Infrared Spectroscopy of NH₄⁺(NH₃)<i>_n</i>_-1 (<i>n</i> = 6−9):  Shell Structures and Collective ν₂ Vibrations

Abstract

Vibrational predissociation spectra of NH 4 + (NH 3 ) n -1 ( n = 6−9) were measured in the wavenumbers of 1045−1091 cm -1 by using a tandem mass spectrometer equipped with a serpentine octopole ion guide and a line-tunable cw CO 2 laser. Each spectrum consists of an intense and broad peak and a weak one. The dissociation cross sections were in the order of 10 -17 −10 -18 cm 2 . As n increases, the wavenumber of the intense and broad peak red-shifts gradually, and its intensity increases. The Hartree−Fock and the second-order Møller−Plesset calculations were performed to obtain the wavenumbers and the intensities of the optical absorption by these cluster ions with n = 2−8. The comparison of the experimental results with the calculations leads us to conclude that NH 4 + (NH 3 ) n -1 forms a shell structure and causes the absorption band associated with the collective ν 2 vibration of NH 3 to split.