Mechanistic Insights into the Delayed Fragmentation of Photoionized Ethanol

Delayed fragmentation of polyatomic molecular ions offers a powerful probe of unimolecular dissociation dynamics, yet the underlying mechanisms remain elusive when singly charged intermediates are involved. Here, we investigate the delayed fragmentation of isolated ethanol molecules using vacuum-ultraviolet photoionization combined with ion-neutral coincidence detection and quantum-chemical calculations. Long-lived dehydrogenated metastable ions are identified, with thresholds only a few eV above the vertical ionization potential. Measured 1/t decay profiles reveal that the metastability arises from statistical fragmentation of vibrationally hot ground states of dehydrogenated ions. Ultrafast hydrogen loss is found to stabilize intermediates by dissipating excess energy and introducing moderate energy barriers, thereby restricting subsequent bond cleavage. These findings establish a clear mechanistic picture of how ethanol ions redistribute excess energy and undergo delayed fragmentation. The combined experiment-theory approach presented here provides a broadly applicable framework for disentangling barrier-mediated delayed fragmentation in polyatomic molecular ions.