Inelastic Loss Research Articles

Local vs. non-local energy loss of low energy ions: Influence of charge exchange processes in close collisions

We investigate the contribution of charge exchange processes in close collisions between projectile and target atoms to the electronic energy loss of low energy ions. We measure the energy loss of slow hydrogen and He ions in ultrathin Al films through which the ions are transmitted before and after backscattering by the atoms of a Ta substrate. The individual contributions to the energy loss are analyzed. The roles of thresholds for reionization and of scattering kinematics as key parameters for the coupling between elastic and inelastic losses are discussed. The implications of the obtained results for different experimental approaches to deduce stopping cross sections are outlined.

Evaporative cooling of the dipolar hydroxyl radical

Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose-Einstein condensation, quantum-degenerate Fermi gases and ultracold optical lattice simulations of condensed-matter phenomena. More recently, substantial progress has been made in the production of cold molecular gases. Their permanent electric dipole moment is expected to generate systems with varied and controllable phases, dynamics and chemistry. However, although advances have been made in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH(•)) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

The role of localized recoil in the formation of Kikuchi patterns

In electron scattering from crystals, diffraction spots are replaced by Kikuchi patterns at high momentum transfer. Kikuchi pattern formation is based on the concept of effective incoherent electron sources (or detectors) inside a crystal. The resulting incoherence is a consequence of energy transfer connected with the momentum transfer in large-angle scattering events. We identify atomic recoil as a key incoherent process giving rise to electron Kikuchi patterns in the scope of the "channeling-in and channeling-out" model of electron backscatter diffraction (EBSD) and electron channeling patterns (ECP) in the scanning electron microscope (SEM). Using model calculations, we explore the characteristic role of the localization of the incoherent scattering event at specific places within the unit cell. In this way, we explain why sometimes inelastic losses do cause Kikuchi-type contrast, and sometimes inelastic losses result in the disappearance of this contrast in the SEM.

Spin-dependent collision of ultracold metastable atoms

Spin-polarized metastable atoms of ultracold ytterbium are trapped at high density and their inelastic collisional properties are measured. We reveal that in collisions of Yb(3P2) with Yb(1S0) there is relatively weak inelastic loss, but with a significant spin-dependence consistent with Zeeman sublevel changes as being the dominant decay process. This is in strong contrast to our observations of Yb(3P2)-Yb(3P2) collisional loss, which are, at low field, much more rapid and have essentially no spin dependence. Our results give a guideline to use the 3P2 states in many possible applications.

A New Contact Force Model for Low Coefficient of Restitution Impact

Impact problems arise in many practical applications. The need for obtaining an accurate model for the inelastic impact is a challenging problem. In general, two approaches are common in solving the impact problems: the impulse-momentum and the compliance based methods. The former approach included the coefficient of restitution which provides a mechanism to solve the problem explicitly. While the compliance methods are generally tailored to solve elastic problems, researchers in the field have proposed several mechanisms to include inelastic losses. In this paper, we present correlations between the coefficient of restitution in the impulse-momentum based method and the contact stiffness in the compliance methods. We conducted numerical analysis to show that the resulting solutions are indeed identical for a specific range of impact conditions. The impulse-momentum based model is considered as a reference case to compare the post impact velocities. The numerical results showed that, the impulse-momentum and the compliance based methods can produce similar outcomes for specific range of coefficient of restitution if they satisfied a set of end conditions. The correlations lead to introduce a new contact force model with hysteresis damping for low coefficient of restitution impact.

Multichannel ultracold collisions between metastable bosonic88Sr and fermionic87Sr atoms

Ultracold collisions between metastable bosonic ${}^{88}$Sr [$(5s5p){\phantom{\rule{0.16em}{0ex}}}^{3\phantom{\rule{-0.16em}{0ex}}}\phantom{\rule{-0.16em}{0ex}}{P}_{2}$] and fermionic ${}^{87}$Sr [$(5s5p){\phantom{\rule{0.16em}{0ex}}}^{3\phantom{\rule{-0.16em}{0ex}}}\phantom{\rule{-0.16em}{0ex}}{P}_{2}$ $(i=9/2)$] atoms are investigated based on an approach of tensorial analysis. The scattering physics of two atoms in fully polarized $s$-wave entrance channels is studied in detail. In the Born-Oppenheimer approximation, the strong anisotropic interatomic interaction is demonstrated to induce formations of long-range molecular potential wells. However, besides significantly modifying the elastic scattering, the anisotropic interatomic interaction leads to the strong multichannel coupling between different partial waves, which triggers a high rate of inelastic losses. By applying an external static magnetic field ${\mathbf{B}}_{0}={\mathcal{B}}_{0}{\mathbf{e}}_{\mathrm{z}}$, the inelastic scattering can be suppressed while the elastic scattering is significantly enhanced. Additionally, the energy-dependent complex $s$-wave scattering lengths at a given relative collision energy strongly depend on the strength of magnetic field. We, moreover, self-consistently investigate ultracold collisions of two atoms at low temperatures in an optical lattice site. In the $\ensuremath{\delta}$-function pseudopotential approximation, we derive the effective $s$-wave scattering lengths, energy eigenvalues, elastic and inelastic scattering rates, and their dependence on the external magnetic field. We find that (i) the effective scattering lengths of ultracold atoms in entrance channels of interest display resonance behavior at certain values of magnetic field and (ii) the extra Zeeman interaction not only leads to the suppression of inelastic scattering but also enlarges the elastic scattering rates.

Fermi liquid of two-dimensional polar molecules

We study Fermi-liquid properties of a weakly interacting two-dimensional gas of single-component fermionic polar molecules with dipole moments $d$ oriented perpendicularly to the plane of their translational motion. This geometry allows the minimization of inelastic losses due to chemical reactions for reactive molecules and, at the same time, provides a possibility of a clear description of many-body (beyond mean-field) effects. The long-range character of the dipole-dipole repulsive interaction between the molecules, which scales as $1/{r}^{3}$ at large distances $r$, makes the problem drastically different from the well-known problem of the two-species Fermi gas with repulsive contact interspecies interaction. We solve the low-energy scattering problem and develop a many-body perturbation theory beyond the mean field. The theory relies on the presence of a small parameter ${k}_{F}{r}_{*}$, where ${k}_{F}$ is the Fermi momentum and ${r}_{*}=m{d}^{2}/{\ensuremath{\hbar}}^{2}$ is the dipole-dipole length, with $m$ being the molecule mass. We obtain thermodynamic quantities as a series of expansion up to the second order in ${k}_{F}{r}_{*}$ and argue that many-body corrections to the ground-state energy can be identified in experiments with ultracold molecules, as it has been recently done for ultracold fermionic atoms. Moreover, we show that only many-body effects provide the existence of zero sound and calculate the sound velocity.

Optical Control of Feshbach Resonances in Fermi Gases Using Molecular Dark States

We propose a general method for optical control of magnetic Feshbach resonances in ultracold atomic gases with more than one molecular state in an energetically closed channel. Using two optical frequencies to couple two states in the closed channel, inelastic loss arising from spontaneous emission is greatly suppressed by destructive quantum interference at the two-photon resonance, i.e., dark-state formation, while the scattering length is widely tunable by varying the frequencies and/or intensities of the optical fields. This technique is of particular interest for a two-component atomic Fermi gas, which is stable near a Feshbach resonance.

Theoretical optical and x-ray spectra of liquid and solid H2O

Theoretical optical and x-ray spectra of model structures of water and ice are calculated using a many-body perturbation theory, Bethe-Salpeter equation (BSE) approach implemented in the valence- and core-excitation codes AI2NBSE and OCEAN. These codes use ab initio density functional theory wave functions from a plane-wave, pseudopotential code, quasi-particle self energy corrections, and a BSE treatment of particle-hole interactions. The approach improves upon independent-particle methods through the inclusion of a complex, energy-dependent self energy and screened particle-hole interactions to account for inelastic losses and excitonic effects. These many-body effects are found to be crucial for quantitative calculations of ice and water spectra.

Inelastic analysis of semi-rigid composite structures under fire conditions

This paper presents the application of a proposed numerical approach, denoted as SAAFE Program (System for Advanced Analysis for Fire Engineering), developed to provide an inelastic analysis of steel and composite (steel-concrete) 2D semi-rigid framed-structures under fire conditions. The proposed structural model allows an accurate description of the structural non-linear response, with less computational effort when compared to the general FEM formulation. The method, although similar in concept to earlier plastic-hinge approaches, differs with regards to numerical implementation methodology and precision. The proposed plastic-hinge model is formulated in a succinct format based on the following characteristics: (i) a refined plastic lumped formulation with interaction surfaces, (ii) a tangent modulus model which includes both gradual inelastic loss of stiffness and ultimate strength of column members, (iii) a second-order large-displacement formulation, based on the Stability Functions concept, (iv) non-homogeneous temperature distribution over the cross-section, (v) a connection semi-rigid model. Obtained results for connection model calibration examples are compared to reported experimental data, showing reasonable agreement. In addition, results of a proposed case-of-study demonstrate the efficiency and robustness of the SAAFE model to perform inelastic analysis semi-rigid members, outlining the advantage of considering advanced analysis in the current fire-design practice of structures.

Cold-collision-shift cancellation and inelastic scattering in a Yb optical lattice clock

Recently, p-wave cold collisions were shown to dominate the density-dependent shift of the clock transition frequency in a 171Yb optical lattice clock. Here we demonstrate that by operating such a system at the proper excitation fraction, the cold collision shift is canceled below the 5x10^{-18} fractional frequency level. We report inelastic two-body loss rates for 3P0-3P0 and 1S0-3P0 scattering. We also measure interaction shifts in an unpolarized atomic sample. Collision measurements for this spin-1/2 171Yb system are relevant for high performance optical clocks as well as strongly-interacting systems for quantum information and quantum simulation applications.

Measurement of Optical Feshbach Resonances in an Ideal Gas

Using a narrow intercombination line in alkaline earth atoms to mitigate large inelastic losses, we explore the optical Feshbach resonance effect in an ultracold gas of bosonic (88)Sr. A systematic measurement of three resonances allows precise determinations of the optical Feshbach resonance strength and scaling law, in agreement with coupled-channel theory. Resonant enhancement of the complex scattering length leads to thermalization mediated by elastic and inelastic collisions in an otherwise ideal gas. Optical Feshbach resonance could be used to control atomic interactions with high spatial and temporal resolution.

Bound States and Scattering Resonances Induced by Spatially Modulated Interactions

We study the two-body problem with a spatially modulated interaction potential using a two-channel model, in which the interchannel coupling is provided by an optical standing wave and its strength modulates periodically in space. As the modulation amplitudes increase, there will appear a sequence of bound states. Part of them will cause a divergence of the effective scattering length, defined through the phase shift in the asymptotic behavior of scattering states. We also discuss how the local scattering length, defined through short-range behavior of scattering states, modulates spatially in different regimes. These results provide a theoretical guideline for a new control technique in the cold atom toolbox, in particular, for alkaline-earth(-like) atoms where the inelastic loss is small.

Prospects for sympathetic cooling of molecules in electrostatic, ac and microwave traps

We consider how trapped molecules can be sympathetically cooled by ultracold atoms. As a prototypical system, we study LiH molecules co-trapped with ultracold Li atoms. We calculate the elastic and inelastic collision cross sections of 7LiH + 7Li with the molecules initially in the ground state and in the first rotationally excited state. We then use these cross sections to simulate sympathetic cooling in a static electric trap, an ac electric trap, and a microwave trap. In the static trap we find that inelastic losses are too great for cooling to be feasible for this system. The ac and microwave traps confine ground-state molecules, and so inelastic losses are suppressed. However, collisions in the ac trap can take molecules from stable trajectories to unstable ones and so sympathetic cooling is accompanied by trap loss. In the microwave trap there are no such losses and sympathetic cooling should be possible.

Improved Tougaard background calculation by introduction of fittable parameters for the inelastic electron scattering cross‐section in the peak fit of photoelectron spectra with UNIFIT 2011

The shape of the background in x‐ray photoemission spectra is strongly affected by scattered electrons from inelastic energy loss processes. A polynomial of low order has very often been applied to model the secondary‐electron background, giving satisfying results in some cases. An improved analysis employing the Tougaard background model has been successfully used to characterize the inelastic loss processes. However, the correct usage of the Tougaard background needs a well defined inelastic electron scattering cross‐section function λ(E) · K(E, T) (λ = inelastic mean free path, E = kinetic energy, T = energy loss). This paper presents a four‐parameter loss function λ(E) · K(E, T) = B · T/(C + C′ · T2)2 + D · T2 with the fitting parameters B, C, C′ and D implemented in the background function allowing the improved estimation of the λ(E) · K(E, T) function for homogenous materials. The fit of the background parameters is carried out parallel to the peak fit. The results will be compared with the parameters recommended by Tougaard. The calculation of inelastic electron scattering cross‐sections of clean surfaces from different materials using UNIFIT 2011 will be demonstrated. Copyright © 2011 John Wiley & Sons, Ltd.

Studies of EPES REELS spectra of polyethylenes aided by line shape analysis—Effect of electron irradiation

Polyethylenes of different density, branching structure, crystallographic order, and their degradation due to electron beam were studied using elastic peak electron spectroscopy (EPES) and reflection electron energy loss spectroscopy (REELS) aided with line shape analysis by the pattern recognition (PR) method. This approach offers an algorithm of classification derived from a reference set, i.e. set of spectra recorded from standards exposed to low electron dose (about few C m −2), i.e. polyethylene (100% of C sp 3 bonds) and polystyrene (75% of C sp 2 bonds). Then, the obtained classifier is applied for identification of spectra recorded from polyethylenes exposed to electron beam (doses from 40 to 60 C m −2). The EPES REELS spectra are analyzed in the vicinity of electron quasi-elastic and inelastic losses. Due to electrons undergoing a quasi-elastic scattering from atoms of different atomic numbers, i.e. carbon and hydrogen, for undamaged polymers the surface hydrogen content can be evaluated. Changes due to electron irradiation in polyethylenes are indicated by decreasing content of hydrogen, increasing C sp 2 content and changes in the π loss peak in the REELS spectra. Results of PR method are consistent with results obtained from the C 1s XPS spectra fitting and the width of C KLL XAES spectra (parameter D). Highest stability under electron irradiation was shown by polyethylene of largest molecular weight and most linear branching structure. Application of the PR method to EPES REELS spectra allows to distinguish different polyethylenes and quantify the C sp 2 content.

Interlayer Superfluidity in Bilayer Systems of Fermionic Polar Molecules

We consider fermionic polar molecules in a bilayer geometry where they are oriented perpendicularly to the layers, which permits both low inelastic losses and superfluid pairing. The dipole-dipole interaction between molecules of different layers leads to the emergence of interlayer superfluids. The superfluid regimes range from BCS-like fermionic superfluidity with a high Tc to Bose-Einstein (quasi-)condensation of interlayer dimers, thus exhibiting a peculiar BCS-Bose-Einstein condensation crossover. We show that one can cover the entire crossover regime under current experimental conditions.

Probability of inelastic nuclear interactions of high-energy protons in a bent crystal

Probability of inelastic nuclear interactions in a short bent silicon crystal for its orientations optimal for channeling and volume reflection was investigated using 400 GeV/c protons of the CERN SPS. The contribution of nuclear interactions from channeled protons was observed to be about 3–4% of the probability for the amorphous orientation. For the crystal orientation optimal for volume reflection the nuclear interaction probability of protons was a few percents larger than in the amorphous case. It was shown that in the limiting case of a quasi parallel beam realizing for the collider beam halo the inelastic nuclear losses should decrease by more than five times, which is an additional advantage of a crystal as a primary collimator for the LHC collimation system.

Bose-Einstein condensation and spin mixtures of optically trapped metastable helium

We report the realization of a BEC of metastable helium-4 atoms (4He*) in an all optical potential. Up to 10^5 spin polarized 4He* atoms are condensed in an optical dipole trap formed from a single, focused, vertically propagating far off-resonance laser beam. The vertical trap geometry is chosen to best match the resolution characteristics of a delay-line anode micro-channel plate detector capable of registering single He* atoms. We also confirm the instability of certain spin state combinations of 4He* to two-body inelastic processes, which necessarily affects the scope of future experiments using optically trapped spin mixtures. In order to better quantify this constraint, we measure spin state resolved two-body inelastic loss rate coefficients in the optical trap.

On a peculiarity of low-energy ion scattering from a well-ordered bcc W(211) surface

Abstract A peculiarity in the backscattering of keV He+ ions by a well-ordered high-purity W(2 1 1) surface is reported. Besides the normal elastic binary collision peak and the low-energy tail due to backscattering in deeper layers, an extra peak is observed for an inelastic loss of about 95 eV. This unusually large loss has a constant value over a wide range of primary energies (1.5–4.5 keV). An extra peculiarity is that the peak is only observed for the scattering in normal incidence towards the (2 1 1) plane. It is also not seen for polycrystalline W. The energy loss may be due to a quasi-double or -triple collisions of He particles with the row-trough structure of W(2 1 1) involving electronic excitation of both He and W atoms. Alternatively it may be due to a special channeling/dechanneling process for the incident ions.