Multiphoton ionization and proton transfer dynamics in (pyrazine) n clusters and (pyrazine) n (water) m ( n = 1–3, m = 1, 2) mixed clusters

Multiphoton ionization of pyrazine (Pyr) clusters generated in supersonic expansion was investigated using 266 nm UV excitation combined with time-of-flight mass spectrometry. Mass spectra revealed proton migration during the formation and dissociation of cluster cations, yielding protonated monomer [H+(Pyr)], dehydrogenated dimer [d-(Pyr)2+], and protonated trimer [H+(Pyr)3] ions. In addition, we report the first experimental observation of pyrazine-water mixed clusters [(Pyr)n(H2O)m, n = 1-3, m = 1, 2] in a supersonic expansion, which had remained undetected in previous mass spectrometric studies. Their absence in earlier experiments is attributed to rapid relaxation and dissociation following single-photon excitation prior to ionization, whereas multiphoton excitation enables the formation and detection of bound ionic mixed-cluster complexes. The unusual ion-yield distribution of the mixed clusters, characterized by very weak signals for (Pyr)n(H2O)+ (n = 1 and 2) and a comparatively strong yield for (Pyr)3(H2O)+, is explained in terms of excited-state relaxation dynamics and cluster stability. Furthermore, photostability studies under intense focused laser irradiation (∼1011 W cm-2) demonstrate complete suppression of the parent cluster ions, accompanied by dominant formation of smaller fragment ions, including C3H3N+. These results provide insight into the photodissociation pathways of nitrogen-containing heterocyclic clusters and may have implications for the chemical evolution of such species in the interstellar medium.