The sub-ps lifetime of the 2A state of Z-hexatriene as deduced from its transient absorption spectrum

Exciting Z-hexatriene in ethanol at its UV absorption band (1B2) gives rise to two types of transient absorption. Below 325 nm the time constants depend on the wavelength; they can be assigned to ground-state processes such as cooling. The (hot) ground state forms within 470 fs as determined from the wavelength-independent decay of the absorption between 325 and 450 nm. This absorption starts from the lowest excited state (2A). This state undergoes internal conversion and Z → E isomerization. The 2A absorption is clearly different from the 2A spectrum of the E isomer. The Z → E and E → Z isomerizations thus do not have a common intermediate.