36 research outputs found
Molecules Near Absolute Zero and External Field Control of Atomic and Molecular Dynamics
This article reviews the current state of the art in the field of cold and
ultracold molecules and demonstrates that chemical reactions, inelastic
collisions and dissociation of molecules at subKelvin temperatures can be
manipulated with external electric or magnetic fields. The creation of
ultracold molecules may allow for spectroscopy measurements with extremely high
precision and tests of fundamental symmetries of nature, quantum computation
with molecules as qubits, and controlled chemistry. The probability of chemical
reactions and collisional energy transfer can be very large at temperatures
near zero Kelvin. The collision energy of ultracold atoms and molecules is much
smaller than perturbations due to interactions with external electric or
magnetic fields available in the laboratory. External fields may therefore be
used to induce dissociation of weakly bound molecules, stimulate forbidden
electronic transitions, suppress the effect of centrifugal barriers in outgoing
reaction channels or tune Feshbach resonances that enhance chemical reactivity
Rotational predissociation of extremely weakly bound atom-molecule complexes produced by Feshbach resonance association
We study the rotational predissociation of atom - molecule complexes with very small binding energy. Such complexes can be produced by Feshbach resonance association of ultracold molecules with ultracold atoms. Numerical calculations of the predissociation lifetimes based on the computation of the energy dependence of the scattering matrix elements become inaccurate when the binding energy is smaller than the energy width of the predissociating state. We derive expressions that represent accurately the predissociation lifetimes in terms of the real and imaginary parts of the scattering length and effective range for molecules in an excited rotational state. Our results show that the predissociation lifetimes are the longest when the binding energy is positive, i.e. when the predissociating state is just above the excited state threshold
Magnetic Feshbach resonances in collisions of non-magnetic closed-shell molecules
Magnetic Feshbach resonances play a central role in experimental research of atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali-metal molecules in the ground rovibrational state of the ground electronic state. Unlike alkali metal atoms, diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for molecules may exceed 1 milliGauss, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances
Laser cooling of a diatomic molecule
It has been roughly three decades since laser cooling techniques produced
ultracold atoms, leading to rapid advances in a vast array of fields.
Unfortunately laser cooling has not yet been extended to molecules because of
their complex internal structure. However, this complexity makes molecules
potentially useful for many applications. For example, heteronuclear molecules
possess permanent electric dipole moments which lead to long-range, tunable,
anisotropic dipole-dipole interactions. The combination of the dipole-dipole
interaction and the precise control over molecular degrees of freedom possible
at ultracold temperatures make ultracold molecules attractive candidates for
use in quantum simulation of condensed matter systems and quantum computation.
Also ultracold molecules may provide unique opportunities for studying chemical
dynamics and for tests of fundamental symmetries. Here we experimentally
demonstrate laser cooling of the molecule strontium monofluoride (SrF). Using
an optical cycling scheme requiring only three lasers, we have observed both
Sisyphus and Doppler cooling forces which have substantially reduced the
transverse temperature of a SrF molecular beam. Currently the only technique
for producing ultracold molecules is by binding together ultracold alkali atoms
through Feshbach resonance or photoassociation. By contrast, different proposed
applications for ultracold molecules require a variety of molecular
energy-level structures. Our method provides a new route to ultracold
temperatures for molecules. In particular it bridges the gap between ultracold
temperatures and the ~1 K temperatures attainable with directly cooled
molecules (e.g. cryogenic buffer gas cooling or decelerated supersonic beams).
Ultimately our technique should enable the production of large samples of
molecules at ultracold temperatures for species that are chemically distinct
from bialkalis.Comment: 10 pages, 7 figure
Dipolar collisions of polar molecules in the quantum regime
Ultracold polar molecules offer the possibility of exploring quantum gases
with interparticle interactions that are strong, long-range, and spatially
anisotropic. This is in stark contrast to the dilute gases of ultracold atoms,
which have isotropic and extremely short-range, or "contact", interactions. The
large electric dipole moment of polar molecules can be tuned with an external
electric field; this provides unique opportunities such as control of ultracold
chemical reactions, quantum information processing, and the realization of
novel quantum many-body systems. In spite of intense experimental efforts aimed
at observing the influence of dipoles on ultracold molecules, only recently
have sufficiently high densities been achieved. Here, we report the observation
of dipolar collisions in an ultracold molecular gas prepared close to quantum
degeneracy. For modest values of an applied electric field, we observe a
dramatic increase in the loss rate of fermionic KRb molecules due to ultrcold
chemical reactions. We find that the loss rate has a steep power-law dependence
on the induced electric dipole moment, and we show that this dependence can be
understood with a relatively simple model based on quantum threshold laws for
scattering of fermionic polar molecules. We directly observe the spatial
anisotropy of the dipolar interaction as manifested in measurements of the
thermodynamics of the dipolar gas. These results demonstrate how the long-range
dipolar interaction can be used for electric-field control of chemical reaction
rates in an ultracold polar molecule gas. The large loss rates in an applied
electric field suggest that creating a long-lived ensemble of ultracold polar
molecules may require confinement in a two-dimensional trap geometry to
suppress the influence of the attractive dipolar interactions
Sisyphus Cooling of Electrically Trapped Polyatomic Molecules
The rich internal structure and long-range dipole-dipole interactions
establish polar molecules as unique instruments for quantum-controlled
applications and fundamental investigations. Their potential fully unfolds at
ultracold temperatures, where a plethora of effects is predicted in many-body
physics, quantum information science, ultracold chemistry, and physics beyond
the standard model. These objectives have inspired the development of a wide
range of methods to produce cold molecular ensembles. However, cooling
polyatomic molecules to ultracold temperatures has until now seemed
intractable. Here we report on the experimental realization of opto-electrical
cooling, a paradigm-changing cooling and accumulation method for polar
molecules. Its key attribute is the removal of a large fraction of a molecule's
kinetic energy in each step of the cooling cycle via a Sisyphus effect,
allowing cooling with only few dissipative decay processes. We demonstrate its
potential by reducing the temperature of about 10^6 trapped CH_3F molecules by
a factor of 13.5, with the phase-space density increased by a factor of 29 or a
factor of 70 discounting trap losses. In contrast to other cooling mechanisms,
our scheme proceeds in a trap, cools in all three dimensions, and works for a
large variety of polar molecules. With no fundamental temperature limit
anticipated down to the photon-recoil temperature in the nanokelvin range, our
method eliminates the primary hurdle in producing ultracold polyatomic
molecules. The low temperatures, large molecule numbers and long trapping times
up to 27 s will allow an interaction-dominated regime to be attained, enabling
collision studies and investigation of evaporative cooling toward a BEC of
polyatomic molecules
Controlling the quantum stereodynamics of ultracold bimolecular reactions
Chemical reaction rates often depend strongly on stereodynamics, namely the
orientation and movement of molecules in three-dimensional space. An ultracold
molecular gas, with a temperature below 1 uK, provides a highly unusual regime
for chemistry, where polar molecules can easily be oriented using an external
electric field and where, moreover, the motion of two colliding molecules is
strictly quantized. Recently, atom-exchange reactions were observed in a
trapped ultracold gas of KRb molecules. In an external electric field, these
exothermic and barrierless bimolecular reactions, KRb+KRb -> K2+Rb2, occur at a
rate that rises steeply with increasing dipole moment. Here we show that the
quantum stereodynamics of the ultracold collisions can be exploited to suppress
the bimolecular chemical reaction rate by nearly two orders of magnitude. We
use an optical lattice trap to confine the fermionic polar molecules in a
quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along
the tight confinement direction. With the combination of sufficiently tight
confinement and Fermi statistics of the molecules, two polar molecules can
approach each other only in a "side-by-side" collision, where the chemical
reaction rate is suppressed by the repulsive dipole-dipole interaction. We show
that the suppression of the bimolecular reaction rate requires quantum-state
control of both the internal and external degrees of freedom of the molecules.
The suppression of chemical reactions for polar molecules in a
quasi-two-dimensional trap opens the way for investigation of a dipolar
molecular quantum gas. Because of the strong, long-range character of the
dipole-dipole interactions, such a gas brings fundamentally new abilities to
quantum-gas-based studies of strongly correlated many-body physics, where
quantum phase transitions and new states of matter can emerge.Comment: 19 pages, 4 figure
Molecules cooled below the Doppler limit
The ability to cool atoms below the Doppler limit -- the minimum temperature reachable by Doppler cooling -- has been essential to most experiments with quantum degenerate gases, optical lattices and atomic fountains, among many other applications. A broad set of new applications await ultracold molecules, and the extension of laser cooling to molecules has begun. A molecular magneto-optical trap has been demonstrated, where molecules approached the Doppler limit. However, the sub-Doppler temperatures required for most applications have not yet been reached. Here we cool molecules to 50 uK, well below the Doppler limit, using a three-dimensional optical molasses. These ultracold molecules could be loaded into optical tweezers to trap arbitrary arrays for quantum simulation, launched into a molecular fountain for testing fundamental physics, and used to study ultracold collisions and ultracold chemistry
Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas
The Boltzmann averaged rate constants for total vibrational relaxation of HF(v = 1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v = 1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v = 0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v = 0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v = 0,jres) may in fact be a rate-limiting stage of vibrational relaxation
Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas
The Boltzmann averaged rate constants for total vibrational relaxation of HF(v = 1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v = 1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v = 0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v = 0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v = 0,jres) may in fact be a rate-limiting stage of vibrational relaxation