Multiphoton ionization and ab initio calculation of C4H5N-(NH3)n hydrogen-bond clusters

Multiphoton ionization of binary clusters (C4H5N) -NH3 at 355 and 532 nm have been investigated using a TOF mass spectrometer, The experimenal results showed that normal cluster ions and protonated cluster ions were produced at both laser wavelengths. The protonated products came from the intracluster proton transfer reaction. The existence of NH3 in the clusters increased the ionization cross sections of the clusters, The higher ionization efficiency at 355 nm results from the (2 + 1) resonance multiphoton ionization of the pyrrole molecule. A peculiar low abundance of (C4H5N)- (NH3)(2)(+) was observed, ab initio calculations indicated that when ammonia number n greater than or equal to 3, two hydrogen bonds between ammnia chain and pyrrole ring can be formed, and the cluster become more stable. The channel ratio for the formation of protonated and unprotonated products increases with the increase of n, in agreement with the trend of proton affinity of (C4H5N) - (NH3)(n),