Infrared photodissociation action spectra of protonated ammonia cluster ions, N H4+ (N H3) n (n=5-8), were measured in the range of 1020-1210 cm-1 by using a tunable infrared free electron laser. Analyses by the density functional theory (DFT) show that the spectral features observed can be assigned to the 2 vibrational mode of the N H3 molecules in N H4+ (N H3) n. Size dependence of the spectra supports structural models obtained by the DFT calculations, in which the N H4+ ion is solvated by the four nearest-neighbor N H3 molecules. For N H4+ (N H3) 5, the spectrum between 1000 and 1700 cm-1 was measured. The 4 bands of the N H3 molecules and the N H4+ ion were found in the range of 1420-1700 cm-1.
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