title: Infrared spectroscopy of large-sized neutral and protonated ammonia clusters.
abstract: Size-selective infrared spectroscopy was applied to neutral and protonated ammonia clusters, (NH3)n (n = ∼5-∼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 ν1 (NH sym stretch)/ν3 (NH asym stretch) band intensity ratio shows a rapid decrease in the size range n ≤ ∼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 ≥ ∼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 = ∼40.
authors.affiliation: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. asukafujii@m.tohoku.ac.jp.
authors.forename: Marusu
authors.initials: M
authors.lastname: Katada
authors.forename: Ryunosuke
authors.initials: R
authors.lastname: Shishido
authors.forename: Asuka
authors.initials: A
authors.lastname: Fujii