Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)(n) clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes < n >=29, 80, 212, 447, and 989 and by depletion spectroscopy for < n >=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu(3) NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for < n >=8 and 29, but only about 0.40 of the intensity of the nu(3) peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2 nu(4) and the symmetric NH-stretch nu(1) in the range from 3150 to 3450 cm(-1) is changed between n=10 and 100, while the asymmetric NH-stretch nu(3) only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones. (c) 2006 American Institute of Physics.