Internal Vibrational Excitation Research Articles

Numerical analysis of lubricant viscosity variations on operating condition of helical gear system

This study presents a dynamic model to investigate the effects of lubricant viscosity due to oil degradations or oxidations on the vibration signal of helical gear system. The modelling has been extended to consider the effect of elastohydrodynamic lubrication (EHL) to include frictional effects between meshed gears. A ten degree-of-freedom (10-DOF) model has been developed to combine lateral, torsional and axial vibrations of helical gear transmissions with supporting bearings, powerful motor and applied load. The variation of viscosity takes into account through the time-varying of EHL friction coefficient. The results conclude that higher lubricant viscosity could increase internal fluid friction and vibrational excitation, where higher motor power is required to overcome the higher friction of thicker lubricants. The vibration signal can be a good indicator for lubrication condition and it is possible to be used for monitoring the lubrication condition and obtain accurate diagnostic result for tooth surface defects.

Dynamics of Carbon Nanotube Chains Residing on Flat Substrates

Stationary states of a single-walled carbon nanotube array lying on a flat substrate formed by the surface of a molecular crystal are investigated. Numerical modeling showed that in the case of weak interaction with the substrate it is more energetically favorable for nanotubes to get assembled into multilayer packing, while in the case of strong interaction (e.g., the interaction with the h–BN crystal surface) they form monolayer packing (i.e., a chain on the substrate surface). The dynamics of nanotube chains is modeled. We show that acoustic supersonic solitons can exist only in small-diameter nanotubes (D < 0.8 nm), and soliton motion is always accompanied by energy loss to excitation of internal vibrations in nanotubes. Low-amplitude vibrations are analyzed. For a finite nanotube chain residing on a flat substrate, it is shown that vibrations are confined to only the end nanotubes or a nanotube that forms a structural defect in the chain.

Splitting of two-component solitary waves from collisions with narrow potential barriers

We consider the interaction of two-component bright-bright solitons with a narrow potential barrier (splitter) in the framework of a system of two Gross-Pitaevskii (nonlinear-Schr\"{o}dinger) equations modeling a binary Bose-Einstein condensate, with self-attraction in each component and cross-attraction between them. The objective is to study splitting of composite solitons, which may be used in the design of two-component soliton-interferometer schemes. We produce approximate analytic results, assuming a weak barrier and applying the perturbation theory in the limit in which the system is integrable and the solitary waves may be considered as exact solitons. We do this in the case of negligible interspecies interactions, and also when the nonlinearities are strongly asymmetric, allowing one to neglect the self-interaction in one of the species. Then, we use systematic simulations to study the transmissions of both components in regions outside these approximations and, in particular, to compare numerical results with their analytical countertparts. We concluded that there is an appreciable parameter range where one component is almost entirely transmitted through the barrier, while the other one is reflected. Excitation of internal vibrations in the passing and rebounding solitons is explored too, with a conclusion that it is weak in the regime of high-quality splitting.

(Invited) Properties of Metal/High-k Oxide/Graphene Structures

The use of graphene as a material for metal-oxide-graphene field-effect transistors (MOGFETs) is a possible overture to an epoch of faster electronics with properties similar to the traditional MOSFET, yet including interesting novel restrictions and possibilities [1]. This is apparent for the gate capacitor, where a two-dimensional graphene layer, constituting the transistor channel, is in intimate contact with a high-k oxide under a gate metal. Thermal activation of trapped charge carriers from interface states and border traps into the energy bands is a common feature of oxide-semiconductor systems. In contrast, due to the lack of an energy bandgap in monolayer graphene, the transfer of charge carriers in oxide traps into states of the channel material takes place as tunneling processes, serving the electron and hole exchange. In the paper, a background is given for modelling capacitance versus voltage data for MOG capacitors and transfer characteristics for MOGFETs. The derivation is founded on the position of the Fermi-level in the graphene layer, which determines all dynamic charge in the system at thermal equilibrium. In the first step, C-V and transfer characteristics for structures with oxides, comprising single electron trap states, will be given. This includes the dependence on temperature and on the frequency used for the probing signal of the capacitance meter. For high-k oxides, the dominating trap constellations consist of oxygen vacancies with varying atomic geometries, depending on the number of occupying carriers. Capturing charge carriers into electron and hole states in the core of the trap volume, the bonds between the surrounding atoms may change character. Accordingly, when an electron is captured into such traps, the (free) energy exchange in the transfer sequence not merely includes electron energy, but also the change in lattice energy needed for the process. Besides the energy carried by the charge carrier from the “sheet of electrons” in the graphene, also mechanical energy is required to change the atomic part of the trap assembly. This latter quantity is supplied by local phonons within the trap volume [2]. Converting the atomic arrangement when turning from one electronic state to another thus includes local vibrational energy, which requires thermal activation for the atomic transition. Therefore, the emission and capture rates for transitions between the oxide traps and the graphene are limited by a combination of the tunneling between electron states and the change in atomic arrangement. An example for a single electron trap is shown in Fig. 1, depicting the electron tunneling transition from graphene into the electron state E 1 for a trap able to capture one electron. Simultaneously, this requires a transition of the atomic vibrational states from Osc. 0 to Osc. 1, which includes thermal activation to the crossing point between these energy potentials, and thus an atomic energy exchange of U 0as noted in the figure. The discussion will be further extended to multiple electron traps, where we find that thermal activation occurs as a stepping process between atomic potentials, depending on internal vibrational excitations of the atoms surrounding the electronic trap state. It will be demonstrated that the emission and capture rates of charge carriers may depend on the ramping direction of the gate voltage in a measurement cycle, which gives rise to specific hysteresis effects for traps with atomic relaxation properties. Finally, single traps positioned close to the graphene layer may create electrical potential variations (“puddles”) along the channel [3]. This will locally change the position of the Fermi-level in the graphene, generate a varying energy position of the graphene Dirac-point and influence the shape of C-V and transfer curves.

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.

Three-stage classical molecular dynamics model for simulation of heavy-ion fusion

A three-stage Classical Molecular Dynamics (3S-CMD) approach for heavy-ion fusion is developed. In this approach the Classical Rigid-Body Dynamics simulation for heavy-ion collision involving light deformed nucleus is initiated on their Rutherford trajectories at very large initial separation. Collision simulation is then followed by relaxation of the rigid-body constrains for one or both the colliding nuclei at distances close to the barrier when the trajectories of all the nucleons are obtained in a Classical Molecular Dynamics approach. This 3S-CMD approach explicitly takes into account not only the long range Coulomb reorientation of the deformed collision partner but also the internal vibrational excitations of one or both the nuclei at distances close to the barrier. The results of the dynamical simulation for 24 Mg+208 Pb collision show significant modification of the fusion barrier and calculated fusion cross sections due to internal excitations.

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: an ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U.

Based on the non-local van der Waals density functional (vdW-DF)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H2 impingement against the stoichiometric PuO2(111), the reduced PuO2(111), and the stoichiometric α-Pu2O3(111) surfaces. The hydrogen molecular physisorption states, which cannot be captured by pure DFT+U method, are obtained by employing the vdW-DF+U scheme. We show that except for the weak physisorption, PuO 2(111) surfaces are so difficult of access that almost all of the H2 molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H2 on PuO2(111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H2 are calculated to be as high as 3.2 eV and 2.0 eV for stoichiometric and reduced PuO2 surfaces, respectively. Unlike PuO2, our AIMD study directly reveals that the hydrogen molecules can penetrate into α-Pu2O3(111) surface and diffuse easily due to the 25% native O vacancies located along the ⟨111⟩ diagonals of α-Pu2O3 matrix. By examining the temperature effect and the internal vibrational excitations of H2, we provide a detailed insight into the interaction dynamics of H2 in α-Pu2O3. The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers (1.0 eV and 0.53 eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H2 and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.

Vibration Testing of Intermediate Bulk Containers for Dangerous Goods

SUMMARYIntermediate bulk containers (IBCs) for liquid dangerous goods require a ‘vibration test’ for type approval introduced in the United Nations Recommendations on the transport of dangerous goods. The test has to be performed as a repetitive shock test practically identical to the test procedure previously prescribed by the US Department of Transport. It is conceptually simple but has several drawbacks.The kinetics and dynamics of a bouncing IBC have been investigated by means of simple computational models. With increasing complexity, partially plastic impact, elastodynamic response and the excitation of internal vibrations not directly affected by the bounces can be covered. The latter is a model for the sidewall vibrations. The numerical results show that the control over the specimens' response is only partially defined in the test procedure. Chaotic movements at resonance are possible and must be suppressed by changing the test parameters, which is mostly left to the test engineer. Accordingly, it is not possible to interpret the test results with a sound relation to real transport vibration environment.Experimental results, however, suggest that vibration testing of IBCs improves safety because resonance effects in a frequency range excited by typical transportation loads exist. A more detailed numerical model is necessary for the assessment of the fatigue damage imposed on IBCs by the prescribed or alternative, more realistic test procedures. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Optically levitating dielectrics in the quantum regime: Theory and protocols

We provide a general quantum theory to describe the coupling of light with the motion of a dielectric object inside a high-finesse optical cavity. In particular, we derive the total Hamiltonian of the system as well as a master equation describing the state of the center-of-mass mode of the dielectric and the cavity-field mode. In addition, a quantum theory of elasticity is used to study the coupling of the center-of-mass motion with internal vibrational excitations of the dielectric. This general theory is applied to the recent proposal of using an optically levitating nanodielectric as a cavity optomechanical system [see Romero-Isart et al., New J. Phys. 12, 033015 (2010); Chang et al., Proc. Natl. Acad. Sci. USA 107, 1005 (2010)]. On this basis, we also design a light-mechanics interface to prepare non-Gaussian states of the mechanical motion, such as quantum superpositions of Fock states. Finally, we introduce a direct mechanical tomography scheme to probe these genuine quantum states by time-of- flight experiments.

News on sputter theory: Molecular targets, nanoparticle desorption, rough surfaces

Sputtering theory has existed as a mature and well-understood field of physics since the theory of collision-cascade sputtering has been developed in the late 1960s. In this presentation we outline several directions, in which the basic understanding of sputter phenomena has been challenged and new insight has been obtained recently. Sputtering of molecular solids: after ion impact on a molecular solid, not all of the impact energy is available for inducing sputtering. Part of the energy is converted into internal (rotational and vibrational) excitation of the target molecules, and part is used for molecule dissociation. Furthermore, exothermic or endothermic chemical reactions may further change the energy balance in the irradiated target. Nanoparticle desorption: usually, the flux of sputtered particles is dominated by monatomics; in the case of a pronounced spike contribution to sputtering, the contribution of clusters in the sputtered flux may become considerable. Here, we discuss the situation that nanoparticles were present on the surface, and outline mechanisms of how these may be desorbed (more or less intact) by ion or cluster impact. Rough surfaces: real surfaces are rough and contain surface defects (adatoms, surface steps, etc.). For grazing ion incidence, these influence the energy input into the surface dramatically. For such incidence angles sputtering vanishes for a flat terrace; however, ion impact close to a defect may lead to sputter yields comparable to those at normal incidence. In such cases sputtering also exhibits a pronounced azimuth and temperature dependence.

Ultraviolet Spectroscopy of HD 44179

We have re-analyzed the ultraviolet spectrum of HD 44179, the central star(s) of the Red Rectangle nebula, providing improved estimates of the column density, rotational, and vibrational temperatures of the 4th Positive A-X system of CO in absorption. The flux shortward of 2200 A is a complex blend of CO features with no discernible stellar photosphere, making the identification of other molecular species difficult, and the direct derivation of the dust extinction curve impossible. We confirm that the spin-forbidden CO (a-X) Cameron bands are likely produced by either collisional excitation or a chemical reaction, not photoexcitation, but with a higher internal vibrational excitation than previously determined. We also detect the spin-forbidden CO a'-X, d-X, and e-X absorption features. The hot CO (A-X) bands exhibit a blue-shift of ~300 km/s, likely occurring close to the white dwarf star(s) suspected as the original source of the ultraviolet flux in the system, and forming the base of the outflow of material in the Red Rectangle. The OH comet-band system near 3000 A is also analyzed, and estimates of its rovibrational temperatures determined. The source of the molecules studied in this system is still unknown, but may be a combination of gaseous material associated with the star(s), or processed material from the surrounding dust torus.

High-resolution infrared spectroscopy of jet-cooled vinyl radical: Symmetric CH2 stretch excitation and tunneling dynamics

First high-resolution IR spectra of jet-cooled vinyl radical in the C-H stretch region are reported. Detailed spectral assignments and least squares fits to an A-reduction Watson asymmetric top Hamiltonian yield rotational constants and vibrational origins for three A-type bands, assigned to single quantum excitation of the symmetric CH(2) stretch. Two of the observed bands arise definitively from ground state vinyl radical, as rigorously confirmed by combination differences predicted from previous midinfrared CH(2) wagging studies of Kanamori et al. [J. Chem. Phys. 92, 197 (1990)] as well as millimeter wave rotation-tunneling studies of Tanaka et al. [J. Chem. Phys. 120, 3604 (2004)]. The two bands reflect transitions out of symmetric (0(+)) and antisymmetric (0(-)) tunneling levels of vinyl radical populated at 14 K slit-jet expansion temperatures. The band origins for the lower-lower (0(+)<--0(+)) and upper-upper (0(-)<--0(-)) transitions occur at 2901.8603(7) and 2901.9319(4) cm(-1), respectively, which indicates an increase in the tunneling splitting and therefore a decrease in the effective tunneling barrier upon CH(2) symmetric stretch excitation. The third A-type band with origin at 2897.2264(3) cm(-1) exhibits rotational constants quite close to (but at high-resolution distinguishable from) the vinyl radical ground state, consistent with a CH(2) symmetric stretch hot band built on one or more quanta of excitation in a low frequency vibration. The observed CH(2) symmetric stretch bands are in excellent agreement with anharmonically scaled high level density functional theory (DFT) calculations and redshifted considerably from previous low resolution assignments. Of particular dynamical interest, Boltzmann analysis indicates that the pair of 0(+) and 0(-) tunneling bands exhibits 1:1 nuclear spin statistics for K(a)=even:odd states. This differs from the expected 3:1 ratio for feasible exchange of the two methylenic H atoms but is consistent with a 4:4 ratio predicted for interchange between all three H atoms. This suggests the novel dynamical possibility of large amplitude "roaming" of all three H atoms in vinyl radical, promoted by high internal vibrational excitation arising from dissociative electron attachment in the discharge.

Nonlinear mobility of a driven system: Temperature and disorder effects

We consider the dissipative nonlinear dynamics of a model of interacting atoms driven over a substrate potential. The substrate parameters can be suitably tuned in order to introduce disorder effects starting from two geometrically opposed ideal cases: commensurate and incommensurate interfaces. The role of temperature is also investigated through the inclusion of a stochastic force via a Langevin molecular dynamics approach. Here, we focus on the most interesting tribological case of underdamped sliding dynamics. For different values of the chain stiffness, we evaluate the static friction threshold and consider the depinning transition mechanisms as a function of the applied driving force. As experimentally observed in QCM frictional measurements of adsorbed layers, we find that disorder operates differently depending on the starting geometrical configuration. For commensurate interfaces, randomness lowers considerably the chain depinning threshold. On the contrary, for incommensurate mating contacts, disorder favors static pinning destroying the possible frictionless (superlubric) sliding states. Interestingly, thermal and disorder effects strongly influence also the occurrence of parametric resonances inside the chain, capable of converting the kinetic energy of the center-of-mass motion into internal vibrational excitations. We comment on the nature of the different dynamical states and hysteresis (due to system bi-stability) observed at different increasing and decreasing strengths of the external force.

Molecule scattering from insulator and metal surfaces

Calculations are carried out and compared with data for the scattering ofCH4 molecules from aLiF(001) surface and for O2 scattering from Al(111). The theory is a mixed classical-quantum formalism thatincludes energy and momentum transfers between the surface and projectile fortranslational and rotational motions as well as internal mode excitation of theprojectile molecule. The translational and rotational degrees of freedom couple moststrongly to multiphonon excitations of the surface and are treated with classicaldynamics. Internal vibrational excitations of the molecules are treated with asemiclassical formalism with extension to arbitrary numbers of modes and arbitraryquantum numbers. Calculations show good agreement for the dependence onincident translational energy, incident beam angle and surface temperature whencompared with data for energy-resolved intensity spectra and angular distributions.

Calculations for methane scattering from LiF(001)

Calculations are presented for the scattering of ${\mathrm{CH}}_{4}$ molecules from a LiF(001) surface. The theory utilized is a mixed classical-quantum model that includes energy and momentum transfers between the surface and projectile for translational and rotational motions as well as internal mode excitation of the projectile molecule. The translation and rotation motions, including multiphonon excitations with the surface, are treated with classical dynamics. Internal vibrational mode excitation of the molecules is treated quantum mechanically with extension to arbitrary numbers of modes and arbitrary excitation quantum numbers. The results of calculations are compared with recent high-precision measurements of the scattering of ${\mathrm{CH}}_{4}$ molecules from clean, ordered LiF(001). The calculated results for energy-resolved spectra and for the angular distributions are in good agreement with experiment.

Theory of molecule-surface scattering

A theory of molecule-surface scattering is developed which includes energy and momentum transfers between the surface and projectile for translational motion, rotational exchange and internal mode excitation for the molecule. The translation and rotation motions are treated with classical mechanics, while a semiclassical quantum treatment for internal vibrational mode excitation is used. Calculations are presented for diatomic molecules and other small molecules scattering from a LiF(001) surface.

The formation and ejection of endohedral Cs@C60+ by low energy collisions (35–220 eV) of Cs+ ions with surface adsorbed C60 molecules

The collisional insertion of Cs+ ions into surface adsorbed C60 molecules was studied by scattering Cs+ ion beams from a C60 layer deposited on gold over the 35–220 eV impact energy range. Both Cs@C60+ and C60+ ions were ejected from the surface following the Cs+ impact but each species was characterized by different impact energy dependent yields and internal temperatures. Clear evidence for the endohedral nature of the complex is given. Both the scattering dynamics (at impact energies up to ∼100 eV) and the instant rise of the Cs@C60+ signal with the Cs+ beam onset clearly demonstrate that the insertion/ejection process is basically a single collision event. The outgoing Cs@C60+ and C60+ ions fragment during their flight time, after leaving the surface, via sequential emission of C2 units down to Cs@C50+ and C44+, respectively. Relative impact energy dependent yields were measured for both parent species and for all fragments. The yield curves are kinetically shifted with respect to each other as expected. Comparing the impact energy dependent fragmentation patterns of C60+ and Cs@C60+ we conclude that the ejected Cs@C60+ ion is much hotter than the C60+ ion. The internal vibrational excitation for both species is reaching a maximal value around 90–110 eV impact energy and than gradually decreases with increase in impact energy. The integrated Cs@C60+ yield is strongly peaked at around 80±5 eV impact energy. At impact energies above 120 eV also a C60− signal is observed but no Cs@C60− could be detected.

Theory of molecule-surface scattering at thermal and hyperthermal energies.

A theoretical model of molecule-surface scattering is developed which includes energy and momentum transfers between the surface and projectile for both translational and rotational motions and internal mode excitation for the projectile molecule. The translation and rotation motions are treated in the classical limit, while a quantum treatment for internal vibrational mode excitation is used. The results of calculations are compared with recent high-precision measurements of the scattering of a beam of C(2)H(2) molecules from a clean, ordered LiF(001) surface at energies of up to nearly 1 eV. The calculated results for angular distributions and rotational excitations are in good agreement with experiment.

Internal energy of sputtered clusters: The influence of bombarding conditions

We have investigated the internal (vibrational) excitation of nascent clusters formed during sputtering of metal surfaces. On one hand, theoretical model calculations were performed by Molecular Dynamics computer simulation using the MD/MC-CEM many body interaction potential. On the other hand, experimental data were collected by studying the fragmentation lifetimes of sputtered cluster ions on time scales of 10−9–10−6 s after their ejection and conversion of the resulting lifetime distributions into the distributions of internal energies. It is found that sputtered clusters possess average internal energies of about 1 eV per constituent atom, corresponding to vibrational temperatures of several thousand K. Special emphasis was put on the influence of the bombarding conditions on these results. Surprisingly, neither the nature or kinetic energy of the primary ions nor the bond strength of the ejected clusters seem to significantly influence the average internal energy.