558 research outputs found
Formation of molecular oxygen in ultracold O + OH reaction
We discuss the formation of molecular oxygen in ultracold collisions between
hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based
on hyperspherical coordinates is employed for the calculations. Elastic,
inelastic and reactive cross sections as well as the vibrational and rotational
populations of the product O2 molecules are reported. A J-shifting
approximation is used to compute the rate coefficients. At temperatures T = 10
- 100 mK for which the OH molecules have been cooled and trapped
experimentally, the elastic and reactive rate coefficients are of comparable
magnitude, while at colder temperatures, T < 1 mK, the formation of molecular
oxygen becomes the dominant pathway. The validity of a classical capture model
to describe cold collisions of OH and O is also discussed. While very good
agreement is found between classical and quantum results at T=0.3 K, at higher
temperatures, the quantum calculations predict a larger rate coefficient than
the classical model, in agreement with experimental data for the O + OH
reaction. The zero-temperature limiting value of the rate coefficient is
predicted to be about 6.10^{-12} cm^3 molecule^{-1} s^{-1}, a value comparable
to that of barrierless alkali-metal atom - dimer systems and about a factor of
five larger than that of the tunneling dominated F + H2 reaction.Comment: 9 pages, 8 figure
Dynamics of ultracold dipolar particles in a confined geometry and tilted fields
We develop a collisional formalism adapted for the dynamics of ultracold
dipolar particles in a confined geometry and in fields tilted relative to the
confinement axis. Using tesseral harmonics instead of the usual spherical
harmonics to expand the scattering wavefunction, we recover a good quantum
number which is conserved during the collision. We derive the
general expression of the dipole-dipole interaction in this convenient basis
set as a function of the polar and azimuthal angles of the fields. We apply the
formalism to the collision of fermionic and bosonic polar KRb molecules in a
tilted electric field and in a one-dimensional optical lattice. The presence of
a tilted field drastically changes the magnitude of the reactive and inelastic
rates as well as the inelastic threshold properties at vanishing collision
energies. Setting an appropriate strength of the confinement for the fermionic
system, we show that the ultracold particles can even further reduce their
kinetic energy by inelastic excitation to higher states of the confinement
trap.Comment: 13 page
Electric field suppression of ultracold confined chemical rates
We consider ultracold collisions of polar molecules confined in a one
dimensional optical lattice. Using a quantum scattering formalism and a frame
transformation method, we calculate elastic and chemical quenching rate
constants for fermionic molecules. Taking KRb molecules as a prototype, we find
that the rate of quenching collisions is enhanced at zero electric field as the
confinement is increased, but that this rate is suppressed when the electric
field is turned on. For molecules with 500 nK of collision energy, for
realistic molecular densities, and for achievable experimental electric fields
and trap confinements, we predict lifetimes of KRb molecules of 1 s. We find a
ratio of elastic to quenching collision rates of about 100, which may be
sufficient to achieve efficient experimental evaporative cooling of polar KRb
molecules.Comment: 4 pages, 3 figure
Dynamics of ultracold molecules in confined geometry and electric field
We present a time-independent quantum formalism to describe the dynamics of
molecules with permanent electric dipole moments in a two-dimensional confined
geometry such as a one-dimensional optical lattice, in the presence of an
electric field. Bose/Fermi statistics and selection rules play a crucial role
in the dynamics. As examples, we compare the dynamics of confined fermionic and
bosonic polar KRb molecules under different confinements and electric fields.
We show how chemical reactions can be suppressed, either by a "statistical
suppression" which applies for fermions at small electric fields and
confinements, or by a "potential energy suppression", which applies for both
fermions and bosons at high electric fields and confinements. We also explore
collisions that transfer molecules from one state of the confining potential to
another. Although these collisions can be significant, we show that they do not
play a role in the loss of the total number of molecules in the gas.Comment: 13 pages, 6 figure
Unified model of ultracold molecular collisions
A scattering model is developed for ultracold molecular collisions, which
allows inelastic processes, chemical reactions, and complex formation to be
treated in a unified way. All these scattering processes and various
combinations of them are possible in ultracold molecular gases, and as such
this model will allow the rigorous parametrization of experimental results. In
addition we show how, once extracted, these parameters can be related to the
physical properties of the system, shedding light on fundamental aspects of
molecular collision dynamics.Comment: 16 Pages, 5 Figure
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