Kinetic Energy Of Atoms Research Articles

Efficient extension of the trapping lifetime of single atoms in an optical tweezer by laser cooling

Optical tweezers have become powerful tools for the confinement and manipulation of neutral atoms, molecules, mesoscopic biological molecules and living cells. In our experiment, a single caesium atom was prepared in a large-magnetic-gradient magneto-optical trap (MOT). It was then efficiently transferred back and forth between the MOT and a 1064 nm microscopic optical tweezer. The atomic transfer between the MOT and the tweezer can be employed to measure the trapping lifetime and the energy distribution of the single atom in the tweezer. In order to extend the trapping lifetime, laser cooling is used to decrease the atom's kinetic energy. The trapping lifetime was extended from ∼75 to ∼130 s by applying a 10 ms laser cooling phase just after the single atom is transferred into the tweezer.

A slice imaging and multisurface wave packet study of the photodissociation of CH3I at 304 nm

The role of the conical intersection between the (1)Q(1) and (3)Q(0) excited states in the photodissociation of CH(3)I at 304 nm is investigated drawing a comparison between the adiabatic--through direct absorption to the (3)Q(1) state--and non-adiabatic--via the (1)Q(1)→(3)Q(0) conical intersection--production of I atoms in the ground (2)P(3/2) state. The versatility of the slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of I((2)P(3/2)) atoms allow distinct measurements of the competing processes. The I((2)P(3/2)) atom kinetic energy distributions (KEDs) obtained in both cases reflect inverted vibrational progressions of the ν(2) umbrella mode of the CH(3) co-product. The experimental results show a satisfactory agreement with multisurface wave packet calculations using a reduced dimensionality (pseudotriatomic) model carried out on the available ab initio potential energy surfaces.

Experimental measurement of the van der Waals binding energy of X–O2 clusters (X=Xe,CH3I,C3H6,C6H12)

Van der Waals binding energies for the X-O(2) complexes (X=Xe, CH(3)I, C(3)H(6), C(6)H(12)) are determined by analysis of experimental velocity map imaging data for O((3)P(2)) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X-O(2) into X+2O((3)P) fragments, which is present for complexes of O(2) with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X-O(2) and O-O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110±20 cm(-1), 280±20 cm(-1), 135±30 cm(-1), and 585±20 cm(-1) are determined for Xe-O(2), CH(3)I-O(2), C(3)H(6)-O(2), and C(6)H(12)-O(2) clusters, respectively.

Dependency of temperature on polarization in CH4/N2 dielectric barrier discharge plasma: A crude assumption

We have investigated the variations of polarization (P) and the temperature (ΔT) at the electrode surfaces during the deposition of C–N layer in CH4/N2 (1:2) dielectric barrier discharge plasma. The reactive deposition process influences the surface temperature, polarization, and the value of the in situ dielectric constant. We have developed a crude model that correlates the surface temperature and surface polarization with thin film properties. We assume that during the thin film deposition process, the atomic mean kinetic energy is equal to the electrostatic energy stored in the electrode surface area. Theoretically estimated temperature is found to agree well with the experimental results. However, the linear model thus developed cannot be used to explain the phenomena in the interfacial polarization stage that requires a nonlinear theory.

Energy dissipation and defect generation in nanocrystalline silicon carbide

Large-scale molecular-dynamics simulations have been employed to study defect generation and primary damage state in nanocrystalline (NC) SiC of average grain diameters from 5 to 21 nm. Primary knock-on atom (PKA) kinetic energies of 10 keV are simulated and cascade structures in NC SiC with a grain size smaller than 12 nm are generally different from those generated in single-crystalline SiC. It is found that the local stresses near the grain boundaries (GBs) strongly affect the behavior of the PKA and secondary recoil atoms (SRAs), and the GBs act as sinks for deposition of kinetic energy. A striking feature is that the PKA and SRAs preferentially deposit energy along the GBs for grains with average size less 12 nm, which results in atomic displacements primarily within the GBs; whereas for larger grain sizes, most defects are produced within the grains. The defect production within gains generally increases with increasing grain size, which is manifested in switching from grain boundary damage to grain damage. The most common defects created in NC SiC are antisite defects, following by vacancies and interstitials, in contrast to those produced in a single-crystalline SiC, where the dominate defects are Frenkel pairs. Defect production efficiency increases with increasing grain size, with a typical value of 0.18 for small grains and rising to 0.5 for larger grains.

Properties of a Method for Performing Adaptive, Multilevel QM Simulations of Complex Chemical Reactions in the Gas-Phase.

The properties of a new method of performing molecular dynamic simulations of complex chemical processes are presented. The method is formulated to give a time-dependent, multilevel representation of the total potential that is derived from spatially resolved quantum mechanical regions. An illustrative simulation is performed on a 110 atom system to demonstrate the continuity and energy conserving properties of the method. The effect of a discontinuous total potential upon the kinetic energy of the system is examined. The discontinuities in the magnitude of atomic force vectors due to changing the electronic structure during the simulation are examined as well as the effect that these discontinuities have upon the atomic kinetic energies. The method, while not conserving total energy, does yield canonical (NVT) simulations. The time reversibility property of the simulation with an extremely discontinuous total potential is discussed. The computational scaling associated with the formation of the spatially resolved, time-dependent groups is also investigated.

Coupling internal atomic states in a two-component Bose-Einstein condensate via an optical lattice: Extended Mott-state–superfluid transitions

An ultracold gas of coupled two-component atoms in an optical field is studied. Due to the internal two-level structure of the atoms, three competing energy terms exist: atomic kinetic, atomic internal, and atom-atom interaction energies. A novel outcome of this interplay, not present in the regular Bose-Hubbard model, is that in the single band and tight-binding approximations four different phases appear: two superfluid and two Mott phases. When passing through the critical point between the two superfluid or the two Mott phases, a swapping of the internal atomic populations takes place. By means of the strong-coupling expansion, we find the full phase diagram for the four different phases.

A numerical study of energy transfer mechanisms in materials following irradiation by swift heavy ions

Swift heavy ions interact with electrons in materials and this may yield permanent atomic displacements; the energy transfer mechanisms that bring electronic excitations into atomic motion are not fully understood, and are generally discussed in terms of two theories, viz. Coulomb explosion and heat exchange between excited electrons and atoms, which is limited by electron-phonon coupling. We address this problem for a “generic” material using a semi-classical numerical approach where the dynamics of the evolving electron density is calculated by using molecular dynamics simulations applied to pseudo-electrons. The forces exerted on the nuclei are then used to calculated the trajectories of the nuclei. From the temporal evolution of the atomic kinetic energy, we find that the energy transfer between the electrons and the nuclei can be divided in two parts. First, a Coulomb heating starts the motion of the atoms by giving them a radial speed; this process differs from Coulomb explosion because the atoms are not displaced over interatomic distances. Second, a thermal energy transfer, as described in linear transport theory, takes place. Our study thus confirms the domination of thermal energy exchange mechanisms over Coulomb explosion models.

Continuous measurement feedback control of a Bose-Einstein condensate using phase-contrast imaging

We consider the theory of feedback control of a Bose-Einstein condensate (BEC) confined in a harmonic trap under a continuous measurement constructed via nondestructive imaging. A filtering theory approach is used to derive a stochastic master equation (SME) for the system from a general Hamiltonian based upon system-bath coupling. Numerical solutions for this SME in the limit of a single atom show that the final steady-state energy is dependent upon the measurement strength, the ratio of photon kinetic energy to atomic kinetic energy, and the feedback strength. Simulations indicate that for a weak measurement strength, feedback can be used to overcome heating introduced by the scattering of light, thereby allowing the atom to be driven toward the ground state.

Single-photon cooling at the limit of trap dynamics: Maxwell's demon near maximum efficiency

We demonstrate a general and efficient informational cooling technique for atoms that is an experimental realization of a one-dimensional Maxwell's demon. The technique transfers atoms from a magnetic trap into an optical trap via a single spontaneous Raman transition that is discriminatively driven near each atom's classical turning point. In this way, nearly all of the atomic ensemble's kinetic energy in one dimension is removed. We develop a simple analytical model to predict the efficiency of transfer between the traps and provide evidence that the performance is limited only by particle dynamics in the magnetic trap. Transfer efficiencies up to 2.2% are reported. We show that efficiency can be traded for phase-space compression, and we report compression up to a factor of 350. Our results represent a 15-fold improvement over our previous demonstration of the cooling technique.

Search for supersolid 4He in neutron scattering experiments at ISIS

The observation of a non-classical rotation inertia (NCRI) fraction in bulk solid 4He by M. Chan and E. Kim [1, 2] attracted significant interest as a possible manifestation of supersolid state of matter. Despite numerous experimental and theoretical studies inspired by this observation, an explicit explanation for this phenomenon is still missing. Neutron scattering experiments on solid helium may help to shed light on the physical grounds of NCRI and answer the question on whether this phenomenon could be caused by Bose-Einstein Condensation. In this paper we are going to discuss the results obtained in experiments involving neutron scattering on solid 4He. Microscopic quantitative data such as mean kinetic energy, mean square momentum and mean square displacement of helium atoms as well as the lattice parameter have been obtained for the first time for solid 4He in temperature range 70mK - 500 mK. No change was seen in the single atom kinetic energy (within statistical error better than 1%) as well as change in the lattice parameter (within 0.03%). The mean square displacement did not change in the region of expected supersolid transition either. All these results suggest that the NCRI transition is quite different from the superfluid transition in liquid 4He.

Analysis of anisotropic suprathermal ion distributions using multidirectional measurements of escaping neutral atom fluxes

A feasible approach in obtaining experimental data on the angular dependence of the ion distribution function in a fusion plasma is to perform angle-resolved measurements of kinetic energy spectra of escaping neutral atoms. A general calculation scheme has been developed and realized as a FORTRAN code that has a predictive force to simulate the experimentally measurable anisotropic distributions and random samples of escaping neutral atom kinetic energies for any given angle-dependent ion distribution law, electron density, and temperature profiles, plasma composition, magnetic surface structure, and experiment geometry on any toroidal plasma device with magnetic confinement. As a particular application of the method to a specific experiment, measured signals for all 20 channels of the angle-resolved multisightline neutral particle analyzer on Large Helical Device have been numerically simulated for certain predefined model fast ion distribution functions.

Motion of Rydberg atoms induced by resonant dipole–dipole interactions

We show that nuclear motion of Rydberg atoms can be induced by resonant dipole–dipole interactions that trigger the energy transfer between two energetically close Rydberg states. How and if the atoms move depends on their initial arrangement as well as on the initial electronic excitation. Using a mixed quantum/classical propagation scheme, we obtain the trajectories and kinetic energies of atoms, initially arranged in a regular chain and prepared in excitonic eigenstates. The influence of the off-diagonal disorder on the motion of the atoms is examined and it is shown that irregularity in the arrangement of the atoms can lead to an acceleration of the nuclear dynamics.

Multiscale simulations: application to the heat transfer simulation of sliding solids

Molecular dynamics is a powerful tool allowing the simulation of matter behaviour at the atomic scale. Due to computation time, it is clearly not possible to use molecular dynamics to simulate a forming process. However, atomistic simulations can be used to study and understand the physical phenomena that occur during matter deformation. As an example, heat transfer between the contacting solids in forming processes is one of the important physics phenomena that have to be taken into account in order to do realistic simulations. A multiscale analysis of heat transfer is presented. This analysis leads to two kinds of models: a macroscopic model which can be used for the simulation of the process itself and a microscopic model that is used to determine the parameters of the macroscopic model. In this microscopic model, the friction heat generation phenomena has to be described quite accurately. Friction heat is mainly due to plastic and elastic deformation and adhesion. Thus, to understand the underlying friction heat generation phenomena, atomistic simulations using molecular dynamics are carried out. It is shown that friction heat is the transformation of mechanical work given to the system at the macroscopic scale into potential energy during elastic deformation. This potential energy which is stored in the system is finally transformed into atomic kinetic energy (friction heat) during plastic transformation.

Atomic kinetic energy, momentum distribution, and structure of solid neon at zero temperature

We report on the calculation of the ground-state atomic kinetic energy ${E}_{k}$ and momentum distribution of solid Ne by means of the diffusion Monte Carlo method and Aziz HFD-B pair potential. This approach is shown to perform notably for this crystal since we obtain very good agreement with respect to experimental thermodynamic data. Additionally, we study the structural properties of solid Ne at densities near the equilibrium by estimating the radial pair-distribution function, Lindemann's ratio, and atomic density profile around the positions of the perfect crystalline lattice. Our value for ${E}_{k}$ at the equilibrium density is $41.51(6)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which agrees perfectly with the recent prediction made by Timms et al., $41(2)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, based on their deep-inelastic neutron scattering experiments carried out over the temperature range $4--20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and also with previous path integral Monte Carlo results obtained with the Lennard-Jones and Aziz HFD-C2 atomic pairwise interactions. The one-body density function of solid Ne is calculated accurately and found to fit perfectly, within statistical uncertainty, to a Gaussian curve. Furthermore, we analyze the degree of anharmonicity of solid Ne by calculating some of its microscopic ground-state properties within traditional harmonic approaches. We provide insightful comparison to solid $^{4}\mathrm{He}$ in terms of the Debye model in order to assess the relevance of anharmonic effects in Ne.

Ion Distribution Function Evaluation Using Escaping Neutral Atom Kinetic Energy Samples

A reliable method to evaluate the probability density function of escaping atom kinetic energies is required for analyzing neutral particle diagnostic data used to study the fast ion distribution function in fusion plasmas. In this paper, digital processing of solid state detector signals is proposed as an improvement of the simple histogram approach. Probability density function of kinetic energies of neutral particles escaping from plasma has been derived in a general form, taking into consideration the plasma ion energy distribution, electron capture and loss rates, superposition along the diagnostic sight line, and the magnetic surface geometry. A pseudorandom number generator has been realized to simulate a sample of escaping neutral particle energies for given plasma parameters and experimental conditions. Empirical probability density estimation code has been developed and tested to reconstruct the probability density function from simulated samples assuming Maxwellian ion energy distribution shapes for different temperatures and classical slowing down distributions with different slowing down times. The application of the developed probability density estimation code to the analysis of experimental data obtained by the novel Angular-Resolved Multi-Sightline Neutral Particle Analyzer has been studied to obtain the suprathermal particle distributions. The optimum bandwidth parameter selection algorithm has also been realized.

Kinetic-energy density functionals with nonlocal terms with the structure of the Thomas-Fermi functional

We study two families of approximate nonlocal kinetic-energy functionals that include a full von Weizs\"acker functional, and that have nonlocal terms with the mathematical structure of the Thomas-Fermi functional. The functionals recover the exact kinetic energy and the linear response function of a homogeneous electron system. The first family is a generalization of a successful previous nonlocal functional. The second family is proposed in the paper, and is designed to obtain functionals suitable for use in both localized and extended systems. Furthermore, this family has been designed to be evaluated by a single integration in momentum space when a constant reference density is used. The atomic total kinetic energies are in good agreement with the exact calculations. The kinetic-energy density corresponding to each functional has been assessed to control its quality. The results show that, in general, these functionals behave better than both the Thomas-Fermi and all semilocal generalized gradient approximation functionals when describing the kinetic-energy density of atoms, providing a better description of the nonlocal effects of the kinetic energy of electron systems.

Josephson dynamics for coupled polariton modes under the atom–field interaction in the cavity

We consider a new approach to the problem of Bose–Einstein condensation (BEC) of polaritons for atom–field interaction under the strong coupling regime in the cavity. We investigate the dynamics of two macroscopically populated polariton modes corresponding to the upper and lower branch energy states coupled via Kerr-like nonlinearity of atomic medium. We found out the dispersion relations for new type of collective excitations in the system under consideration. Various temporal regimes like linear (nonlinear) Josephson transition and/or Rabi oscillations, macroscopic quantum self-trapping (MQST) dynamics for population imbalance of polariton modes are predicted. We also examine the switching properties for time-averaged population imbalance depending on initial conditions, effective nonlinear parameter of atomic medium and kinetic energy of low-branch polaritons.

Atomic Partitioning of the Dissociation Energy of the P−O(H) Bond in Hydrogen Phosphate Anion (HPO42-): Disentangling the Effect of Mg2+

This paper has three goals: (1) to provide a first step in understanding the atomic basis of the role of magnesium in facilitating the dissociation of the P-O bond in phosphorylated biochemical fuel molecules (such as ATP or GTP), (2) to compare second-order Møller-Plesset perturbation theory (MP2) results with those obtained at the more economical density functional theory (DFT) level for a future study of larger more realistic models of ATP/GTP, and (3) to examine the calculation of atomic total energies from atomic kinetic energies within a Kohn-Sham implemention of DFT, as compared to ab initio methods. A newly described method based on the quantum theory of atoms in molecules (QTAIM), which is termed the "atomic partitioning of the bond dissociation energy" (APBDE), is applied to a simple model of phosphorylated biological molecules (HPO42-). The APBDE approach is applied in the presence and in the absence of magnesium. It is found that the P-O(H) bond in the magnesium complex is shorter, exhibits a higher stretching frequency, and has a higher electron density at the bond critical point than in the magnesium-free hydrogen phosphate anion. Though these data would seem to suggest a stronger P-O(H) bond in the magnesium complex compared to the magnesium-free case, the homolytic breaking of the P-O(H) bond in the complex is found to be easier, i.e., has a lower BDE. This effect is the result of the balance of several atomic contributions to the BDE induced by the magnesium cation, which stabilizes the dissociation product more than it stabilizes the intact model molecule.

Testing General Relativity with Atom Interferometry

The unprecedented precision of atom interferometry will soon lead to laboratory tests of general relativity to levels that will rival or exceed those reached by astrophysical observations. We propose such an experiment that will initially test the equivalence principle to 1 part in 10(15) (300 times better than the current limit), and 1 part in 10(17) in the future. It will also probe general relativistic effects - such as the nonlinear three-graviton coupling, the gravity of an atom's kinetic energy, and the falling of light - to several decimals. In contrast with astrophysical observations, laboratory tests can isolate these effects via their different functional dependence on experimental variables.