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We consider the infinite hierarchy of local collective rearrangements on bond networks that preserves the valency of each atom and explicitly enumerate those involving 4, 5, and 6 particles. The only 4-particle rearrangement is identical to the Wooten-Winer-Weaire (WWW) mechanism. Each rearrangement mechanism is applied in a Monte Carlo (MC) algorithm in order to determine the rate at which it equilibrate a network and relax the structure at equilibrium. At low temperature the 4-particle mechanism provides the fastest relaxation rate but we find that there is a crossover with increasing temperature to 5-particle mechanisms. [ABSTRACT FROM AUTHOR]

The authors present a new method for the computation of vibrationally resolved optical spectra of large molecules, including the Duschinsky rotation of the normal modes and the effect of thermal excitation. The method automatically selects the relevant vibronic contributions to the spectrum, independently of their frequency, and it is able to provide fully converged spectra with moderate computational times, both in vacuo and in solution. By describing the electronic states in the frame of the density functional theory and its time-dependent extension, they computed the room temperature absorption spectra of coumarin C153 and trans-stilbene in cyclohexane and the phosphorescence spectrum of porphyrazine in gas phase, showing that the method is fast and efficient. The comparison with experiment for trans-stilbene and coumarin C153 is very satisfactory, confirming the progress made toward a reliable method for the computation and interpretation for the optical spectra of large molecules. [ABSTRACT FROM AUTHOR]

The ionization of a beam of H2 Rydberg molecules in collision with a metal surface (evaporated Au or Al) is studied. The Rydberg states are excited in an ultraviolet–vacuum ultraviolet double-resonant process and are state selected with a core rotational quantum number N+=0 or 2 and principal quantum numbers n=17–22 (N+=2) or n=41–45 (N+=0). It is found that the N+=0 states behave in a very similar manner to previous studies with atomic xenon Rydberg states, the distance of ionization from the surface scaling with n2. The N+=2 states, however, undergo a process of surface-induced rotational autoionization in which the core rotational energy transfers to the Rydberg electron. In this case the ionization distance scales approximately with ν02, the effective principal quantum number with respect to the adiabatic threshold. This process illustrates the close similarity between field ionization in the gas phase and the surface ionization process which is induced by the field due to image charges in the metal surface. The surface ionization rate is enhanced at certain specific values of the field, which is applied in the time interval between excitation and surface interaction. It is proposed here that these fields correspond to level crossings between the N+=0 and N+=2 Stark manifolds. The population of individual states of the N+=2, n=18 Stark manifold in the presence of a field shows that the surface-induced rotational autoionization is more facile for the blueshifted states, whose wave function is oriented away from the surface, than for the redshifted states. The observed processes appear to show little dependence on the chemical nature of the metallic surface, but a significant change occurs when the surface roughness becomes comparable to the Rydberg orbit dimensions. [ABSTRACT FROM AUTHOR]

This paper presents the propagation of waves along a one-dimensional “diatomic” chain made up to welded spheres, i.e., with two steel spheres of different diameters alternating. First, a theoretical analysis is presented, which gives the vibration modes of an infinite chain, leading to two low-frequency branches, separated by a band gap. A theoretical analysis is then performed on a finite chain, containing an even or an odd number of spheres. Depending on the parity of the number of spheres in the finite chain and on the ratio between the masses of the spheres, it points out that localized modes may appear in the band gap. The theoretical results have been validated by a comparison between numerical and experimental results. Many applications of such systems can therefore be found: acoustic filters, noise and vibration isolation, acoustic wave guiding, etc. [ABSTRACT FROM AUTHOR]

The architecture and operation of a trace hydrogen cyanide (HCN) gas sensor based on quartz-enhanced photoacoustic spectroscopy and using a λ=1.53 μm telecommunication diode laser are described. The influence of humidity content in the analyzed gas on the sensor performance is investigated. A kinetic model describing the vibrational to translational (V–T) energy transfer following the laser excitation of a HCN molecule is developed. Based on this model and the experimental data, the V–T relaxation time of HCN was found to be (1.91±0.07)10-3 s Torr in collisions with N2 molecules and (2.1±0.2)10-6 s Torr in collisions with H2O molecules. The noise-equivalent concentration of HCN in air at normal indoor conditions was determined to be at the 155-ppbv level with a 1-s sensor time constant. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the atomic geometries and electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq3)/Al interface using density functional calculations. The calculated interface dipoles agree reasonably well with the experiments and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq3 molecules. The HOMO levels of the Alq3 molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the co-existence of the two orientations of the molecular dipoles. [Copyright &y& Elsevier]

First-principles calculations show that AB defects substituting on an O site in ZnO where A, B=N, O, or C are an important class of defects whose physical properties cannot be described by the usual split interstitials but rather by substitutional diatomic molecules. The molecular natures of the (AB)O defects are reflected in their vibrational frequencies which are redshifted from those of the corresponding free molecules but only by about 10%. These calculated results agree with the frequency range recently observed by IR measurement on N-doped ZnO. Moreover, most (AB)O defects are donors in p-type samples. The (NC)O and (N2)O defects have sufficiently low energies to convert substituional NO acceptors into donors, thereby hindering the efforts of doping ZnO p type. [ABSTRACT FROM AUTHOR]

The article presents a research on a separation method based on acoustic forces that works on a continuous flow of sample in a microfluidic device. The researchers had applied acoustic forces to move particle from one laminar stream to another in a microfluidic channel. The method works in a device that has a central microfuidic channel with an inlet jutting out from either side. They also demonstrated that acoustically inseparable particles could be separated by manipulating the density of the suspending buffer.