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Physical Sciences, Atomic & Molecular Physics; quantum gravity; vacuum polarization; gravitational dipole; Majorana particle
Online: 4 December 2018 (09:54:13 CET)
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In this article various heuristic approaches are discussed to solve the dark matter phenomenon by the concept of vacuum polarization. They are compared with a more fundamental approach, based upon an entropy model of the visible universe. They all make use of some kind of gravitational dipole. These dipoles seem to violate Einstein’s equivalence principle between inertial mass and gravitational mass. It is shown how the paradox can be solved by a quantum mechanical principle.
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Physical Sciences, Atomic & Molecular Physics; precision measurements; fundamental constants; variation of constants; proton-to-electron mass ratio; molecular ions; forbidden transition; two-photon transition; vibrational overtone
Online: 2 November 2018 (12:58:06 CET)
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Vibrational overtones in deeply bound molecules are sensitive probes for variation of the proton-to-electron mass ratio μ. In nonpolar molecules, these overtones may be driven as two-photon transitions. Here, we present procedures for experiments with O2+, including state-preparation through photoionization, a two-photon probe, and detection. We calculate transition dipole moments between all X2Пg vibrational levels and those of the A2Пu excited electronic state. Using these dipole moments, we calculate two-photon transition rates and AC-Stark-shift systematics for the overtones. We estimate other systematic effects and statistical precision. Two-photon vibrational transitions in O2+ provide multiple routes to improved searches for μ variation.
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Physical Sciences, Atomic & Molecular Physics; zero-degree Auger projectile spectroscopy; mixed-state beams; metastable states; He-like states; Li-like states; Be-like states; cascade feeding; electron transfer excitation; electron excitation; electron transfer; hollow states; SIMION
Online: 30 October 2018 (03:53:32 CET)
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The use of mixed-state ionic beams in collision dynamics investigations is examined. Using high resolution Auger projectile spectroscopy involving He-like ($1s^2\thinspace^1\!S, 1s2s\thinspace^{3,1}\!S$) mixed-state beams, the spectrum contributions of the $1s2s\thinspace^3\!S$ metastable beam component is effectively separated and clearly identified. This is performed with a technique that exploits two independent spectrum measurements under the same collision conditions, but with ions having quite different metastable fractions, judiciously selected by varying the ion beam charge-stripping conditions. Details of the technique are presented together with characteristic examples. In collisions of 4 MeV B$^{3+}$ with H$_2$ targets, the Auger electron spectrum of the separated $1s2s\thinspace^{3}S$ boron beam component allows for a detailed analysis of the formation of the $1s2s(^3\!S)nl \thinspace^2\!L$ states by direct $nl$ transfer. In addition, the production of hollow $2s2p\thinspace^{1,3}\!P$ doubly- and $2s2p^2\thinspace^2\!D$ triply-excited states, by direct excitation and transfer-excitation processes, respectively, can also be independently studied. In similar mixed-state beam collisions of 15 MeV C$^{4+}$ with H$_2$, He, Ne and Ar targets, the contributions of the $1s^2$, $1s2s\thinspace^{3,1}S$ beam components to the formation of the $2s2p\thinspace^{3,1}\!P$ states by double-excitation, $1s\rightarrow2p$ excitation and transfer-loss processes can be clearly identified, facilitating comparisons with theoretical calculations.
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Physical Sciences, Atomic & Molecular Physics; laser induced plasma spectroscopic (LIPS); optical emission spectroscopic (OES); spectroscopy diagnostic; cadmium telluride (CdTe)
Online: 18 October 2018 (04:26:08 CEST)
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In this work, the parameters of plasma (electron temperature (Te), electron density ne, plasma frequency (fp) , Debye length (λD) and Debye number (ND)) have been studied by using the spectrometer that collect the spectrum of Laser produce Cadmium telluride plasma at different energies. The results of electron temperature for CdTe range 0.93-1.18 eV also the electron density 5 × 1010 – 3.8 × 1011 cm-3 have been measured under vacuum reaching 2.5 × 10-2 mbar .Optical properties of CdTe were determined through the optical transmission method using ultraviolet visible spectrophotometer within the range 190 – 1100 nm.
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Physical Sciences, Atomic & Molecular Physics; Schrödinger equation; vibrational spectrum; collocation; inverse multiquadratic function; rectangular matrix
Online: 28 September 2018 (10:15:42 CEST)
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We explore the use of inverse multiquadratic (IMQ) functions as basis functions when solving the vibrational Schrödinger equation with the rectangular collocation method. The quality of the vibrational spectrum of formaldehyde (in six dimensions) is compared to that obtained using Gaussian basis functions when using different numbers of width-optimized IMQ functions. The effects of the ratio of the number of collocation points to the number of basis functions and of the choice of the IMQ exponent are studied. We show that the IMQ basis can be used with parameters where the IMQ function is not integrable. We find that the quality of the spectrum with IMQ basis functions is somewhat lower that that with a Gaussian basis when the basis size is large and for a range of IMQ exponents. The IMQ functions are, however, advantageous when a small number of functions is used or with a small number of collocation points e.g. when using square collocation.
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Physical Sciences, Atomic & Molecular Physics; photoacoustic spectroscopy; PAS; hydrocarbons; optical-parametric oscillator; OPO; gas sampling; spectral deconvolution; EUREQA
Online: 5 September 2018 (04:01:48 CEST)
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Photoacoustic spectroscopy allows the identification of specific molecules in gases. We evaluate the spectral resolution and detection limits for a PAS system in the broadband infrared wavelength region 3270 nm ≲ λ ≲ 3530 nm driven by a continuous wave optical parametric oscillator with P ≈ 1.26 W by measuring the absorption of diluted propane, ethane and methane test gases at low concentrations c ~ 100 ppm for ~1350 discrete wavelengths λi. The resulting spectra IPAS(λi) were compared to the high resolution cross section data σFTIR obtained by Fourier Transform Infrared Spectroscopy from the HITRAN database. Deviations as little as 7.1(6)% for propane, 8.7(11)% for ethane and 15.0(14)% for methane with regard to the average uncertainty between IPAS(λi) and the expected reference values based on σFTIR were recorded. The wavelengths λres of the characteristic absorption lines can be pinpointed with a high relative accuracy <5 × 10−5 corresponding to a resolution of λres ~ 0.16 nm. Detection limits range between 7.1 ppb (ethane) to 13.6 ppb (methane) coinciding with high experimental signal-to-noise ratios. Moreover, using EUREQA, an artificial intelligence program, simulated mixed gas samples at low limits of detection could be deconvoluted. These results justify a further development of PAS technology to support, e.g., biomedical research.
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Physical Sciences, Atomic & Molecular Physics; laser-induced plasma; stark broadening; electron density; nanomaterial; atomic spectroscopy; silver; hydrogen
Online: 1 August 2018 (08:02:03 CEST)
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This work communicates results from optical emission spectroscopy following laser-induced optical breakdown at or near nanomaterial. Selected atomic lines of silver are evaluated for consistent determination of electron density. Comparisons are presented with Balmer series hydrogen results. Of particular interest are measurements free of self-absorption effects. For several silver lines, asymmetries are observed in the recorded line profiles. Electron densities of interest range from 0.5 to 3 × 1017 cm-3, for 5 nanosecond Q-switched Nd:YAG radiation at wavelengths of 1064, 532, and 355 nm, and for selected silver emission lines including 328.0, 338.2, 768.7, and 827.3 nm, and the hydrogen alpha Balmer series line at 656.3 nm. Line asymmetries are presented for the 328.0 nm Ag I line that is measured following generation of the plasma due to multiple photon absorption. This work explores electron density variations for different irradiance levels, and reports spectral line asymmetry of resonance lines for different laser fluence levels.
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Physical Sciences, Atomic & Molecular Physics; atomic physics; transition probabilities; hyperfine and isotopic substructure; stellar spectroscopy; chemical compositions
Online: 27 July 2018 (04:34:38 CEST)
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This brief review highlights some current issues in Galactic stellar nucleosynthesis, and some recent laboratory studies by the Wisconsin atomic physics group that have direct application to stellar spectroscopy, in order to advance our understanding of the chemical evolution of our Galaxy. The relevant publication history of the lab studies are summarized, and investigations into the abundances of neutron-capture and iron-peak elements in low metallicity stars are described. Finally, new initatives in near-infrared spectroscopy are briefly explored.
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Physical Sciences, Atomic & Molecular Physics; laser-induced plasma; atomic spectroscopy; self-absorption; nanoparticles; silver; hydrogen
Online: 20 July 2018 (05:49:30 CEST)
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The resonance spectra of neutral silver indicate self-absorption for the studied Ag I lines at the wavelengths of 327.9 nm and 338.2 nm. The center dip is associated with self-reversal due to self-absorption in the plasma. The Q-switched radiation of 355 nm, 532 nm, or 1064 nm from a Nd:YAG laser device generates the plasma at the surface of silver nano-material targets, with experiments conducted in standard ambient temperature and pressure laboratory air. Procedures for recovery of the spectral line shapes confirm that over and above the effects of self-reversal, line shape distortion are important in the analysis. The work discusses parameters describing self-absorption when using fluence levels of 2 to 33 J/cm2 to generate the plasma. Furthermore, subsidiary calibration efforts that utilize the hydrogen alpha line of the Balmer series show that the Ag I lines at 827.35 nm and 768.7 nm are optically thin.
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Physical Sciences, Atomic & Molecular Physics; laser-plasma interactions; plasma dynamics and flow; hypersonic flows; emission spectra
Online: 13 July 2018 (10:16:46 CEST)
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Micro-plasma is generated in ultra-high-pure hydrogen gas filled inside a cell at a pressure of (1.08 ± 0.033) × 105 Pa (810 ± 25 Torr) by using a Q-switched Nd:YAG laser device operated at 1064 nm wavelength and 14 ns pulse duration. Micro-plasma emission spectra of the hydrogen Balmer alpha line, Hα, are recorded with a Czerny-Turner type spectrometer and an intensified charge-coupled device. The spectra are calibrated for wavelength and corrected for detector sensitivity. During the first few tens of nanoseconds after initiating optical breakdown, significantly Stark-broadened and Stark-shifted Hα lines mark the well-above hypersonic outward expansion. The vertical diameters of the spectrally resolved plasma images are measured for time delays of 10 ns to 35 ns to determine expansion speeds of the order of 100 km/s to 10 km/s. For time delays of the order of 0.5 µs to 1 µs, the expansion decreases to the speed of sound of 1.3 km/s in the near ambient temperature and pressure hydrogen gas.
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Physical Sciences, Atomic & Molecular Physics; time variation of constants; proton-electron mass ratio; molecular ion; laser cooling
Online: 12 July 2018 (08:04:03 CEST)
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Molecular overtone transitions provide optical frequency transitions sensitive to variation in the proton-to-electron mass ratio (μ = mp/me). However, robust molecular state preparation presents a challenge critical for achieving high precision. Here, we characterize infrared and optical-frequency broadband laser cooling schemes for TeH+, a species with multiple electronic transitions amenable to sustained laser control. Using rate equations to simulate laser cooling population dynamics, we estimate the fractional sensitivity to μ attainable using TeH+. We find that laser cooling of TeH+ can lead to significant improvements on current μ variation limits.
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Physical Sciences, Atomic & Molecular Physics; atomic data
Online: 3 July 2018 (11:25:13 CEST)
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The rich emission and absorption line spectra of Fe I may be used to extract crucial information on astrophysical plasmas, such as stellar metallicities. There is currently a lack, in quality and quantity, of accurate level-resolved effective electron-impact collision strengths and oscillator strengths for radiative transitions. Here, we discuss the challenges in obtaining a sufficiently good structure for neutral iron and compare our theoretical fine-structure energy levels with observation for several increasingly large models. Radiative data is presented for several transitions for which the atomic data is accurately known.
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Physical Sciences, Atomic & Molecular Physics; DFT; periodic simulations; amorphous minerals; physico-chemical properties; super-cell
Online: 26 June 2018 (13:10:49 CEST)
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Silicates are among the most abundant and important inorganic materials, not only in the Earth’s crust but also in the interstellar medium in the form of micro-/nano-particles or embedded in the matrices of comets, meteorites and other asteroidal bodies. Although the crystalline phases of silicates are indeed present in Nature, amorphous forms are also highly abundant. Here we report a theoretical investigation of the structural, dielectric and vibrational properties of the amorphous bulk for forsterite (Mg2SiO4) as a silicate test case by a combined approach of classical molecular dynamics (MD) simulations for structure evolution, and periodic quantum mechanical DFT calculations for electronic structure analysis. Using classical MD based on an empirical partial charge rigid ionic model within a melt-quenching scheme at different temperatures performed with the GULP 4.0 code amorphous bulk structures for Mg2SiO4 were generated using as initial guess the crystalline phase. This has been done for bulks with three different unit cell sizes adopting a super-cell approach; i.e., 1×1×2, 2×1×2 and 2×2×2. The radial distribution functions indicated a good degree of amorphisation of the structures. Periodic B3LYP-geometry optimizations performed with the CRYSTAL14 code on the generated amorphous systems were used to analyze their structure, to calculate their high frequency dielectric constants (ε∞), and to simulate the IR, Raman and reflectance spectra, which were compared with the experimental and theoretical crystalline Mg2SiO4. The most significant changes of the physico-chemical properties of the amorphous systems compared to the crystalline ones are presented and discussed (e.g., larger deviations in the bond distances and angles, broadening of the IR bands, etc.), which are consistent with their disordered nature. It is also shown that by increasing the unit cell size the bulk structures present larger degree of amorphisation.
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Physical Sciences, Atomic & Molecular Physics; weak measurement; transition probability amplitude; atomic metastable states
Online: 29 April 2018 (10:01:25 CEST)
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A method for measuring the real part of the weak value of spin for non-zero rest mass atoms is presented using a variant on the original Stern-Gerlach apparatus. The experiment utilises helium in the metastable 23S1 state. A full simulation for observing the real part of the weak value using the impulsive approximation has been carried out and it predicts a displacement of the beam, Δw, that is within the resolution of our detector. It also indicates how this shift might be increased. The full analysis also indicated that there is a limit, L, to the applicability of the weak value approximation and has been evaluated for our apparatus. This experiment has the possibility to be expanded to utilise other nobal gas species such as neon and argon in the 3P2 metastable state, but we shall restrict this paper to metastable helium only.
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Physical Sciences, Atomic & Molecular Physics; symmetry; linear molecules; group theory; finite symmetry; point groups
Online: 11 April 2018 (08:07:38 CEST)
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A numerical application of linear-molecule symmetry properties, described by the D∞h point group, is formulated in terms of lower-order symmetry groups Dnh with finite n. Character tables and irreducible representation transformation matrices are presented for Dnh groups with arbitrary n-values. These groups are subsequently used in the construction of symmetry-adapted ro-vibrational basis functions for solving the Schrödinger equations of linear molecules as part of the variational nuclear motion program TROVE. The TROVE symmetrisation procedure is based on a set of "reduced" vibrational eigenvalue problems with simplified Hamiltonians. The solutions of these eigenvalue problems have now been extended to include the classification of basis-set functions using ℓ, the eigenvalue (in units of ℏ) of the vibrational angular momentum operator . This facilitates the symmetry adaptation of the basis set functions in terms of the irreducible representations of Dnh. 12C2H2 is used as an example of a linear molecule of D∞h point group symmetry to illustrate the symmetrisation procedure.
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Physical Sciences, Atomic & Molecular Physics; atomic structure; hartree fock; exchange; line broadening; scattering
Online: 8 March 2018 (02:10:55 CET)
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Atomic structure of N-electron atoms is often determined using the Hartree-Fock method, which is an integro-differential equation. The exchange term of the Hartree-Fock equations is usually treated as an inhomogeneous term of a differential equation, or with a local density approximation. This work uses matrix methods to solve for the Hartree-Fock equations, rather than the more commonly-used shooting method to integrate an inhomogeneous differential equation. It is well known that a derivative operator can be expressed as a matrix made of finite-difference coefficients; energy eigenvalues and eigenvectors can be obtained by using computer linear-algebra packages. We extend the same technique to integro-differential equations, where a discretized integral can be written as a sum in matrix form. This method is compared against experiment and standard atomic structure calculations. We also can use this method for free-electron wavefunctions. This technique is important for spectral line broadening in two ways: improving the atomic structure calculations, and improving the motion of the plasma electrons that collide with the atom.
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Physical Sciences, Atomic & Molecular Physics; Stark broadening; van der Waals broadening; line shapes; helium plasma; corona discharge; plasma diagnostics; code comparison; neutral broadening; pressure broadening
Online: 7 March 2018 (13:36:47 CET)
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Passive plasma spectroscopy is a well-established non-intrusive diagnostic technique. Depending on the emitter and its environment which determine the dominant interactions and effects governing emission line shapes, passive spectroscopy allows the determination of electron densities, emitter and perturber temperatures as well as other quantities like abundances. However, using spectroscopy needs appropriate line shape codes retaining all the physical effects governing the emission line profiles. This requires for line shape code developers to continuously correct or improve them to increase their accuracy when applied for diagnostics. This is exactly the aim expected from code-code and code-data comparisons. In this context, the He I 492 nm emitted in a helium corona discharge at room temperature represents an ideal case since its profile results from several broadening mechanisms: Stark, Doppler, resonance and van der Waals. The importance of each broadening mechanism depends on the plasma parameters. Here the profiles of the He I 492 nm in a helium plasma computed by various codes are compared for a selected set of plasma parameters. In addition, preliminary results related to plasma parameter determination using experimental spectra from a helium corona discharge at low pressure 1- 2 bars, are presented.
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Physical Sciences, Atomic & Molecular Physics; stark broadening; van der waals broadening; line shapes; helium plasma; corona discharge; plasma diagnostics; code comparison; neutral broadening; pressure broadening
Online: 6 March 2018 (03:51:17 CET)
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Many spectroscopic diagnostics are routinely used as a technique to infer the plasma parameters from line emission spectra but their accuracy depends on the numerical model or code used for the fitting process. However, the validation of a line shape code requires some steps : comparison of the line shape code with other similar codes for some academic (simple) cases and then more complex ones, comparison of the fitting parameters obtained from the best fit of the experimental spectra with those obtained with other diagnostic techniques and/or comparison of the fitting parameters obtained by different codes to fit the same experimental data. Here we compare the profiles of the hydrogen Balmer β line in a helium plasma computed by six codes for a selected set of plasma parameters and we report on the plasma parameters inferred by each of them from the fitting to a number of experimental spectra measured in a helium corona discharge where the pressure was in the range 1- 5 bar.
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Physical Sciences, Atomic & Molecular Physics; Plasma spectroscopy; Line broadening; Series limit; Balmer lines; Inglis-Teller limit; line merging.
Online: 1 March 2018 (16:38:57 CET)
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For a given set of plasma parameters, along a single series (Lyman, Balmer etc) the lines with higher
principal quantum number(n) lines get progressively wider, closer to each other,
and start merging for a certain critical n.
In the present work four different codes(with further options) are used to calculate the entire Balmer series for moderate and high electron densities. Particular attention is paid to the relevant physics, such as the cutoff criteria, strong and penetrating electron collisions.
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Physical Sciences, Atomic & Molecular Physics; electron scattering; cross sections; Rosetta mission; atomic and molecular databases
Online: 21 October 2017 (15:30:59 CEST)
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The emission of [O I] lines in the coma of Comet 67P/Churyumov-Gerasimenko during the Rosetta mission have been explained by electron impact dissociation of water rather than the process of photodissociation. This is the direct evidence for the role of electron induced processing has been seen on such a body. Analysis of other emission features is handicapped by a lack of detailed knowledge of electron impact cross sections which highlights the need for a broad range of electron scattering data from the molecular systems detected on the comet. In this paper we present an overview of the needs for electron scattering data relevant for the understanding of observations in coma, the tenuous atmosphere and on the surface of 67P/Churyumov-Gerasimenko during the Rosetta mission. The relevant observations for elucidating the role of electrons come from optical spectra, particle analysis using the ion and electron sensors and mass spectrometry measurements. To model these processes electron impact data should be collated and reviewed in an electron scattering database and an example is given in the BEAMD, which is a part of a larger consortium of Virtual Atomic and Molecular Data Centre – VAMDC.
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Physical Sciences, Atomic & Molecular Physics; Markov chain Monte Carlo; stochastic dynamics integrators; decorrelation time; integrated autocorrelation time
Online: 7 September 2017 (03:43:35 CEST)
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Markov chain Monte Carlo sampling propagators, including numerical integrators for stochastic dynamics, are central to the calculation of thermodynamic quantities and determination of structure for molecular systems. Efficiency is paramount, and to a great extent, this is determined by the integrated autocorrelation time (IAcT). This quantity varies depending on the observable that is being estimated. It is suggested that it is the maximum of the IAcT over all observables that is the relevant metric. Reviewed here is a method for estimating this quantity. For reversible propagators (which are those that satisfy detailed balance), the maximum IAcT is determined by the spectral gap in the forward transfer operator, but for irreversible propagators, the maximum IAcT can be far less than or greater than what might be inferred from the spectral gap. This is consistent with recent theoretical results (not to mention past practical experience) suggesting that irreversible propagators generally perform better if not much better than reversible ones. Typical irreversible propagators involve a parameter controlling the mix of ballistic and diffusive movement. To gain insight into the effect of the damping parameter for Langevin dynamics, its optimal value is obtained here for a multidimensional quadratic potential energy function.