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Search Results: 1 - 10 of 340564 matches for " H. R. Sadeghpour "
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Hyperfine-changing transitions in $^3$He II and other one-electron ions by electron scattering
Klaus Bartschat,H. R. Sadeghpour
Physics , 2014, DOI: 10.1088/0004-637X/788/1/69
Abstract: We consider the spin-exchange (SE) cross section in electron scattering from $^3$He\,{\scriptsize II}, which drives the hyperfine-changing \hbox{3.46 cm} (8.665 GHz) line transition. Both the analytical quantum defect method --- applicable at very low energies --- and accurate R-matrix techniques for electron-He$^+$ scattering are employed to obtain SE cross sections. The quantum defect theory is also applied to electron collisions with other one-electron ions in order to demonstrate the utility of the method and derive scaling relations. At very low energies, the hyperfine-changing cross sections due to e$-$He$^+$ scattering are much larger in magnitude than for electron collisions with neutral hydrogen, hinting at large rate constants for equilibration. Specifically, we obtain rate coefficients of $K(10\,{\rm K}) = 1.10 \times 10^{-6}\,\rm cm^3/s$ and $K(100\,{\rm K}) = 3.49\times 10^{-7}\,\rm cm^3/s$.
Angular momentum changing transitions in proton-Rydberg atom collisions
D. Vrinceanu,R. Onofrio,H. R. Sadeghpour
Physics , 2012, DOI: 10.1088/0004-637X/747/1/56
Abstract: Collisions between electrically charged particles and neutral atoms are central for understanding the dynamics of neutral gases and plasmas in a variety of physical situaziones of terrestrial and astronomical interest. Specifically, redistribution of angular momentum states within the degenerate shell of highly excited Rydberg atoms occurs efficiently in distant collisions with ions. This process is crucial in establishing the validity of the local thermal equilibrium assumption and may also play a role in determining a precise ionization fraction in primordial recombination. We provide an accurate expression for the non-perturbative rate coefficient of collsions between protons and H(n_l) ending in a final state H(n_l'), with n being the principal quantum number and l,l' the initial and final angular momentum quantum numbers, respectively. The validity of this result is confirmed by results of classical trajectory Monte Carlo simulations. Previous results, obtained by Pengelly and Seaton only for dipole-allowed transitions, l--->l+-1, overestimate the l-changing collisional rate approximately by a factor of six, and the physical origin of this overestimation is discussed.
Ultracold giant polyatomic Rydberg molecules: coherent control of molecular orientation
Seth T. Rittenhouse,H. R. Sadeghpour
Physics , 2010, DOI: 10.1103/PhysRevLett.104.243002
Abstract: We predict the existence of a class of ultracold giant molecules formed from trapped ultracold Rydberg atoms and polar molecules. The interaction which leads to the formation of such molecules is the anisotropic charge-dipole interaction ($a/R^2$). We show that prominent candidate molecules such as KRb and deuterated hydroxyl (OD) should bind to Rydberg rubidium atoms, with energies $E_b\simeq 5-25$ GHz at distances $R\simeq 0.1-1 \ \mu$m. These molecules form in double wells, mimicking chiral molecules, with each well containing a particular dipole orientation. We prepare a set of correlated dressed electron-dipole eigenstates which are used in a resonant Raman scheme to coherently control the dipole orientation and to create cat-like entangled states of the polar molecule.
Controlling two-species Mott-insulator phses in an optical lattice to form an array of dipolar molecules
M. G. Moore,H. R. Sadeghpour
Physics , 2002, DOI: 10.1103/PhysRevA.67.041603
Abstract: We consider the transfer of a two-species Bose-Einstein condensate into an optical lattice with a density such that that a Mott-insulator state with one atom per species per lattice site is obtained in the deep lattice regime. Depending on collision parameters the result could be either a `mixed' or a `separated' Mott-insulator phase. Such a `mixed' two-species insulator could then be photo-associated into an array of dipolar molecules suitable for quantum computation or the formation of a dipolar molecular condensate. For the case of a $^{87}$Rb-$^{41}$K two-species BEC, however, the large inter-species scattering length makes obtaining the desired `mixed' Mott insulator phase difficult. To overcome this difficulty we investigate the effect of varying the lattice frequency on the mean-field interaction and find a favorable parameter regime under which a lattice of dipolar molecules could be generated.
Nucleation and stabilization of carbon-rich structures in interstellar media
N. Patra,P. Kral,H. R. Sadeghpour
Physics , 2014, DOI: 10.1088/0004-637X/785/1/6
Abstract: We study conditions under which carbon clusters of different sizes form and stabilize. {We describe an approach to equilibrium by simulating tenuous carbon gas dynamics to long times.} First, we use reactive molecular dynamics simulations to describe the nucleation of long chains, large clusters, and complex cage structures in carbon and hydrogen rich interstellar gas phases. We study how temperature, particle density, presence of hydrogen, and carbon inflow affect the nucleation of molecular moieties with different characteristics, in accordance with astrophysical conditions. We extend the simulations to densities which are orders of magnitude lower than current laboratory densities, to temperatures relevant to circumstellar environments of planetary nebulae, and to longtime (microsecond) formation timescales. We correlate cluster size distributions from dynamical simulations with thermodynamic equilibrium intuitions, where at low temperatures and gas densities, entropy plays a significant role.
Cold and Ultracold Rydberg Atoms in Strong Magnetic Fields
T. Pohl,H. R. Sadeghpour,P. Schmelcher
Physics , 2009, DOI: 10.1016/j.physrep.2009.10.001
Abstract: Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms such as giant dipole states are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for trapping and cooling of such atoms are investigated.
Quantum synchronization of quantum van der Pol oscillators with trapped ions
Tony E. Lee,H. R. Sadeghpour
Physics , 2013, DOI: 10.1103/PhysRevLett.111.234101
Abstract: Van der Pol oscillators are prototypical self-sustaining oscillators which have been used to model nonlinear processes in biological and other classical processes. In this work, we investigate how quantum fluctuations affect phase-locking in one or many van der Pol oscillators. We find that phase-locking is much more robust in the quantum model than in the equivalent classical model. Trapped-ion experiments are ideally suited to simulate van der Pol oscillators in the quantum regime via sideband heating and cooling of motional modes. We provide realistic experimental parameters for ${}^{171}\text{Yb}^+$ achievable with current technology.
Potential energy curves for the interaction of Ag(5s) and Ag(5p) with noble gas atoms
J. Loreau,H. R. Sadeghpour,A. Dalgarno
Physics , 2013, DOI: 10.1063/1.4790586
Abstract: We investigate the interaction of ground and excited states of a silver atom with noble gases (NG), including helium. Born-Oppenheimer potential energy curves are calculated with quantum chemistry methods and spin-orbit effects in the excited states are included by assuming a spin-orbit splitting independent of the internuclear distance. We compare our results with experimentally available spectroscopic data, as well as with previous calculations. Because of strong spin-orbit interactions, excited Ag-NG potential energy curves cannot be fitted to Morse-like potentials. We find that the labeling of the observed vibrational levels has to be shifted by one unit.
A mesoscopic Rydberg impurity in an atomic quantum gas
Richard Schmidt,H. R. Sadeghpour,E. Demler
Physics , 2015,
Abstract: Giant impurity excitations with large binding energies are powerful probes for exploring new regimes of far out of equilibrium dynamics in few- and many-body quantum systems, as well as for in-situ observations of correlations. Motivated by recent experimental progress in spectroscopic studies of Rydberg excitations in ensembles of ultracold atoms, we develop a new theoretical approach for describing multiscale dynamics of Rydberg excitations in quantum Bose gases. We find that the crossover from few- to many-body dynamics manifests in a dramatic change in spectral profile from resolved molecular lines to broad Gaussian distributions representing a superpolaronic state in which many atoms bind to the Rydberg impurity. We discuss signatures of this crossover in the temperature and density dependence of the spectra.
Simulating the Formation of Carbon-rich Molecules on an idealised Graphitic Surface
David W. Marshall,H. R. Sadeghpour
Physics , 2015,
Abstract: There is accumulating evidence for the presence of complex molecules, including carbon-bearing and organic molecules, in the interstellar medium. Much of this evidence comes to us from studies of chemical composition, photo- and mass-spectroscopy in cometary, meteoritic and asteroid samples, indicating a need to better understand the surface chemistry of astrophysical objects. There is also considerable interest in the origins of life-forming and life-sustaining molecules on Earth. Here, we perform reactive molecular dynamics simulations to probe the formation of carbon-rich molecules and clusters on carbonaceous surfaces resembling dust grains and meteoroids. Our results show that large chains form on graphitic surfaces at low temperatures (100K - 500K) and smaller fullerene-like molecules form at higher temperatures (2000K - 3000K). The formation is faster on the surface than in the gas at low temperatures but slower at high temperatures as surface interactions prevent small clusters from coagulation. We find that for efficient formation of molecular complexity, mobility about the surface is important and helps to build larger carbon chains on the surface than in the gas phase at low temperatures. Finally, we show that the temperature of the surface strongly determines what kind of structures forms and that low turbulent environments are needed for efficient formation.
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