Classical, non-linear, internal dynamics of large, isolated, vibrationally excited molecules

This work reports numerical experiments intended to clarify the internal equilibration process in large molecules, following vibrational excitation. A model of amorphous and oxygenated hydrocarbon macromolecule (about 500 atoms)--simulating interstellar dust-- is built up by means of a chemical simulation code. Its structure is optimized, and its normal modes determined. About 4.5 eV of potential energy is then deposited locally by perturbing one of the C-H peripheral bonds, thus simulating the capture of a free H atom by a dangling C bond. The ensuing relaxation of the system is followed for up to 300 ps, using a molecular mechanics code. When steady state is reached, spectra and time correlation functions of kinetic energy and bond length fluctuations indicate that most normal modes have been activated, but the motion remains quasi-periodic or regular. By contrast, when the molecule is violently excited or embedded in a thermal bath (modelled by Langevin dynamics), the same markers clearly depict chaotic motions. Thus it appears that even such a large system of oscillators is unable to provide the equivalent of a thermal bath to any one of these, unless there are strong resonances between some of them. In general, therefore, an energy of a few eV's deposited in an isolated molecule will not be immediately thermalized. This conclusion is of consequence for the interpretation of astronomical UIB spectra. Key Words:IS dust--UIBs--Excitation, relaxation processes.Comment: 19 pages, 9 figures, J. of Phys. B 2002, vol 35(17

Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction.

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with firstprinciples calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA)produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials