Vibrational excitation of ammonia clusters by helium atom scattering

Buck, U.; Krohne, R.; Schütte, S.

doi:10.1063/1.473036

Cited by 16 publications

(14 citation statements)

References 17 publications

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“…This observable is dominated by the diffraction oscillations of scattered monomers, and near their minima we expect the largest contributions from the clusters [2]. In contrast to the results obtained for argon [2] and ammonia [13] clusters, the spectra exhibit three well-resolved peaks. In order to assign these peaks, we first calculate the corresponding energy transfer.…”

Section: Surface Vibrations Of Large Water Clusters By He Atom Scattecontrasting

confidence: 79%

Surface Vibrations of Large Water Clusters by He Atom Scattering

Brudermann¹,

Lohbrandt²,

Buck³

et al. 1998

Phys. Rev. Lett.

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The low energy intermolecular vibrational modes of water clusters for the average size n 110 have been measured by high resolution inelastic helium atom scattering. The water clusters are generated in adiabatic expansions through conical nozzles. By accompanying quantum and classical calculations the excited mode at 5.0 meV has been identified as O? ?O? ?O bending motion between adjacent hydrogen bonds, involving 3-coordinated water molecules on the amorphous cluster surface. Experiments for different sizes show that the frequency and thus the force constant of this mode increase for larger cluster sizes. [S0031-9007 (98)05739-1] PACS numbers: 36.40.Sx, 34.50.Dy, 68.35.JaWeakly bonded clusters have been a focus of intense interest during the past few years. One of the major objectives is to understand how the particles evolve as a function of size towards condensed phase behavior. The evolution of particle properties is influenced strongly by the large surface-to-bulk ratio, and therefore experimental tools sensitive specifically to the cluster surface properties are of interest. A unique tool of this kind has been developed in our laboratory, which probes surface vibrations by inelastic energy exchange with He atoms scattered from the particles [1]. In fact, we were able to demonstrate that in helium atom -argon cluster collisions surface modes were predominantly excited [2]. One can distinguish between single phonon and multiphonon excitation [3] using the measured angular dependence, in conjunction with calculations. Here the method is applied to the investigation of vibrational surface properties of water particles. Water clusters are of particular interest because of the importance of water in terrestrial phenomena, and because of the role of icy particles in atmospheric and extraterrestrial physics and chemistry. Electron diffraction studies of ͑H 2 O͒ n up to sizes of several hundred molecules demonstrated a noncrystalline structure [4], while spectroscopic studies of ice nanocrystal surfaces and surface-adsorbate systems showed persistence of amorphous surface structure to larger sizes, for which the particle interior is crystalline [5]. Thus, past studies indicate the properties of small ice particles that are quite distinct from those of bulk ice, and surface properties that are distinct from the interior. The present study focuses on these aspects of cluster behavior, for low energy, hydrogen bond vibration frequencies.For bulk hexagonal ice these modes have been measured by neutron scattering. In the range from 2 to 40 meV the peaks are attributed to the hindered intermolecular motion and interpreted as acoustical (7.1 meV) and optical (28.4 and 37.9 meV) modes [6], the splitting between the latter ones still being a subject of controversy [6,7]. The modes near 7.1 meV have also been interpreted as O? ?O? ?O bending between adjacent hydrogen bonds [8], while the higher frequency modes have been assigned to hydrogen bond stretching [6,8]. For clusters, extensive calculations are available for some sele...

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Section: Surface Vibrations Of Large Water Clusters By He Atom Scattecontrasting

confidence: 79%

Surface Vibrations of Large Water Clusters by He Atom Scattering

Brudermann¹,

Lohbrandt²,

Buck³

et al. 1998

Phys. Rev. Lett.

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“…9 Here we add a new measurement at a lower energy of 50.5 eV. 12 In the course of this work we discovered that the determination of the lowest size in one of the measurements of Ref.…”

Section: Methodsmentioning

confidence: 99%

“…We also added a measurement for the average size ͗n͘ϭ92 at a smaller collision energy of 50 meV, in contrast to the 91-100 meV that was used in the previous experiments. 9 The cluster size distribution was measured with the help of a newly established method; namely, by doping the cluster by one sodium atom. 10 This gives, especially for smaller cluster sizes, more reliable, nearly fragmentation-free results.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Intermolecular vibrations of large ammonia clusters from helium atom scattering

Beu¹,

Steinbach²,

Buck³

2002

The Journal of Chemical Physics

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The excitation of the low-energy intermolecular modes of ammonia clusters by helium atom scattering has been calculated using classical trajectories. The energy transfer is investigated as a function of scattering angle ͑from 10°to 90°͒, collision energy ͑94.8 and 50.5 meV͒, cluster size (nϭ18, 100, 1000͒, and cluster temperature (T c ϭ1 K, 30-50 K, and 105 K͒. It is observed that predominantly the mode at 7 meV and to a lesser extent also the one at 12 meV are excited. These are surface modes that mainly originate from the angular motion of three adjacent N atoms. The excitation is nearly independent of the cluster size and the probability for multiphonon excitation steadily increases with increasing deflection angle. This trend is even strengthened by increasing the collision energy. The role of the cluster temperature is to broaden the energy transfer distribution with increasing values. The calculations are compared with previous and new measurements presented here of the double-differential cross sections (d/d) ⌬E of ammonia clusters of average size ͗n͘ϭ92 at two collision energies and ͗n͘ϭ1040 at one energy. While the general trends in the angular and energy dependence could be well reproduced, the correct cluster temperature was crucial in getting good agreement at the lower collision energy for nϭ100. At the higher collision energy, the large energy transfer is not reproduced, probably a shortcoming of the potential models to account correctly for the anharmonicity of the strong multiquantum excitations.

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“…Clustering can significantly influence both the terminal energy and the terminal energy spread of a supersonic beam and its characterization is therefore obligatory, if for no other reason than to delimit and avoid the clustering regime. There exists an extensive literature on the clustering of ammonia in both neat and seeded beams, including basic mass spectrometrometric studies, 14,15 cluster spectroscopy, 15,16 photodissociation of clusters, 17-20 characterization of cluster fragmentation upon electron impact, 21,22 ultraviolet ͑UV͒ photoelectron spectroscopy of clusters, 23 vibrational excitation of clusters through helium atom impact, [24][25][26] electron diffraction studies of solid clusters, 27 and spectroscopy of ammonia on or in larger inert gas clusters. 28,29 Some time-of-flight ͑TOF͒ 30 energy analysis of clustered ammonia seeded beams has been carried out by the Buck and co-workers 22,25,26 within the context of crossed-beam scattering experiments.…”

Section: Seeded Beams Of Ammonia In Heliummentioning

confidence: 99%

Selected energy epitaxial deposition of GaN and AlN on SiC(0001) using seeded supersonic free jets of NH3 in helium

Torres

Doak

Wilkens

et al. 1999

Journal of Vacuum Science &Amp; Technology A: Vacuum, Surfaces, and Films

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By expanding a gas mixture into vacuum through a supersonic nozzle, a heavy “seed” species in a light diluent gas can be aerodynamically accelerated to suprathermal translational energies. Such beams are intense, directional, easily tuneable in energy, and narrowly distributed in energy. They thereby offer the means of selectively promoting activated gas-surface reactions. We report the use of 10% NH3 in He seeded beams to grow GaN and AlN epitaxially on 6H–SiC(0001) and to grow GaN on AlN buffer layers deposited on SiC(0001). The III–N films were grown under a variety of incident energies and angles of the NH3 beam, with the III metal species supplied from an effusive evaporator source. Film thickness and morphology were characterized ex situ with Rutherford backscattering spectroscopy, Auger spectroscopy, transmission electron microscopy, and atomic force microscopy. Of particular relevance to the III–N growth are the following results: (1) Selected energy epitaxial growth was observed, evidently via a direct reaction channel over a barrier of 0.25±0.1 eV. A comparison of films grown at 0° (normal) and 30° angles of NH3 incidence indicated total energy scaling of this chemisorption process. (2) A low energy reaction channel (<0.10 eV) was explicitly confirmed. The mechanism by which this might occur is discussed.

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Vibrational excitation of ammonia clusters by helium atom scattering

Cited by 16 publications

References 17 publications

Surface Vibrations of Large Water Clusters by He Atom Scattering

Surface Vibrations of Large Water Clusters by He Atom Scattering

Intermolecular vibrations of large ammonia clusters from helium atom scattering

Selected energy epitaxial deposition of GaN and AlN on SiC(0001) using seeded supersonic free jets of NH3 in helium

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