Multiphoton Ionization and ab Initio Calculation Studies of the Hydrogen-Bonded Clusters C₄H₅N−(H₂O)n

The multiphoton ionization of the hydrogen-bonded clusters C 4 H 5 N−(H 2 O) n was studied using a time-of-flight mass spectrometer at the laser wavelengths of 355 and 532 nm. At both wavelengths, a series of C 4 H 5 N−(H 2 O) n + and the protonated C 4 H 5 N−(H 2 O) n H + were obtained. The two-photon resonance ionization processes at 355 nm make this wavelength produce obviously more abundant ions of pyrrole and the clusters than 532 nm. Ab initio calculations show that in the protonated products, the proton prefers to link with the α-C of pyrrole rather than with the N atom. The production of the protonated products requires an intracluster proton transfer reaction. The protonated products obtained at 532 nm are suggested to arise from an intracluster penning ionization or a charge transfer process. The abnormally higher intensities of C 4 H 4 N−(H 2 O) n + ( n ≥ 1) than those of C 4 H 4 N + are attributed to the stabilization effects of the cluster formation on the dissociation products C 4 H 4 N + of the pyrrole molecule.