Ultrafast Evolution of Imidazole after Electronic Excitation

The ultrafast dynamics of the imidazole chromophore has been tracked after electronic excitation in the 250-217 nm energy region, by time delayed ionization with 800 nm laser pulses. The time-dependent signals collected at the imidazole mass channel show the signature of femtosecond dynamics, originating on the pi sigma*- and pi pi*-type states located in the explored energy region. The fitting of the transients, which due to the appearance of nonresonant coherent adiabatic excitation requires a quantum treatment based in the Bloch equations, yields two lifetimes of 18 +/- 4 and 19 +/- 4 fs. The first is associated with the pi sigma* <- pi pi* internal conversion, while the second reflects the loss of ionization cross-section as the system evolves along the dissociative pi sigma* surface. This study provides a comprehensive picture of the photophysics of the molecule that agrees with previous experimental and theoretical findings.