coordinates for the five lowest electronic states using the RS2C method. Our simulations indicate a moderately fast population transfer from the A²A_u and B²A_g to the X²B_g state, taking place on a time scale of 70-80 fs. Furthermore, the computed second and third bands of the photoelectron spectrum are in good agreement with the corresponding experimental ones. Our calculation identifies the symmetric stretching of the central C-C bond S₅ and the symmetric C-C-C bending S₁₃ to be the main progression modes of the second and third bands of the photoelectron spectrum of (cationic) s-trans-1,3-butadiene.