Time-resolved measurements of subpicosecond excited-state lifetimes of high-lying Rydberg states in pyrrole†

Abstract

We report the ultrafast decay dynamics of pyrrole upon excitation in the vacuum ultraviolet region using femtosecond time-resolved photoelectron spectroscopy in combination with two-photon absorption. With the two-photon pump energy up to ∼6.78 eV, pyrrole is excited to the ¹B₂ valence and Rydberg states, i.e., the first ¹B₂(ππ*) valence state and the ¹B₂(π3d) Rydberg state. The former is at high levels of vibrational excitation and has an extremely short lifetime of <30 fs, while the latter is in the vibrational ground state and decays with a lifetime of about 400 fs. As the excitation energy slightly increases, the ¹B₂(π3d) vibrational states are populated and decay in 210–260 fs. We propose that the ultrafast deactivation pathway of the ¹B₂(π3d) Rydberg state is internal conversion to the lower-lying ¹B₂(ππ*) state. At higher excitation energies, other valence states, such as the second ¹B₂(ππ*) state, should make a main contribution to the absorption and a series of other higher-lying Rydberg states with lifetimes of hundreds of femtoseconds are also involved. This study provides direct time-resolved measurements of subpicosecond excited-state lifetimes for high-lying Rydberg states in bare pyrrole.