949 research outputs found
Long-range interactions between an atom in its ground S state and an open-shell linear molecule
Theory of long-range interactions between an atom in its ground S state and a
linear molecule in a degenerate state with a non-zero projection of the
electronic orbital angular momentum is presented. It is shown how the
long-range coefficients can be related to the first and second-order molecular
properties. The expressions for the long-range coefficients are written in
terms of all components of the static and dynamic multipole polarizability
tensor, including the nonadiagonal terms connecting states with the opposite
projection of the electronic orbital angular momentum. It is also shown that
for the interactions of molecules in excited states that are connected to the
ground state by multipolar transition moments additional terms in the
long-range induction energy appear. All these theoretical developments are
illustrated with the numerical results for systems of interest for the
sympathetic cooling experiments: interactions of the ground state Rb(S)
atom with CO(), OH(), NH(), and CH() and of the
ground state Li(S) atom with CH().Comment: 30 pages, 3 figure
ReWE: Regressing Word Embeddings for Regularization of Neural Machine Translation Systems
Regularization of neural machine translation is still a significant problem,
especially in low-resource settings. To mollify this problem, we propose
regressing word embeddings (ReWE) as a new regularization technique in a system
that is jointly trained to predict the next word in the translation
(categorical value) and its word embedding (continuous value). Such a joint
training allows the proposed system to learn the distributional properties
represented by the word embeddings, empirically improving the generalization to
unseen sentences. Experiments over three translation datasets have showed a
consistent improvement over a strong baseline, ranging between 0.91 and 2.54
BLEU points, and also a marked improvement over a state-of-the-art system.Comment: Accepted at NAACL-HLT 201
Laser Induced Selective Alignment of Water Spin Isomers
We consider laser alignment of ortho and para spin isomers of water molecules
by using strong and short off-resonance laser pulses. A single pulse is found
to create a distinct transient alignment and antialignment of the isomeric
species. We suggest selective alignment of one isomeric species (leaving the
other species randomly aligned) by a pair of two laser pulses.Comment: 6 pages, 4 figures, 3 table
Global visualization and quantification of compressible vortex loops
The physics of compressible vortex loops generated due to the rolling up of the shear layer upon the diffraction of a shock wave from a shock tube is far from being understood, especially when shock-vortex interactions are involved. This is mainly due to the lack of global quantitative data available which characterizes the flow. The present study involves the usage of the PIV technique to characterize the velocity and vorticity of compressible vortex loops formed at incident shock Mach numbers ofM=1.54 and1.66. Another perk of the PIV technique over purely qualitative methods, which has been demonstrated in the current study, is that at the same time the results also provide a clear image of the various flow features. Techniques such as schlieren and shadowgraph rely on density gradients present in the flow and fail to capture regions of the flow influenced by the primary flow structure which would have relatively lower pressure and density. Various vortex loops, namely, square, elliptic and circular, were generated using different shape adaptors fitted to the end of the shock tube. The formation of a coaxial vortex loop with opposite circulation along with the generation of a third stronger vortex loop ahead of the primary with same circulation direction are of the interesting findings of the current study
Classification of quantum relativistic orientable objects
Started from our work "Fields on the Poincare Group and Quantum Description
of Orientable Objects" (EPJC,2009), we consider here a classification of
orientable relativistic quantum objects in 3+1 dimensions. In such a
classification, one uses a maximal set of 10 commuting operators (generators of
left and right transformations) in the space of functions on the Poincare
group. In addition to usual 6 quantum numbers related to external symmetries
(given by left generators), there appear additional quantum numbers related to
internal symmetries (given by right generators). We believe that the proposed
approach can be useful for description of elementary spinning particles
considering as orientable objects. In particular, their classification in the
framework of the approach under consideration reproduces the usual
classification but is more comprehensive. This allows one to give a
group-theoretical interpretation to some facts of the existing phenomenological
classification of known spinning particles.Comment: 24 page
Doppler-free ion imaging of hydrogen molecules produced in bimolecular reactions
Abstract We report on the use of two-color Doppler-free [(1 + 1 0 ) + 1/1 0 ] resonance enhanced multiphoton ionization (REMPI) for threedimensional imaging of state-selected product molecules from bimolecular reactions. We demonstrate the viability of this method by measuring differential cross sections for the reaction H + D 2 ! D + HD(v 0 = 1,j 0 = 1,5,8) at 1.7 eV collision energy. We achieve higher resolution allowing us to observe oscillations that were not resolved by previous experiments; these oscillations agree closely with quantum mechanical calculations
Optical angular momentum: Multipole transitions and photonics
The premise that multipolar decay should produce photons uniquely imprinted with a measurably corresponding angular momentum is shown in general to be untrue. To assume a one-to-one correlation between the transition multipoles involved in source decay and detector excitation is to impose a generally unsupportable one-to-one correlation between the multipolar form of emission transition and a multipolar character for the detected field. It is specifically proven impossible to determine without ambiguity, by use of any conventional detector, and for any photon emitted through the nondipolar decay of an atomic excited state, a unique multipolar character for the transition associated with its generation. Consistent with the angular quantum uncertainty principle, removal of a detector from the immediate vicinity of the source produces a decreasing angular uncertainty in photon propagation direction, reflected in an increasing range of integer values for the measured angular momentum. In such a context it follows that when the decay of an electronic excited state occurs by an electric quadrupolar transition, for example, any assumption that the radiation so produced is conveyed in the form of âquadrupole photonsâ is experimentally unverifiable. The results of the general proof based on irreducible tensor analysis invite experimental verification, and they signify certain limitations on quantum optical data transmission
Sub-Doppler spectroscopy of Rb atoms in a sub-micron vapor cell in the presence of a magnetic field
We report the first use of an extremely thin vapor cell (thickness ~ 400 nm)
to study the magnetic-field dependence of laser-induced-fluorescence excitation
spectra of alkali atoms. This thin cell allows for sub-Doppler resolution
without the complexity of atomic beam or laser cooling techniques. This
technique is used to study the laser-induced-fluorescence excitation spectra of
Rb in a 50 G magnetic field. At this field strength the electronic angular
momentum J and nuclear angular momentum I are only partially decoupled. As a
result of the mixing of wavefunctions of different hyperfine states, we observe
a nonlinear Zeeman effect for each sublevel, a substantial modification of the
transition probabilities between different magnetic sublevels, and the
appearance of transitions that are strictly forbidden in the absence of the
magnetic field. For the case of right- and left- handed circularly polarized
laser excitation, the fluorescence spectra differs qualitatively. Well
pronounced magnetic field induced circular dichroism is observed. These
observations are explained with a standard approach that describes the partial
decoupling of I and J states
Theory of x-ray absorption by laser-aligned symmetric-top molecules
We devise a theory of x-ray absorption by symmetric-top molecules which are
aligned by an intense optical laser. Initially, the density matrix of the
system is composed of the electronic ground state of the molecules and a
thermal ensemble of rigid-rotor eigenstates. We formulate equations of motion
of the two-color (laser plus x rays) rotational-electronic problem. The
interaction with the laser is assumed to be nonresonant; it is described by an
electric dipole polarizability tensor. X-ray absorption is approximated as a
one-photon process. It is shown that the equations can be separated such that
the interaction with the laser can be treated independently of the x rays. The
laser-only density matrix is propagated numerically. After each time step, the
x-ray absorption is calculated. We apply our theory to study adiabatic
alignment of bromine molecules (Br2). The required dynamic polarizabilities are
determined using the ab initio linear response methods coupled-cluster singles
(CCS), second-order approximate coupled-cluster singles and doubles (CC2), and
coupled-cluster singles and doubles (CCSD). For the description of x-ray
absorption on the sigma_g 1s --> sigma_u 4p resonance, a parameter-free
two-level model is used for the electronic structure of the molecules. Our
theory opens up novel perspectives for the quantum control of x-ray radiation.Comment: 14 pages, 4 figures, 1 table, RevTeX4, revise
Scattering polarization of hydrogen lines from electric-induced atomic alignment
We consider a gas of hydrogen atoms illuminated by a broadband, unpolarized
radiation with zero anisotropy. In the absence of external fields, the atomic
J-levels are thus isotropically populated. While this condition persists in the
presence of a magnetic field, we show instead that electric fields can induce
the alignment of those levels. We also show that this electric alignment cannot
occur in a two-term model of hydrogen (e.g., if only the Ly-alpha transition is
excited), or if the level populations are distributed according to Boltzmann's
law.Comment: 10 pages, 4 figures. Accepted by J.Phys.B: At.Mol.Opt.Phy
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