Full observation of ultrafast cascaded radiationless transitions from S₂(ππ^-) state of pyrazine using vacuum ultraviolet photoelectron imaging

A photoexcited molecule undergoes multiple deactivation and reaction processes simultaneously or sequentially, which have been observed by combinations of various experimental methods. However, a single experimental method that enables complete observation of the photo-induced dynamics would be of great assistance for such studies. Here we report a full observation of cascaded electronic dephasing from S₂(ππ^∗) in pyrazine (C₄N₂H₄) by time-resolved photoelectron imaging (TRPEI) using 9.3-eV vacuum ultraviolet pulses with a sub-20 fs time duration. While we previously demonstrated a real-time observation of the ultrafast S₂(ππ^∗) → S₁(nπ^∗) internal conversion in pyrazine using TRPEI with UV pulses, this study presents a complete observation of the dynamics including radiationless transitions from S₁ to S₀ (internal conversion) and T₁(nπ^∗) (intersystem crossing). Also discussed are the role of ¹A_u(nπ^∗) in the internal conversion and the configuration interaction of the S₂(ππ^∗) electronic wave function.