arXiv subjects

Jeremy M. Hutson

Publications and source records attributed to Jeremy M. Hutson.

At least 127 records · Page 7Linked to original sources

arXiv2009

Hyperfine structure in the microwave spectra of ultracold polar molecules

We investigate the microwave spectra of ultracold alkali metal dimers in magnetic, electric and combined fields, taking account of the hyperfine structure due to the nuclear spins. We consider the molecules 41K87Rb and 7Li133Cs, which are the targets of current experiments and demonstrate two extremes of large and small nuclear quadrupole coupling. We calculate the frequencies and intensities of transitions that may be used to transfer ultracold molecules between hyperfine states in a magnetic field, employing different polarizations of microwave radiation. In an electric field, the hyperfine levels display narrow avoided crossings at specific fields that we explain in terms of molecular alignment. The hyperfine splittings that arise in electric fields may hinder individual addressing in schemes to use ultracold molecules in quantum computation, but the structure of the spectra is suppressed in combined fields.

physics.chem-ph↗

arXiv2009

Manipulating ultracold polar molecules with microwave radiation: the influence of hyperfine structure

We calculate the microwave spectra of ultracold KRb alkali metal dimers, including hyperfine interactions and in the presence of electric and magnetic fields. We show that microwave transitions may be used to transfer molecules between different hyperfine states, but only because of the presence of nuclear quadrupole interactions. Hyperfine splittings may also complicate the use of ultracold molecules for quantum computing. The spectrum of molecules oriented in electric fields may be simplified dramatically by applying a simultaneous magnetic field.

physics.chem-ph↗

arXiv2009

Dramatic reductions in inelastic cross sections for ultracold collisions near Feshbach resonances

We show that low-energy inelastic cross sections can decrease as well as increase in the vicinity of a zero-energy Feshbach resonance. When an external field is used to tune across such a resonance, the real and imaginary parts of the scattering length show asymmetric oscillations, with both peaks and troughs. In favorable circumstances the inelastic collision rate can be reduced almost to zero. This may be important for efforts to achieve evaporative and sympathetic cooling for molecules.

physics.atom-ph↗

arXiv2009

Conical intersections in laboratory coordinates with ultracold molecules

For two states of opposite parity that cross as a function of an external magnetic field, the addition of an electric field will break the symmetry and induce an avoided crossing. A suitable arrangement of fields may be used to create a conical intersection as a function of external spatial coordinates. We consider the effect of the resulting geometric phase for ultracold polar molecules. For a Bose-Einstein condensate in the mean-field approximation, the geometric phase effect induces stable states of persistent superfluid flow that are characterized by half-integer quantized angular momentum.

cond-mat.quant-gas↗

arXiv2009

Production of ultracold NH molecules by sympathetic cooling with Mg

We carry out calculations on $M$-changing collisions of NH ($^3Σ^-$) molecules in magnetically trappable states using a recently calculated potential energy surface. We show that elastic collision rates are much faster than inelastic rates for a wide range of fields at temperatures up to 10 mK and that the ratio increases for lower temperatures and magnetic fields. If NH molecules can be cooled to temperatures approaching 10 mK and brought into contact with laser-cooled Mg then there is a good prospect that sympathetic cooling can be achieved.

physics.chem-ph↗

arXiv2009

Low-energy collisions of NH3 and ND3 with ultracold Rb atoms

We carry out quantum inelastic scattering calculations of collisions of Rb atoms with inverting NH3 and ND3 molecules in the energy range between 0 and 100 cm-1, which are important for experiments using velocity-controlled molecular beams to probe scattering resonances. We focus on molecules initially in the upper level of the ammonia inversion doublet, which are low-field-seeking and can be controlled in a Stark decelerator. We calculate the integral elastic and state-to-state inelastic cross sections in the coupled states approximation. We demonstrate the presence of both shape and Feshbach resonances in the elastic and inelastic cross sections at low collision energies and discuss their origin in terms of the bound states of Rb--ND3 complex. We also consider elastic and inelastic cross sections in the ultracold regime, using close-coupling calculations, in order to assess the viability of sympathetic cooling of ND3 by Rb. The inelastic cross section for relaxation to the lower level of the inversion doublet is smaller than expected for such a strongly coupled system, but is still likely to be too large to allow sympathetic cooling for ND3 in low-field-seeking states. However, there is a good prospect that sympathetic cooling will be possible for molecules in high-field-seeking states, even when the collision partner is a magnetically trapped atom in a low-field-seeking state.

physics.chem-ph↗

arXiv2009

Stimulating the production of deeply bound RbCs molecules with laser pulses: the role of spin-orbit coupling in forming ultracold molecules

We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb_2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85Rb and 133Cs atoms colliding on the X^1Sigma+ potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S) + Cs(6P_1/2) asymptote. We explore the nature of the Omega=0^+ levels in this region, which have mixed A^1Sigma^+ and b^3Pi character. We then study the quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A wavepacket is formed by exciting a few vibronic levels and is allowed to propagate on the coupled electronic potential energy curves. For a detuning of 7.5 cm-1, the wavepacket for RbCs reaches the short-range region in about 13 ps, which is significantly faster than for the homonuclear Rb_2 system; this is mostly because of the absence of an R^-3 long-range tail in the excited-state potential curves for heteronuclear systems. We give a simple semiclassical formula that relates the time taken to the long-range potential parameters. For RbCs, in contrast to Rb_2, the excited-state wavepacket shows a substantial peak in singlet density near the inner turning point, and this produces a significant probability of deexcitation to form ground-state molecules bound by up to 1500 cm-1. Our analysis of the role of spin-orbit coupling concerns the character of the mixed states in general and is important for both photoassociation and stimulated Raman deexcitation.

physics.atom-ph↗

arXiv2009

Prospects for sympathetic cooling of polar molecules: NH with alkali-metal and alkaline-earth atoms - a new hope

We explore the potential energy surfaces for NH molecules interacting with alkali-metal and alkaline-earth atoms using highly correlated ab-initio electronic structure calculations. The surfaces for interaction with alkali-metal atoms have deep wells dominated by covalent forces. The resulting strong anisotropies will produce strongly inelastic collisions. The surfaces for interaction with alkaline-earth atoms have shallower wells that are dominated by induction and dispersion forces. For Be and Mg the anisotropy is small compared to the rotational constant of NH, so that collisions will be relatively weakly inelastic. Be and Mg are thus promising coolants for sympathetic cooling of NH to the ultracold regime.

physics.chem-ph↗

arXiv2008

The hyperfine energy levels of alkali metal dimers: ground-state homonuclear molecules in magnetic fields

We investigate the hyperfine energy levels and Zeeman splittings for homonuclear alkali-metal dimers in low-lying rotational and vibrational states, which are important for experiments designed to produce quantum gases of deeply bound molecules. We carry out density-functional theory (DFT) calculations of the nuclear hyperfine coupling constants. For nonrotating states, the zero-field splittings are determined almost entirely by the scalar nuclear spin-spin coupling constant. By contrast with the heteronuclear case, the total nuclear spin remains a good quantum number in a magnetic field. We also investigate levels with rotational quantum number N = 1, which have long-range anisotropic quadrupole-quadrupole interactions and may be collisionally stable. For these states the splitting is dominated by nuclear quadrupole coupling for most of the alkali-metal dimers and the Zeeman splittings are considerably more complicated.

physics.atom-ph↗

arXiv2008

The hyperfine energy levels of alkali metal dimers: ground-state polar molecules in electric and magnetic fields

We investigate the energy levels of heteronuclear alkali metal dimers in levels correlating with the lowest rotational level of the ground electronic state, which are important in efforts to produce ground-state ultracold molecules. We use density-functional theory to calculate nuclear quadrupole and magnetic coupling constants for RbK and RbCs and explore the hyperfine structure in the presence of electric and magnetic fields. For nonrotating states, the zero-field splittings are dominated by the electron-mediated part of the nuclear spin-spin coupling. They are a few kHz for RbK isotopologs and a few tens of kHz for RbCs isotopologs.

physics.atom-ph↗

arXiv2008

Avoided crossings between bound states of ultracold Cesium dimers

We present an efficient new computational method for calculating the binding energies of the bound states of ultracold alkali-metal dimers in the presence of magnetic fields. The method is based on propagation of coupled differential equations and does not use a basis set for the interatomic distance coordinate. It is much more efficient than the previous method based on a radial basis set and allows many more spin channels to be included. This is particularly important in the vicinity of avoided crossings between bound states. We characterize a number of different avoided crossings in Cs_2 and compare our converged calculations with experimental results. Small but significant discrepancies are observed in both crossing strengths and level positions, especially for levels with l symmetry (rotational angular momentum L=8). The discrepancies should allow the development of improved potential models in the future.

physics.chem-ph↗

arXiv2008

The prospects for producing ultracold NH$_3$ molecules by sympathetic cooling: a survey of interaction potentials

We investigate the possibility of producing ultracold NH3 molecules by sympathetic cooling in a bath of ultracold atoms. We consider the interactions of NH3 with alkali-metal and alkaline-earth atoms, and with Xe, using ab initio coupled-cluster calculations. For Rb-NH3 and Xe-NH3 we develop full potential energy surfaces, while for the other systems we characterize the stationary points (global and local minima and saddle points). We also calculate isotropic and anisotropic Van der Waals C6 coefficients for all the systems. The potential energy surfaces for interaction of NH3 with alkali-metal and alkaline-earth atoms all show deep potential wells and strong anisotropies. The well depths vary from 887 1/cm for Mg-NH3 to 5104 1/cm for Li-NH3. This suggests that all these systems will exhibit strong inelasticity whenever inelastic collisions are energetically allowed and that sympathetic cooling will work only when both the atoms and the molecules are already in their lowest internal states. Xe-NH3 is more weakly bound and less anisotropic.

physics.chem-ph↗

arXiv2007

Ultracold collisions in magnetic fields: reducing inelastic cross sections near Feshbach resonances

We have carried out bound-state and low-energy quantum scattering calculations on He + NH (triplet Sigma) in magnetic fields, with the NH molecule in its n=1 rotationally excited states. We have explored the pattern of levels as a function of magnetic field and identified the nearly good quantum numbers in different regimes. We have used the bound-state calculations to locate low-energy Feshbach resonances. When the magnetic field is used to tune across such a resonance, the real and imaginary part of the scattering length show asymmetric oscillations and peaks with amplitude between 1 and 3 Angstroms. The scattering length does not pass through a pole at resonance. The resonant behavior is characterized by a complex resonant scattering length a_res. The corresponding inelastic cross sections show troughs as well as peaks near resonance. This may be important for efforts to achieve evaporative and sympathetic cooling for molecules, because it offers the hope that inelastic trap losses can be reduced by tuning close to a Feshbach resonance.

physics.chem-ph↗

arXiv2007

Interactions and dynamics in Li + Li_2 ultracold collisions

A potential energy surface for the lowest quartet electronic state of lithium trimer is developed and used to study spin-polarized Li + Li_2 collisions at ultralow kinetic energies. The potential energy surface allows barrierless atom exchange reactions. Elastic and inelastic cross sections are calculated for collisions involving a variety of rovibrational states of Li_2. Inelastic collisions are responsible for trap loss in molecule production experiments. Isotope effects and the sensitivity of the results to details of the potential energy surface are investigated. It is found that for vibrationally excited states the cross sections are only quite weakly dependent on details of the potential energy surface.

physics.chem-ph↗

arXiv2007

Feshbach resonances in ultracold atomic and molecular collisions: threshold behaviour and suppression of poles in scattering lengths

In the absence of inelastic scattering, Feshbach resonances produce poles in scattering lengths and very large peaks in elastic cross sections. However, inelastic scattering removes the poles. Whenever the resonant state is coupled comparably to the elastic and inelastic channels, the scattering length exhibits only a small oscillation and peaks in cross sections are dramatically suppressed. A resonant scattering length is defined to characterize the amplitude of the oscillation, and is shown to be small for many collisions of ultracold molecules. The results suggest that cross sections for ultracold molecular inelastic collisions are much less sensitive to details of the potential than has been believed.

physics.atom-ph↗

arXiv2006

Ultracold atom-molecule collisions and bound states in magnetic fields: tuning zero-energy Feshbach resonances in He-NH (3Sigma-)

We have generalized the BOUND and MOLSCAT packages to allow calculations in basis sets where the monomer Hamiltonians are off-diagonal and used the new capability to carry out bound-state and scattering calculations on 3He-NH and 4He-NH as a function of magnetic field. Following the bound-state energies to the point where they cross thresholds gives very precise predictions of the magnetic fields at which zero-energy Feshbach resonances occur. We have used this to locate and characterize two very narrow Feshbach resonances in 3He-NH. Such resonances can be used to tune elastic and inelastic collision cross sections, and sweeping the magnetic field across them will allow a form of quantum control in which separated atoms and molecules are associated to form complexes. For the first resonance, where only elastic scattering is possible, the scattering length shows a pole as a function of magnetic field and there is a very large peak in the elastic cross section. For the second resonance, however, inelastic scattering is also possible. In this case the pole in the scattering length is dramatically suppressed and the cross sections show relatively small peaks. The peak suppression is expected to be even larger in systems with stronger inelasticity. The results suggest that calculations on ultracold molecular inelastic collisions may be much less sensitive to details of the potential energy surface than has been believed.

physics.atom-ph↗

arXiv2006

Molecular collisions in ultracold atomic gases

It has recently become possible to form molecules in ultracold gases of trapped alkali metal atoms. Once formed, the molecules may undergo elastic, inelastic and reactive collisions. Inelastic and reactive collisions are particularly important because they release kinetic energy and eject atoms and molecules from the trap. The theory needed to handle such collisions is presented and recent quantum dynamics calculations on ultracold atom-diatom collisions of spin-polarised Li + Li_2, Na + Na_2 and K + K_2 are described. All these systems have potential energy surfaces on which barrierless atom exchange reactions can occur, and both inelastic and reactive rates are very fast (typically k(inel) > 10^{-10} cm^3 s^-1 in the Wigner regime).

physics.atom-ph↗

arXiv2006

Ab initio potential energy surfaces, bound states and electronic spectrum of the Ar-SH complex

New ab initio potential energy surfaces for the doublet pi ground electronic state of the Ar-SH complex are presented, calculated at the RCCSD(T)/aug-cc-pV5Z level. Weakly bound rotation-vibration levels are calculated using coupled-channel methods that properly account for the coupling between the two electronic states. The resulting wavefunctions are analysed and a new adiabatic approximation including spin-orbit coupling is proposed. The ground-state wavefunctions are combined with those obtained for the excited doublet sigma + state [Phys. Chem. Chem. Phys. 6, 5463 (2004)] to produce transition dipole moments. Modelling the transition intensities as a combination of these dipole moments and calculated lifetime values [J. Chem. Phys. 109, 170 (1998)] leads to a good representation of the experimental fluorescence excitation spectrum [J. Chem. Phys. 98, 4301 (1993)].

physics.chem-ph↗