Infrared spectroscopy of large-sized neutral and protonated ammonia clusters

Size-selective infrared spectroscopy was applied to neutral and protonated ammonia clusters, (NH3)(n) (n = similar to 5-similar to 80) and H+(NH3)(n) (n = 8-100), to observe their NH stretching vibrations. The moderate size selection was achieved for the neutral clusters by the infrared-ultraviolet double resonance scheme combined with mass spectrometry. The size dependence of the observed spectra of (NH3)(n) is similar to that of the average size-controlled clusters doped in He droplets. The nu(1) (NH sym stretch)/nu(3) (NH asym stretch) band intensity ratio shows a rapid decrease in the size range n <= similar to 20. This demonstrates that ammonia begins to form crystalline like hydrogen bond networks at the much smaller size region than water. The precise size selection was achieved for H+(NH3)(n) by infrared photodissociation spectroscopy combined with a tandem type quadrupole mass spectrometer. The spectra of the protonated clusters become almost identical with those of the corresponding neutral clusters at n >= similar to 40, demonstrating that the radial chain structures, which are characteristic of the small-sized protonated clusters, develop into the crystalline like structures seen in the neutral clusters up to n = similar to 40.