Multiphoton ionization and ab initio calculation studies of pyridine–water mixed clusters using time of flight mass spectrometer

Multiphoton ionization of binary mixed clusters (C5H5N)x–(H2O)y at 532, 355 and 266 nm laser wavelengths has been investigated using TOF mass spectrometer. The experiments showed that almost all the products were protonated ions. At 532 and 355 nm, the products were mainly protonated pyridine clusters (C5H5N)n–H+, while at 266 nm, mixed binary cluster ions (C5H5N)m– (H2O)n–H+ appeared. It was found that the abundance of the [(C5H5N)3–H2O–H]+ ions was abnormally high. The calculation indicated that the ion [(C5H5N)3–H2O–H]+ is of a kind of magic number structures with C3v symmetry. A stepwise reaction mechanism is suggested that photoionization is followed by dissociation.