145 research outputs found
Long-range interactions in the ozone molecule: spectroscopic and dynamical points of view
Using the multipolar expansion of the electrostatic energy, we have
characterized the asymptotic interactions between an oxygen atom O and
an oxygen molecule O, both in their electronic ground state.
We have calculated the interaction energy induced by the permanent electric
quadrupoles of O and O and the van der Waals energy. On one hand we
determined the 27 electronic potential energy surfaces including spin-orbit
connected to the O + O dissociation limit of the
O--O complex. On the other hand we computed the potential energy curves
characterizing the interaction between O and a O
molecule in its lowest vibrational level and in a low rotational level. Such
curves are found adiabatic to a good approximation, namely they are only weakly
coupled to each other. These results represent a first step for modeling the
spectroscopy of ozone bound levels close to the dissociation limit, as well as
the low energy collisions between O and O thus complementing the knowledge
relevant for the ozone formation mechanism.Comment: Submitted to J. Chem. Phys. after revisio
Formation of ultracold SrYb molecules in an optical lattice by photoassociation spectroscopy: theoretical prospects
State-of-the-art {\em ab initio} techniques have been applied to compute the
potential energy curves for the SrYb molecule in the Born-Oppenheimer
approximation for the ground state and first fifteen excited singlet and
triplet states within the coupled-cluster framework. The leading long-range
coefficients describing the dispersion interactions at large interatomic
distances are also reported. The electric transition dipole moments have been
obtained as the first residue of the polarization propagator computed with the
linear response coupled-cluster method restricted to single and double
excitations. Spin-orbit coupling matrix elements have been evaluated using the
multireference configuration interaction method restricted to single and double
excitations with a large active space. The electronic structure data was
employed to investigate the possibility of forming deeply bound ultracold SrYb
molecules in an optical lattice in a photoassociation experiment using
continuous-wave lasers. Photoassociation near the intercombination line
transition of atomic strontium into the vibrational levels of the strongly
spin-orbit mixed , , , and states with
subsequent efficient stabilization into the vibrational
level of the electronic ground state is proposed. Ground state SrYb molecules
can be accumulated by making use of collisional decay from
to . Alternatively, photoassociation and stabilization to
can proceed via stimulated Raman adiabatic passage
provided that the trapping frequency of the optical lattice is large enough and
phase coherence between the pulses can be maintained over at least tens of
microseconds
Two-photon coherent control of femtosecond photoassociation
Photoassociation with short laser pulses has been proposed as a technique to
create ultracold ground state molecules. A broad-band excitation seems the
natural choice to drive the series of excitation and deexcitation steps
required to form a molecule in its vibronic ground state from two scattering
atoms. First attempts at femtosecond photoassociation were, however, hampered
by the requirement to eliminate the atomic excitation leading to trap
depletion. On the other hand, molecular levels very close to the atomic
transition are to be excited. The broad bandwidth of a femtosecond laser then
appears to be rather an obstacle. To overcome the ostensible conflict of
driving a narrow transition by a broad-band laser, we suggest a two-photon
photoassociation scheme. In the weak-field regime, a spectral phase pattern can
be employed to eliminate the atomic line. When the excitation is carried out by
more than one photon, different pathways in the field can be interfered
constructively or destructively. In the strong-field regime, a temporal phase
can be applied to control dynamic Stark shifts. The atomic transition is
suppressed by choosing a phase which keeps the levels out of resonance. We
derive analytical solutions for atomic two-photon dark states in both the
weak-field and strong-field regime. Two-photon excitation may thus pave the way
toward coherent control of photoassociation. Ultimately, the success of such a
scheme will depend on the details of the excited electronic states and
transition dipole moments. We explore the possibility of two-photon femtosecond
photoassociation for alkali and alkaline-earth metal dimers and present a
detailed study for the example of calcium
Ab initio calculation of the KRb dipole moments
The relativistic configuration interaction valence bond method has been used
to calculate permanent and transition electric dipole moments of the KRb
heteronuclear molecule as a function of internuclear separation. The permanent
dipole moment of the ground state potential is found to be
0.30(2) at the equilibrium internuclear separation with excess negative
charge on the potassium atom. For the potential the dipole moment
is an order of magnitude smaller (1 Cm) In addition, we
calculate transition dipole moments between the two ground-state and
excited-state potentials that dissociate to the K(4s)+Rb(5p) limits. Using this
data we propose a way to produce singlet KRb molecules by a
two-photon Raman process starting from an ultracold mixture of doubly
spin-polarized ground state K and Rb atoms. This Raman process is only allowed
due to relativistic spin-orbit couplings and the absence of gerade/ungerade
selection rules in heteronuclear dimers.Comment: 16 pages, 7 figure
Dispersion coefficients of the excited states of lithium atoms
The dispersion coefficients of a number of the low-lying states of Li are determined for the homonuclear case. The Li wave functions and energies were computed in a frozen core Hamiltonian with a semiempirical polarization potential. Besides computing the dispersion coefficients, the scalar and tensor polarizabilities and oscillator strengths are computed and generally seen to be in good agreement with other accurate calculations
Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms
We present the design, implementation and characterization of a dual-species
magneto-optical trap (MOT) for fermionic 6Li and 40K atoms with large atom
numbers. The MOT simultaneously contains 5.2x10^9 6Li-atoms and 8.0x10^9
40K-atoms, which are continuously loaded by a Zeeman slower for 6Li and a
2D-MOT for 40K. The atom sources induce capture rates of 1.2x10^9 6Li-atoms/s
and 1.4x10^9 40K-atoms/s. Trap losses due to light-induced interspecies
collisions of ~65% were observed and could be minimized to ~10% by using low
magnetic field gradients and low light powers in the repumping light of both
atomic species. The described system represents the starting point for the
production of a large-atom number quantum degenerate Fermi-Fermi mixture
Ultracold collisions of oxygen molecules
Collision cross sections and rate constants between two ground- state oxygen
molecules are investigated theoretically at translational energies below K and in zero magnetic field. We present calculations for elastic and spin-
changing inelastic collision rates for different isotopic combinations of
oxygen atoms as a prelude to understanding their collisional stability in
ultracold magnetic traps. A numerical analysis has been made in the framework
of a rigid- rotor model that accounts fully for the singlet, triplet, and
quintet potential energy surfaces in this system. The results offer insights
into the effectiveness of evaporative cooling and the properties of molecular
Bose- Einstein condensates, as well as estimates of collisional lifetimes in
magnetic traps. Specifically, looks like a good candidate for
ultracold studies, while is unlikely to survive evaporative
cooling. Since is representative of a wide class of molecules that
are paramagnetic in their ground state we conclude that many molecules can be
successfully magnetically trapped at ultralow temperatures.Comment: 15 pages, 9 figure
Limit on suppression of ionization in metastable neon traps due to long-range anisotropy
This paper investigates the possibility of suppressing the ionization rate in
a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms.
Suppression of the ionization is critical for the possibility of reaching
Bose-Einstein condensation with such atoms. We estimate the relevant long-range
interactions for the system, consisting of electric quadrupole-quadrupole and
dipole-induced dipole terms, and develop short-range potentials based on the
Na_2 singlet and triplet potentials. The auto-ionization widths of the system
are also calculated. With these ingredients we calculate the ionization rate
for spin-polarized and for spin-isotropic samples, caused by anisotropy of the
long-range interactions. We find that spin-polarization may allow for four
orders of magnitude suppression of the ionization rate for Ne. The results
depend sensitively on a precise knowledge of the interaction potentials,
however, pointing out the need for experimental input. The same model gives a
suppression ratio close to unity for metastable xenon in accordance with
experimental results, due to a much increased anisotropy in this case.Comment: 15 pages including figures, LaTex/RevTex, uses epsfig.st
Theoretical study of the absorption spectra of the lithium dimer
For the lithium dimer we calculate cross sections for absorption of radiation
from the vibrational-rotational levels of the ground X [singlet Sigma g +]
electronic state to the vibrational levels and continua of the excited A
[singlet Sigma u +] and B [singlet Pi u] electronic states. Theoretical and
experimental data are used to characterize the molecular properties taking
advantage of knowledge recently obtained from photoassociation spectroscopy and
ultra-cold atom collision studies. The quantum-mechanical calculations are
carried out for temperatures in the range from 1000 to 2000 K and are compared
with previous calculations and measurements.Comment: 20 pages, revtex, epsf, 6 fig
Electronic spectral shift of oxygen-filled (6,6) carbon nanotubes
Electronic state modulation of the armchair (6,6) carbon nanotubes filled
with a linear assembly of oxygen molecules is addressed theoretically.
Ferromagnetic coupling of encapsulated oxygen produces a magnetic field with
cylindrical symmetry, which deviates the electron's eigenenergies from those
prior to the oxygen absorption. An intriguing spectral gap arises near the
Fermi energy, at which the gap formation is allowed only when the tube length
equals to a multiple of three in units of carbon hexagon. A possible means to
detect the selective gap formation is discussed.Comment: 4 pages, 5 figure
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