Infrared spectroscopy of NH4+(NH3)(n-1) (n=6-9): Shell structures and collective nu(2) vibrations

Vibrational predissociation spectra of NH4+(NH3)(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 CO2 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 Moller-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 NH4+(NH3)(n-1) forms a shell structure and causes the absorption band associated with the collective v(2) vibration of NH3 to split.