Intermolecular vibrations of large ammonia clusters from helium atom scattering

Abstract

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.

Large molecular aggregates: from atmospheric aerosols to drug nanoparticles

“…Infrared spectra of large molecular aggregates can be modelled either by classical scattering theory [55][56][57] or by quantum chemical calculations. 6,15,24,[58][59][60][61][62][63][64][65][66] It is not possible to obtain a comprehensive understanding of the various spectroscopic features observed using one particular method alone. One goal of the present contribution is to demonstrate that the combination of the different methods helps us to improve our understanding of various phenomena observed in particle spectra.…”

“…Below this size range it is no longer suitable to describe infrared spectra since the molecular structure of the aggregates starts to dominate them. 15,18,37,48,49,[62][63][64][65][66] Classical scattering theory is particularly useful to describe infrared spectra of large particles (r 4 100 nm) which exhibit in addition to characteristic absorption bands pronounced features due to elastic scattering of the light by the particles. A big disadvantage of this method is the need for independent optical data as input.…”

Charge delocalization dynamics of ammonia in different hydrogen bonding environments: free clusters and in liquid water solution

“…[7][8][9][10][11] Large ammonia clusters have been investigated by helium scattering and molecular dynamics methods, and were shown to retain the ordered structure and coordination number of the solid phase. 12 Neutron diffraction studies of liquid ammonia indicate that less than two hydrogen bonds per molecule are present in the liquid phase. 13 The ammonia liquid and water-ammonia solutions have been investigated with Raman spectroscopy by Ujike and Tominaga, 14 who describe the liquids as consisting of dimers whose hydrogen bonds break and form dynamically.…”

Quantum Monte Carlo simulations of selected ammonia clusters (n = 2–5): Isotope effects on the ground state of typical hydrogen bonded systems

“…[5][6][7] Our work has joined the experimental and theoretical endeavors of a growing community. Experimental [8][9][10][11][12][13][14][15] and theoretical [16][17][18][19][20][21][22] investigations abound in the literature. The main focus of the experimental work has been on the dimer and on the condensed phase.…”