Excitation Of Vibrational Modes Research Articles

Picosecond infrared laser driven sample delivery for simultaneous liquid-phase and gas-phase electron diffraction studies.

Here, we report on a new approach based on laser driven molecular beams that provides simultaneously nanoscale liquid droplets and gas-phase sample delivery for femtosecond electron diffraction studies. The method relies on Picosecond InfraRed Laser (PIRL) excitation of vibrational modes to strongly drive phase transitions under energy confinement by a mechanism referred to as Desorption by Impulsive Vibrational Excitation (DIVE). This approach is demonstrated using glycerol as the medium with selective excitation of the OH stretch region for energy deposition. The resulting plume was imaged with both an ultrafast electron gun and a pulsed bright-field optical microscope to characterize the sample source simultaneously under the same conditions with time synchronization equivalent to sub-micrometer spatial resolution in imaging the plume dynamics. The ablation front gives the expected isolated gas phase, whereas the trailing edge of the plume is found to consist of nanoscale liquid droplets to thin films depending on the excitation conditions. Thus, it is possible by adjusting the timing to go continuously from probing gas phase to solution phase dynamics in a single experiment with 100% hit rates and very low sample consumption (<100 nl per diffraction image). This approach will be particularly interesting for biomolecules that are susceptible to denaturation in turbulent flow, whereas PIRL–DIVE has been shown to inject molecules as large as proteins into the gas phase fully intact. This method opens the door as a general approach to atomically resolving solution phase chemistry as well as conformational dynamics of large molecular systems and allow separation of the solvent coordinate on the dynamics of interest.

Resonances in nitrobenzene probed by the electron attachment to neutral and by the photodetachment from anion.

We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries. In both cases, the anions decay by electron emission and we record the electron spectra. Several types of emission are recognized, differing by the way in which the resulting molecule is vibrationally excited. In the excitation of specific vibrational modes, distinctly different modes are visible in electron collision and photodetachment experiments. The unspecific vibrational excitation, which leads to the emission of thermal electrons following the internal vibrational redistribution, shows similar features in both experiments. A model for the thermal emission based on a detailed balance principle agrees with the experimental findings very well. Finally, a similar behavior in the two experiments is also observed for a third type of electron emission, the vibrational autodetachment, which yields electrons with constant final energies over a broad range of excitation energies. The entrance channels for the vibrational autodetachment are examined in detail, and they point to a new mechanism involving a reverse valence to non-valence internal conversion.

Bolt looseness monitoring based on damping measurement by using a quantitative electro-mechanical impedance method

In-situ bolt looseness monitoring is very important for the stability and safety of engineering structures. The electro-mechanical impedance (EMI) method, which had been intensively studied in structure health monitoring, has offered a feasible solution. However, in traditional EMI method, the lack of modal pre-selection leads to dense resonant peaks which brings difficulty to signal analysis. The commonly used statistical damage indices typically do not have physics meaning and may fail to distinguish the defects from the environmental changes. In this work, a quantitative EMI (Q-EMI) method was proposed to measure the damping of structures for bolt looseness monitoring. The in-plane excitation method based on distributed thickness-shear-mode piezoelectric transducers was used for selective excitation of vibration modes. The modal-related sensitivity variation was theoretically analyzed and then validated by experiments. Three different specimens were tested and the results showed that during bolt loosening, the modal damping increased by 100%–300%, while the resonance frequency of the structure only decreased by several percent. Moreover, it was found that the loosened bolt may be located using the variation of modal-related sensitivity. Due to its good performance and easy mounting, the proposed Q-EMI method is very promising for bolt looseness monitoring and is expected to be widely used in near future.

Accurately Measuring Molecular Rotational Spectra in Excited Vibrational Modes.

Although gas phase rotational spectroscopy is a mature field for which millions of rotational spectral lines have been measured in hundreds of molecules with sub-MHz accuracy, it remains a challenge to measure these rotational spectra in excited vibrational modes with the same accuracy. Recently, it was demonstrated that virtually any rotational transition in excited vibrational modes of most molecules may be made to lase when pumped by a continuously tunable quantum cascade laser (QCL). Here, we demonstrate how an infrared QCL may be used to enhance absorption strength or induce lasing of terahertz rotational transitions in highly excited vibrational modes in order to measure their frequencies more accurately. To illustrate the concepts, we used a tunable QCL to excite v3 R-branch transitions in N2O and either enhanced absorption or induced lasing on 20 v3 rotational transitions, whose frequencies between 299 and 772 GHz were then measured using either heterodyne or modulation spectroscopy. The spectra were fitted to obtain the rotational constants B3 and D3, which reproduce the measured spectra to within the experimental uncertainty of ± 5 kHz. We then show how this technique may be generalized by estimating the threshold power to make any rotational transition lase in any N2O vibrational mode.

KCo2As2 : A new portal for the physics of high-purity metals

High-quality single crystals of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ with the body-centered tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$ structure were grown using KAs self flux. Structural, magnetic, thermal, and electrical transport properties were investigated. No clear evidence for any phase transitions was found in the temperature range 2--300 K. The in-plane electrical resistivity $\ensuremath{\rho}$ versus temperature $T$ is highly unusual, showing a ${T}^{4}$ behavior below 30 K and an anomalous positive curvature up to 300 K, which is different from the linear behavior expected from the Bloch-Gr\"uneisen theory for electron scattering by acoustic phonons. This positive curvature has been previously observed in the in-plane resistivity of high-conductivity layered delafossites such as ${\mathrm{PdCoO}}_{2}$ and ${\mathrm{PtCoO}}_{2}$. The in-plane $\ensuremath{\rho}(T\ensuremath{\rightarrow}0)=0.36\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}$ cm of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ is exceptionally small for this class of compounds. The material also exhibits a magnetoresistance at low $T$ which attains a value of about 40% at $T=2$ K and magnetic field $H=80$ kOe. The magnetic susceptibility $\ensuremath{\chi}$ of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ is isotropic and about an order of magnitude smaller than the values for the related compounds ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{BaCo}}_{2}{\mathrm{As}}_{2}$. The $\ensuremath{\chi}$ increases above 100 K, which is found from our first-principles calculations to arise from a sharp peak in the electronic density of states just above the Fermi energy ${E}_{\mathrm{F}}$. Heat capacity ${C}_{\mathrm{p}}(T)$ data at low $T$ yield an electronic density of states $N({E}_{\mathrm{F}})$ that is about 36% larger than predicted by the first-principles theory. The ${C}_{\mathrm{p}}(T)$ data near room temperature suggest the presence of excited optic vibration modes, which may also be the source of the positive curvature in $\ensuremath{\rho}(T)$. Angle-resolved photoemission spectroscopy measurements are compared with the theoretical predictions of the band structure and Fermi surfaces. Our results show that ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ provides a new avenue for investigating the physics of high-purity metals.

5DOF planar – rotary motion piezoelectric robot

A novel resonant type five degrees of freedom (5DOF) mobile piezoelectric robot is proposed and analysed. The piezoelectric robot performs unlimited linear working motion in the plane and rotates the positioning table or sphere about three axes independently. The piezorobot can be used for precise positioning or transportation of small objects and make complicated locomotion trajectories. The piezoelectric robot is designed as a compound structure and includes a piezoelectric ring and a special ring-shaped bronze layer. Electrodes of the piezoelectric ring are divided into six equal sections. Excitation of the piezoelectric robot is performed by using a single switched harmonic signal applied to the particular electrode. The operation principle of the piezoelectric robot is based on the excitation of the third radial vibration mode of the piezoelectric ring. The numerical and experimental study was performed, and the operating principle was validated. The maximum linear velocity of 18.8 mm/s and rotational speed of 31.3 rpm were achieved when load of 25.1g and excitation voltage of 200 Vp-p were applied.

Enhanced Chemical Sensing with Multiorder Coherent Raman Scattering Spectroscopic Dephasing.

Molecular vibrational spectroscopy is widely used in various sensing and imaging applications, providing intrinsic information at the molecular level. Nonlinear optical interactions using ultrashort laser pulses facilitate the selective coherent excitation of molecular vibrational modes by focusing energy into specific molecular bonds, boosting the signal level for multiple orders of magnitude. The dephasing of such coherence, which is susceptible to the local molecular environment, however, is often neglected. The unique capability of vibrational dephasing dynamics to serve as a unique probe for complex molecular interactions and the effect of local nano- and microenvironments are beyond the reach of conventional, intensity-based spectroscopy. Here, we developed a novel multiorder coherent Raman spectroscopy platform with a special focus on the temporal evolution of molecular vibrational dephasing, termed as time-resolved coherent Raman scattering (T-CRS) spectroscopy. By utilizing a high dynamic range detection, molecular vibrational dynamics and the environmental effects are demonstrated with multidimensional spectroscopic sensing, which promises a new range of applications in biology, materials, and chemical sciences.

Triangular-Shaped 5-DOF Piezoelectric Robot for Optical Lens Positioning

The paper represents numerical and experimental investigations on a 5-DOF piezoelectric robot that can provide rotary and planar motions of the payload. The design of the robot is based on a single piezoelectric ring and a triangular-shaped passive layer made from stainless steel. Six semispherical contacts of alumina oxide were used as contact points for rotary and planar motions. Finally, the top electrode of the piezo ceramic ring was divided into six equal segments to control the 3-DOF angular and 2-DOF planar motions of the payload. Two harmonic signals of different frequencies are used to drive the piezoelectric robot. The robot operation is based on the excitation of the third radial vibration mode of the ring and the first bending mode of the trapezoidal-shaped cantilever. Motion control is performed by switching electric signals between the particular segments of the piezoelectric ring. A numerical investigation was performed to validate the operation principle of the robot and to analyze electrical and mechanical characteristics. Numerical investigations showed that the first bending mode of trapezoidal cantilevers and the third radial mode of the piezo ceramic ring were obtained at a frequency of 13.79 kHz and 95.75 kHz, respectively. Moreover, it was revealed that the coupling ratio between vibration amplitudes of passive and active segments is more than 4 times. The prototype of the piezoelectric robot was made and an experimental study was performed to validate the operating principle of the robot, as well as to investigate the dynamic characteristics. The investigation showed that the highest velocity of the planar motion is 22.3 mm/s while the maximum angular motion speed is 29.3 RPM when an excitation voltage of 200 Vp-p and payload of 25.1 g was applied.

Effect of thermal nonequilibrium on the shock interaction mechanism for carbon dioxide mixtures on double-wedge geometries

The effect of thermal nonequilibrium on shock interactions of carbon dioxide (CO2) hypersonic flows is investigated. Given the relatively low characteristic vibrational temperature of the CO2 molecule, it is expected that excited vibrational modes play a significant role in the physics of shock/shock and shock/boundary layer interactions. The shock interference mechanism resulting from a CO2-dominated flow over different double-wedge geometries is investigated by numerically solving the Navier–Stokes equations within the framework of a two-temperature model that considers translational energy–vibrational energy transfer. To assess the impact of vibrational relaxation, a comparative assessment of the patterns obtained with three thermo-physical models is presented, with the two-temperature model flow pattern being compared to thermally perfect and perfect ideal gas ones. Results obtained with the two-temperature model show that increasing the aft angle significantly enlarges the separated region in the compression corner and generates numerous secondary shock waves and shear layers. Peaks of heat flux and pressure occur along the surface due to boundary layer reattachment downstream of the compression corner, except for the case of the higher angle, which results in the largest peaks due to shock impingement. Different assumptions on the excitation of vibrational modes are shown to largely influence the size of the recirculation bubble in the compression corner, shock interaction mechanism, and surface loads. The more energy transferred to the vibrational mode, the lower post-shock temperatures are obtained, which tends to reduce the post-shock density, leading to weaker shock interactions characterized by delayed onsets of separation, reduced separation regions, and smaller standoff distances.

In-plane selective excitation of arbitrary vibration modes using thickness-shear (d 15) piezoelectric transducers

Experimental modal analysis is of great importance for the dynamic characterization of structures. Existing methods typically employ out-of-plane forces for excitation and measure the acceleration or strain for modal analysis. However, these methods encountered difficulties in some cases. In this work, we proposed an in-plane excitation method based on thickness-shear (d 15) piezoelectric transducers. Through the combination of distributed d 15 PZT strips, arbitrary vibration modes can be selectively excited in a wide frequency range. Both simulations and experiments were conducted and the results validated the proposed method. Specifically, bending, torsional, and longitudinal vibration modes of a rectangular bar were selectively excited. Torsional modes of a shaft were excited without the aid of brackets and bending modes of a circular plate were excited with actuators placed at nodal lines. Furthermore, the electromechanical impedance of the PZT-structure system was measured from which the natural frequency and quality factor were directly extracted. Due to its simplicity and flexibility, the proposed vibration excitation method is expected to be widely used in near future.

Dynamic electron-phonon and spin-phonon interactions due to inertia

THz radiation allows for the controlled excitation of vibrational modes in molecules and crystals. We show that the circular motion of ions introduces inertial effects on electrons. In analogy to the classical Coriolis and centrifugal forces, these effects are the spin-rotation coupling, the centrifugal field coupling, the centrifugal spin-orbit coupling, and the centrifugal redshift. Depending on the phonon decay, these effects persist for various picoseconds after excitation. Potential boosting of the effects would make it a promising platform for vibration-based control of localized quantum states or chemical reaction barriers.

Electronic spectra of positively charged carbon clusters-C2n + (n = 6-14).

Electronic spectra are measured for mass-selected C2n +(n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C2n +(n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C22 + and larger clusters. The C2n + electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C12 +, C16 +, C20 +, C24 +, and C28 + are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C14 +, C18 +, C22 +, and C26 + are narrower, consistent with slower non-radiative deactivation.

Theoretical analysis by Mindlin theory and experimental measurements of the piezoceramic circular bimorphs in resonance

This paper investigates the free vibration characteristics for piezoceramic bimorphs in series connections by using theoretical analysis, numerical calculation, and experimental measurement. Series- and parallel-type piezoceramic bimorphs under fully-clamped boundary conditions are analyzed in this study. The theoretical solutions are derived base on the Mindlin's plate theory to explore the coupled out-of-plane (transverse) and in-plane (planar) resonant vibration characteristics; the numerical calculations based upon finite-element method are also performed and agree very well with the theoretical results for resonant frequency and corresponding mode shape. Two optical techniques, amplitude- fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometer (LDV), and an impedance analysis are used to validate the theoretical and numerical results. It is shown that there is no difference between theoretical and numerical results regarding vibration characteristics of series- and parallel-type piezoceramic bimorphs. However, the experimental findings obtained by impedance analysis reveal that the boundary condition plays an important role for the excited vibration modes of parallel-type piezoceramic bimorphs. The imperfect clamped-circumference simulation in experimental procedure will result in some unexpected vibration modes arising, which can be distinguished between out-of-plane and in-plane modes by AF-ESPI measurement.

High temperature surface graphitization of CVD diamond films and analysis of the kinetics mechanism

A plasma arc generator was used to heat treat the free-standing CVD diamond films at 1450–1900 °C. An Arrhenius curve was fit according to trends in the measured Raman peak intensity ratio of diamond to graphite as a function of heat treatment temperature, and the curve was extrapolated linearly. It was found that the extrapolation lines intersected at the Debye temperature, ΘD = 2021 K. By comparing the activation energies of diamond graphitization in literature and from different measurement modes in our study, it was found that the graphitization mechanisms were different on both sides of Debye temperature due to differences in the excited lattice vibration modes. When the heat treatment temperature was just below the Debye temperature, the graphitization process was promoted by generating vacancies or the breaking of single CC bonds, and the apparent activation energy was 366 ± 60 kJ·mol−1. When the heat treatment temperature was above the Debye temperature, the graphitization process was promoted by the simultaneous breaking of multiple CC bonds, and the apparent activation energy was 887 ± 141 kJ·mol−1. In combination with the morphology changes on the surface of the CVD diamond film after high temperature heat treatment, the mechanism of the micro-cyclic phase transition on the surface of the diamond film at temperatures lower than Debye temperature was summarized as follows. First, CC bonds on the diamond surface were activated and broken at high temperatures, and some diamond carbon atoms dissociated from solid phase sites to the gas phase by forming hydrocarbon free radicals. Then, the carbon atoms from these hydrocarbons entered the graphite lattice step by step to complete the spontaneous deposition of sp2 clusters.

Reducing Vehicle Glass Sensitivity to Turbulent Pressure

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Vehicle interior wind noise is typically managed through the overall exterior geometry of the vehicle, mirror shape and mounting location, sealing features and glass thickness and damping. Prior research has distinguished between contribution of fluctuating pressure due to air turbulence as compared to acoustic pressure to a passenger vehicles exterior at highway speeds. Because of the large difference in propagation speed between turbulent and acoustic pressure for on-road passenger vehicles, the structural response of the glass to turbulent versus acoustic pressure is not the same. The acoustic coincidence frequency of door glass is typically in the 2-3 kHz range. Turbulent coincidence frequency is much lower, and the effective transmission loss (TL) of the glass depends on the mix of turbulent and acoustic pressure on the exterior surface of the glass. Structural changes to the perimeter of the glass itself can reduce turbulent sensitivity, which, at highway speeds, can reduce low frequency noise due to turbulent pressure excitation of door glass vibration modes.&lt;/div&gt;&lt;/div&gt;

Unimolecular Dissociation of C6H6-C6Cl6 Complex and Effect of Mode-Mode Coupling.

The unimolecular dissociation dynamics of the C6H6-C6Cl6 (Bz-HCB) complex is studied with initial excitation of all vibrational modes for a temperature range of 1000-2000 K and with mode-specific excitations at 1500 K. The results are compared with those of the C6H6-C6F6 [Bz- HFB] complex. When all modes of Bz-HCB are initially excited, the rate of dissociation is slower with respect to Bz-HFB. However, the rate of dissociation is faster when simulations with nonrandom excitation of the specific vibrational modes are performed. The rate of dissociation of Bz-HCB is found to become slower when a few intramolecular modes are excited along with all inter-fragment modes compared to the simulation when only inter-fragment modes of the same complex are excited. Such an energy-transfer dynamics is absent if both intramolecular and inter-fragment modes are not initially excited. Thus, a "stimulated" resonance energy-transfer dynamics is observed in Bz-HCB dissociation dynamics.

Gigahertz Nano-Optomechanical Resonances in a Dielectric SiC-Membrane Metasurface Array.

Optically and vibrationally resonant nanophotonic devices are of particular importance for their ability to enhance optomechanical interactions, with applications in nanometrology, sensing, nano-optical control of light, and optomechanics. Here, the optically resonant excitation and detection of gigahertz vibrational modes are demonstrated in a nanoscale metasurface array fabricated on a suspended SiC membrane. With the design of the main optical and vibrational modes to be those of the individual metamolecules, resonant excitation and detection are achieved by making use of direct mechanisms for optomechanical coupling. Ultrafast optical pump-probe studies reveal a multimodal gigahertz vibrational response corresponding to the mechanical modes of the suspended nanoresonators. Wavelength and polarization dependent studies reveal that the excitation and detection of vibrations takes place through the metasurface optical modes. The dielectric metasurface pushes the modulation speed of optomechanical structures closer to their theoretical limits and presents a potential for compact and easily fabricable optical components for photonic applications.

Celanese to expand acetyl chain capacities to strengthen high value, downstream vinyls portfolio

Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics. Sliding-velocity dependence of the frictional force density is characterized by the threshold phenomenon. Further insights into this phenomenon were gained by analyses of ensemble-averaged behavior under the steady sliding condition. An enormous number of simulation runs and frequent validation of their results were carried out so that the interfacial temperature and the pressure, which directly affect the heat generation processes, were made common at different sliding velocities. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by excitation of certain vibrational modes. Such elastic waves hidden in random thermal motion of atoms near the sliding interface were separated by the sampling and averaging. The atomistic reason for the dependence of the threshold speed on the combination of the lattice constants along the sliding direction is discussed based on the patterns of the orderly atomic-displacement fields.

On the in-plane vibrations and electromechanical resonance characteristics of non-uniformly polarized rectangular piezoelectric wafers: Selective mode-type excitation and specific mode enhancement

We investigate the in-plane vibrations and electromechanical resonance characteristics of non-uniformly polarized rectangular piezoelectric wafers. Non-uniform polarization is represented as a non-uniform electromechanical coupling coefficient using a polarization function. Governing equations are derived for the forced in-plane vibrations of a thin wafer under the assumption of generalized plane stress. The effect of non-uniform polarization is explicitly obtained in the forcing terms of the governing equations. These equations are then recast into a variational weak form that is then solved using the finite element method to obtain the displacement fields for different modes. The electromechanical response of a non-uniformly polarized piezoelectric wafer is derived in terms of the out-of-plane displacement profile on the surface of the wafer. Using the derived analytical expression, a necessary and sufficient condition for the presence/absence of a vibrational mode in the electromechanical impedance spectrum is obtained. Based on this condition, criteria for selective mode-type excitation and specific mode enhancement of vibrational modes in the electromechanical impedance spectrum are postulated. Selective mode-type excitation of in-plane extensional and shear modes is demonstrated for a square wafer and that of in-plane bending modes is demonstrated for a rectangular wafer. Specific mode enhancement is demonstrated for both square and rectangular wafers. In addition, it is also demonstrated how the criteria can be used to suppress specific vibrational modes in the electromechanical impedance spectrum. The proposed methodology of using non-uniformly polarized piezoelectric wafers finds application in the design of single element transducers with multi-frequency operation, frequency-tuned receivers/sensors, acoustic holograms, designing acoustic beams of prescribed shape/lobes, and other non-traditional applications such as information storage.

Angular momentum generation in nuclear fission.

When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning1; this phenomenon has been a mystery in nuclear physics for over 40 years2,3. The internal generation of typicallysix or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum4-12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors13,14, for the study of the structure of neutron-rich isotopes15,16, and for the synthesis and stability of super-heavy elements17,18.