Ultrafast structural dynamics of pyrrole trimer probed by Coulomb explosion imaging

The ultrafast structural dynamics of the pyrrole trimer induced by femtosecond laser pulses have been investigated using the Coulomb explosion imaging technique. Coincidence measurements of the three-body dissociation channel (${({\mathrm{C}}_4{\mathrm{H}}_5\mathrm{N})}_3^{3+}\to 3{\mathrm{C}}_4{\mathrm{H}}_5{\mathrm{N}}^{+}$) reveal the geometric configuration and fragmentation behavior of the trimer in the gas phase. Analysis of the kinetic energy release (KER) and fragment angular distributions, combined with Dalitz-plot reconstruction, indicates that the pyrrole trimer predominantly adopts a stable equilateral triangular (cyclic) configuration. Ab initio molecular dynamics (AIMD) simulations show that the experimental KER distribution can be reproduced only when the structural evolution proceeds along the ground state of the trimer dication within ∼50 fs. These results demonstrate that significant intracluster structural rearrangement occurs on an ultrafast timescale during multiple ionization, highlighting the strong coupling between electronic excitation and nuclear motion even in relatively large molecular clusters.