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Two new potential surfaces for CO2–Ar, obtained principally from the spectra of van der Waals complexes, are tested against measurements of pressure broadening and of transport and relaxation properties, none of which was used in the determination of the surfaces. Pressure broadening of both infrared and Raman lines is considered. The coupled states (CS) approximation is used for all line shape calculations. Thermally averaged infrared and Raman cross sections at 523, 296, 160 and 77 K (infrared) and 295 K (Raman) show good agreement with the experimental data available. Generalized transport and relaxation cross sections are obtained via full classical trajectory and classical CS calculations. Properties tested include diffusion, viscosity and nuclear spin relaxation. They provide a different test of the surfaces and agree well with experiment.

The I-NoLLS program is a package for carrying out interactive nonlinear least-squares fits to determine the parameters of physical or mathematical models from experimental or other data, under circumstances where automated least-squares procedures are excessively computationally expensive and the user needs interactive control to apply physical insight to the fitting process. The program was developed to facilitate the fitting of molecular potential energy surfaces (PES) to spectroscopic and scattering data, but is also applicable to a variety of other optimization problems. A range of different algorithms adapted to highly nonlinear least-squares problems may be selected. The interactive nature of the code permits rapid and flexible control over the progress of the fit. I-NoLLS is written in a modular way that allows the easy incorporation of new modules for calculating observable quantities from model parameters. The structure of the program allows straightforward parallelisation of the time-consuming property calculations. In pilot applications, I-NoLLS has been interfaced with programs for calculating bound states of Van der Waals complexes, cross sections for molecular scattering processes, and second virial coefficients of gas mixtures. Parallelisation of the property calculations has been achieved using PVM running on a cluster of workstations.

Variational calculations of vibrational energies, rotational constants and angular expectation values are carried out for the trimers Ar2–HCl, Ar2–DCl, Ar2–HF and Ar2–DF. Calculations are performed on pairwise additive potential energy surfaces and on surfaces including a variety of non-additive contributions. Attention is focused on the bending levels corresponding to hindered internal rotation of the HX molecule in the complex, several of which have been observed by high-resolution spectroscopy. The results confirm that it is crucial to include dispersion, induction and short-range effects when modelling the non-additive forces in molecular systems. It is found that the model of non-additive forces previously proposed by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] works well for the bending bands of Ar2–HCl, Ar2–DCl and Ar2–DF as well as Ar2–HF. In addition, a new distributed model of the non-additive dispersion energy is proposed, in which the triple-dipole energy is partitioned between two anisot...

We show theoretically that ultracold hydrogen atoms have very favorable properties for sympathetic cooling of molecules to microkelvin temperatures. We calculate the potential energy surfaces for spin-polarized interactions of H atoms with the prototype molecules NH(triplet-Sigma-) and OH(doublet-Pi) and show that they are shallow (50 to 80 cm-1) and only weakly anisotropic. We carry out quantum collision calculations on H+NH and H+OH and show that the ratio of elastic to inelastic cross sections is high enough to allow sympathetic cooling from temperatures well over 1 K for NH and around 250 mK for OH., Comment: 6 pages, 3 figures

We consider the prospect of using ultracold hydrogen atoms for sympathetic cooling of fluorine atoms to microkelvin temperatures. We carry out quantum-mechanical calculations on collisions between cold F and H atoms in magnetically trappable states and show that the ratio of elastic to inelastic cross sections remains high across a wide range of temperatures and magnetic fields. For F atoms initially in the spin-stretched state ($^2$P$_{3/2}$, $f=m_f=+2$), sympathetic cooling appears likely to succeed from starting temperatures around 1 K or even higher. This occurs because inelastic collisions are suppressed by p-wave and d-wave barriers that are 600 mK and 3.2 K high, respectively. In combination with recent results on H + NH and H + OH collisions [M. L. Gonz\'alez-Mart\'{\i}nez and J. M. Hutson, arXiv:1305.6282 (2013)], this establishes ultracold H atoms as a very promising and versatile coolant for atoms and molecules that cannot be laser-cooled., Comment: 11 pages, 7 figures

We have investigated magnetically tunable Feshbach resonances arising from the interaction of Li(${}^{2}S$) with metastable Yb(${}^{3}{P}_{2}$) and (${}^{3}{P}_{0}$). For Yb(${}^{3}{P}_{2}$), all the resonance features are strongly suppressed by inelastic collisions that produce Yb in its lower-lying ${}^{3}{P}_{1}$ and ${}^{3}{P}_{0}$ states. For Yb(${}^{3}{P}_{0}$), sharp resonances exist but they are extremely narrow (widths less than 1 mG).

We explore the feasibility of producing ultracold diatomic molecules with nonzero electric and magnetic dipole moments by magnetically associating two atoms, one with zero electron spin and one with nonzero spin. Feshbach resonances arise through the dependence of the hyperfine coupling on internuclear distance.We survey the Feshbach resonances in diatomic systems combining the nine stable alkali-metal isotopes with those of Yb, focusing on the illustrative examples of RbYb and CsYb. We show that the resonance widths may expressed as a product of physically comprehensible terms in the framework of Fermi’s golden rule. The resonance widths depend strongly on the background scattering length, which may be adjusted by selecting the Yb isotope, and on the hyperfine coupling constant and the magnetic field. In favorable cases the resonances may be over 100 mG wide.

We perform radio-frequency dissociation spectroscopy of weakly bound 6Li2 Feshbach molecules using low-density samples of about 30 molecules in an optical dipole trap. Combined with a high magnetic field stability, this allows us to resolve the discrete trap levels in the radio-frequency dissociation spectra. This novel technique allows the binding energy of Feshbach molecules to be determined with unprecedented precision. We use these measurements as an input for a fit to the 6Li scattering potential using coupled-channel calculations. From this new potential, we determine the pole positions of the broad 6Li Feshbach resonances with an accuracy better than 7×10(-4) of the resonance widths. This eliminates the dominant uncertainty for current precision measurements of the equation of state of strongly interacting Fermi gases. As an important consequence, our results imply a corrected value for the Bertsch parameter ξ measured by Ku et al. [Science 335, 563 (2012)], which is ξ=0.370(5)(8).

We report the observation of interspecies Feshbach resonances in an optically trapped mixture of 85Rb and 133Cs. We measure nine resonances in the lowest spin channel for a magnetic field range from 0 to 700 G and show that they are in good agreement with coupled-channel calculations. The interspecies background scattering length is close to zero over a large range of magnetic fields, permitting the sensitive detection of Feshbach resonances through interspecies thermalization. Our results confirm the quality of the Rb-Cs potential curves [ Phys. Rev. A 85 032506 (2012)] and offer promising starting points for the production of ultracold polar molecules.

Two potential energy surfaces for CO2–Ar are obtained by least‐squares fitting to the high‐resolution spectra of Van der Waals complexes and the second virial coefficients of Ar+CO2 gas mixtures. The potentials incorporate a repulsive wall based on monomer ab initio calculations and the assumption that the repulsion potential is proportional to the overlap of the monomer charge densities. The dispersion energy is represented in a two‐site model, with dispersion centers located along the C–O bonds of CO2. The resulting potentials give a good representation of all the experimental data with only three or four adjustable parameters. They are quite different from previous empirical CO2–Ar potentials, which all have either a poor representation of the attractive well or a poor representation of the repulsive wall.

We have observed a microwave spectrum of the HeKr+ ion in which all of the observed levels lie within a few cm−1 of either the first or second dissociation limit. We use an ion beam technique in which HeKr+ ions, formed by electron impact, are mass analyzed. Passage of the ion beam through an electric field lens results in selective fragmentation of energy levels lying close to dissociation. Kr+ ions formed in the lens are separated from all other ions by means of an electrostatic analyzer, and are detected with an electron multiplier. Microwave radiation induces transitions which result in population transfer and produce detected changes in the electric field‐induced Kr+ fragment ion current. Additional transitions have been detected by a microwave–microwave double resonance method, and we have also made extensive use of the Zeeman effects produced by small applied coaxial magnetic fields to identify the J quantum numbers of the levels involved. Coupled channel calculations of the bound states of the He⋯...

Two potential energy surfaces for He–HCN are determined by least‐squares fitting of parameterised functional forms to data from high‐resolution microwave and millimeter‐wave spectroscopy [Drucker et al., J. Phys. Chem. 99, 2646 (1995)]. The two potentials both have significantly deeper wells than suggested by the ab initio supermolecule calculations of Drucker et al. Both potentials have linear or near‐linear equilibrium geometries, He–H–C–N, but the shapes of the well depth functions away from the linear geometry are significantly different. The existing experimental data are thus not sufficient to probe this potential feature in detail. Predictions of spectroscopic properties that would allow the new potentials to be tested and refined are given.

Nine different potential energy surfaces for CO2–Ar taken from the literature are tested for their ability to reproduce the spectra of the Ar–CO2 van der Waals complex and the pressure broadening of CO2 infrared lines by Ar. None of the potentials give a satisfactory account of all the experimental results. All the potentials are found to give significant discrepancies with at least some of the spectroscopic properties of the van der Waals complex. Coupled‐states (CS) and infinite‐order sudden (IOS) calculations of the pressure broadening cross sections are compared for a few of the potential energy surfaces. The IOS approximation is found to be seriously inaccurate for some potential surfaces, especially for high‐j lines, so that CS calculations are essential when comparing with experimental line‐shape data. CS calculations of line‐broadening cross sections are therefore carried out on all nine different potential energy surfaces. For the pressure broadening coefficients, there are substantial uncertaint...

We explore the scattering properties of ultracold ground-state Cs atoms at magnetic fields between 450 G (45 mT) and 1000 G. We identify 17 new Feshbach resonances, including two very broad ones near 549 G and 787 G. We measure the binding energies of several different dimer states by magnetic field modulation spectroscopy. We use least-squares fitting to these experimental results, together with previous measurements at lower field, to determine a new 6-parameter model of the long-range interaction potential, designated M2012. Coupled-channels calculations using M2012 provide an accurate mapping between the s-wave scattering length and the magnetic field over the entire range of fields considered. This mapping is crucial for experiments that rely on precise tuning of the scattering length, such as those on Efimov physics., 19 pages, 18 figures

A classical trajectory study of Ar + Ar2 collisions is described. The system provides a model chaotic scattering system in more than two degrees of freedom. The initial conditions that give rise to ejection of each of the three Ar atoms, and the corresponding collision mechanisms, are investigated. There are some large regions of the initial condition space in which the identity of the atom ejected does not change, and other regions in which it changes rapidly. Attention is focused on long‐lived trajectories, which lie at the boundaries between different product identities. The long‐lived trajectories are associated with sequences of periodic orbits. The different stability possibilities for periodic orbits in three degrees of freedom are discussed, and a sequence of periodic orbits responsible for dividing the initial condition space is identified. These periodic orbits are born at ‘‘avoided bifurcations,’’ at which a saddle‐center bifurcation occurs close to a parent periodic orbit. The generalization to systems with more than three degrees of freedom is discussed.

Calculations of ground‐state energies and rotational constants are carried out for a variety of van der Waals dimers and trimers formed from Ne, Ar, Kr and Xe. It is found that the existing pair potentials for Ne–Ar, Ne–Kr and Ne–Xe do not adequately reproduce the measured rotational constants of the van der Waals dimers. Modified pair potentials, with equilibrium distances that differ from the originals by less than 1% but give much better rotational constants, are then proposed. Calculations of rotational constants for Ne2–Ar, Ne2–Kr and Ne2–Xe are carried out using pairwise‐additive potentials constructed from both the original and the modified pair potentials. The modified pair potentials give much better agreement with experiment for the trimers as well as the dimers. The effect of an Axilrod–Teller triple‐dipole term on the rotational constants is considered, and found to be significant, especially for the A rotational constant. However, the best available Ne–Ne potential is not accurate enough to a...

A new model potential is constructed for Li+-H2O, based largely on ab initio calculations of the monomer wavefunctions. The electrostatic energy is calculated using distributed multipoles, the induction and dispersion energies are calculated using damped multipole series, and the repulsion energy is assumed to be proportional to the charge density overlap integral. The model potential has a shallower well than most previous potentials; the smaller well depth is in agreement with new high-level supermolecule ab initio calculations. Spectroscopic data for Li+-H2O are not yet available, but will provide a rigorous test of the potential, so bound-state calculations of intermolecular vibration-rotation energy levels are performed in this work. The fundamental vibrational wave-numbers are found to be 357 cm-1, 538 cm-1 and 532 cm-1 for the out-of-plane bend, in-plane bend and intermolecular stretch vibrations, respectively.

Femtosecond pump-probe experiments involving pseudorotational motion on the B state of the sodium trimer are simulated using time-dependent wavepacket calculations. The calculations are carried out in two dimensions, corresponding to the radial and angular pseudorotation modes. The pump and probe lasers are modelled using time-dependent first-order perturbation theory. The ion signal is simulated using two different values of the ionisation potential: the first, 32240 cm−1, is close to an older experimental value; the second, 31363 cm−1, was recently obtained from ZEKE spectra. The relative intensities of different peaks depend strongly on the value of the ionisation potential used. The ion signal calculated using the more recent value for the ionisation potential shows closer agreement with the signal found in the experiments.

We have measured and interpreted a microwave spectrum of the HeAr+ ion in which all of the observed energy levels lie within 8 cm−1 of the lowest dissociation limit, He(1S)+Ar+(2P3/2). We use an ion beam technique in which the HeAr+ ions are formed by electron impact, accelerated to kilovolt potentials, and mass‐analyzed. After passage through an appropriate section of waveguide, the ions enter an electric field lens in which state‐selective fragmentation occurs; the Ar+ ions produced in the lens are separated from all other ions by means of an electrostatic analyser and detected with an electron multiplier. Microwave transitions induced in the waveguide section result in population transfer which produces detected changes in the electric field‐induced Ar+ fragment current. Many transitions have also been observed by a microwave–microwave double resonance technique. We have observed 68 lines spanning the frequency range 6–170 GHz; no immediately recognizable pattern is apparent. We have measured the Zeeman splitting produced by a small axial magnetic field for almost every line, which enables us to determine the values of the total angular momentum J involved in each transition, and also effective g factors for the two levels involved. We are therefore able to construct a purely experimental pattern of 37 levels lying within 8 cm−1 of the dissociation limit. The data are treated first by means of a conventional effective Hamiltonian in a case (c) basis, which allows electronic and vibrational quantum numbers to be assigned to most of the levels; the assignments are approximate, however, because very strong rotational‐electronic coupling undermines the Born–Oppenheimer approximation.A more complete theoretical treatment is then presented, using the coupled‐channel method in a case (e) representation to calculate the energy levels without making the Born–Oppenheimer approximation. The microwave transition frequencies and g‐factors are fitted, together with earlier ultraviolet spectra, to provide a new interaction potential (designated MAL1) for He interacting with Ar+(2P3/2 and 2P1/2). The MAL1 potential is substantially more accurate than previous potentials, especially in the long‐range region and for the A1 2Π3/2 state, which had not been observed before. An important new feature of the MAL1 potential is that the long‐range C6 coefficient is strongly anisotropic, so that the different electronic curves have substantially different C6 coefficients.

Fully quantal scattering calculations are carried out for H 35 Cl in Ar, using both close-coupling and coupled-states calculations on the Ar-HCl H6(4,3,0) potential surface of Hutson, which was obtained by fitting to extensive spectroscopic data on the Ar-HCl and Ar-DCl Van der Waals complexes. Line width and shift cross sections are calculated for the pure rotational and infrared fundamental and first overtone bands of HCl. Both widths and shifts are found to be in reasonable agreement with existing measurements. However, there is considerable scatter between the different sets of experimental results, and more precise measurements are needed to provide a sensitive test of the quality of the potential.

The nuclear quadrupole coupling constants of van der Waals complexes contain valuable information on intermolecular forces. In the past, the coupling constants have been interpreted in terms of angular expectation values, involving the projections of the monomer coupling constant onto the inertial axes of the complex. However, this is a significant approximation. The present paper develops the theory needed to calculate nuclear quadrupole splittings without defining an inertial axis system for the complex. The theory is applied to Ar-CO2, Ar-HCl and Ar-N2 as test cases. An improved value of the quadrupole coupling constant of the N2 monomer is obtained, based on the microwave spectra of Ar-N2 and Kr-N2.

Calculations of vibrational energies and rotational constants are carried out for the van der Waals trimer Ar[sub 2]-HF. The calculations include all five intermolecular degrees of freedom. The different intramolecular vibrational states [ital v] of the HF molecule are separated out adiabatically, so that the calculations are carried out on effective intermolecular potentials for each HF vibrational state separately. Calculations are performed both on pairwise-additive potentials, derived from the well-known Ar-Ar and Ar-HF potentials, and on nonadditive potentials, incorporating various different contributions to the three-body forces. The results are compared with experimental results from high-resolution spectroscopy, and provide detailed information on the anisotropy of the nonadditive intermolecular forces. As in previous work on Ar[sub 2]-HCl, it is found that a very important nonadditive term arises from the interaction between the permanent multipoles of the HF molecule and the exchange quadrupole caused by distortion of the two Ar atoms as they overlap. An improved model of this term is described.

Multichannel quantum defect theory (MQDT) can provide an efficient alternative to full coupled-channel calculations for low-energy molecular collisions. However, the efficiency relies on interpolation of the Y matrix that encapsulates the short-range dynamics, and there are poles in Y that may prevent interpolation over the range of energies of interest for cold molecular collisions. We show how the phases of the MQDT reference functions may be chosen so as to remove such poles from the vicinity of a reference energy and dramatically increase the range of interpolation. For the test case of Mg+NH, the resulting optimized Y matrix may be interpolated smoothly over an energy range of several Kelvin and a magnetic field range of over 1000 G. Calculations at additional energies and fields can then be performed at a computational cost that is proportional to the number of channels N and not to N^3., 7 pages, 6 figures

We show that multichannel quantum defect theory (MQDT) can be applied successfully as an efficient computational method for cold molecular collisions in Li+NH, which has a deep and strongly anisotropic interaction potential. In this strongly coupled system, closed-channel poles restrict the range over which the MQDT Y matrix can be interpolated. We present an improved procedure to transform the MQDT reference functions so that the poles are removed from the energy range of interest. Effects due to very long-range spin-dipolar couplings are outside the scope of MQDT, but can be added perturbatively. The new procedure makes it possible to calculate the elastic and inelastic cross sections needed to evaluate the feasibility of sympathetic cooling of NH by Li using coupled-channel calculations at only 5 combinations of energy and field., Comment: 8 pages, 6 figures

We have studied interspecies scattering in an ultracold mixture of $^{87}$Rb and $^{133}$Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming s- and p-wave magnetic Feshbach resonances over the range 0 to 667 G have been catalogued. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 MHz$\times h$. Magnetic moment spectroscopy along the magneto-association pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have created RbCs Feshbach molecules using two of the resonances. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wavefunctions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage (STIRAP)., Comment: 16 pages, 11 figures, submitted to PRA

We present 17 experimentally confirmed Feshbach resonances in optically trapped 85Rb. Seven of the resonances are in the ground-state channel (f,m_f) = (2,+2)+(2,+2), and nine are in the excited-state channel (2,-2)+(2,-2). We find a wide resonance at high field in each of the two channels, offering new possibilities for the formation of larger 85Rb condensates and studies of few-body physics. A detailed coupled-channels analysis is presented to characterize the resonances, and also provides an understanding of the inelastic losses observed in the excited-state channel. In addition we have confirmed the existence of one narrow resonance in a (2,+2)+(3,+3) spin mixture., Comment: 8 pages, 6 Figures

The infrared spectrum of the HeCO2 van der Waals molecule is recorded in the region of the CO2 ν3 asymmetric stretch via direct absorption of a tunable Pb–salt diode laser. HeCO2 is formed in a slit jet supersonic expansion; the slit valve and the stagnation gas must be precooled to −35 °C before substantial formation of the complex is observed. Sixty‐six rovibrational transitions are recorded by exciting the ν3 asymmetric stretch of the CO2 monomer within the complex. Forty‐three of these transitions can be assigned using internally consistent combination differences as a b‐type band of a T‐shaped asymmetric rotor. There are several indications that large amplitude motion is significant in HeCO2, including the poor quality of the fit to an asymmetric rotor model and the large positive inertial defects of Δ=8.54 and 10.98 uA2 in the ground and excited states, respectively. However, a hindered rotor analysis based on these inertial defects demonstrates that the CO2 motion within the complex is far from the...

The coupled channel method for solving the bound-state Schrodinger equation is described. The method has been widely applied to the vibrational-rotational levels of floppy molecules such as Van der Waals complexes, but is also applicable in other areas of physics. It uses a basis set expansion for all degrees of freedom except a radial coordinate, and produces a set of coupled differential equations in the radial coordinate. In modern methods of solution, the coupled equations are solved by propagating the log-derivative of the wavefunction. Eigenvalues are located by searching for zeroes of the determinant of the “log-derivative matching matrix”. The resulting methods are very stable, and are the method of choice for providing eigenvalues to very high precision. Some recent advances in the methods, which provide improved convergence on eigenvalues and allow the calculation of wavefunctions, are described.

A method for obtaining wave functions from coupled‐channel bound state calculations using log‐derivative propagators is presented. Bound states occur at energies for which the inward and outward log‐derivative solutions match at a central point in the propagation; at such energies, the log‐derivative matching matrix has one eigenvalue which is zero. The wave function at the matching point is the eigenvector corresponding to this zero eigenvalue. The wave function at other points can be obtained by back‐substitution in the log‐derivative propagation equations. The method is tested by calculating infrared intensities for the Ar–HF van der Waals complex, using the H6(4,3,2) potential.

The theory needed to carry out calculations on atom–molecule van der Waals complexes containing open‐shell atoms is developed. The discussion concentrates on complexes containing atoms in P states. Several possible expansions of the total wave function are described, and the matrix elements needed to construct the Hamiltonian matrix are set out. Several different angular momentum coupling cases may arise, analogous to Hund’s coupling cases in diatomic molecules. The bending energy levels of Ca–HCl, B–H2, F–H2, Cl–Cl2, and F–N2 are calculated, using simple models of the interaction potentials.

Methods for calculating the rotational constants of floppy triatomic molecules and atom-linear molecule complexes are discussed. It is shown that previously published equations relating the rotational constants to expectation values of moments of inertia correspond to an approximate separation of rotation and vibration. Improved equations, which take account of the Eckart conditions, are presented. The results of the old and new methods are compared with rotational constants obtained by fitting to J -dependent eigenvalues obtained from exact close-coupling calculations on ArCO 2 . The improved equations are found to give substantially more accurate results.

The far-infrared vibration-rotation spectrum of the out-of-plane DCl bending band of Ar2-DCl is observed around 36·0 cm-1. The experimental bending frequency, rotational constants and hyperfine coupling constants are compared with the results of calculations employing both pairwise-additive and non-additive interaction potentials. As found previously for Ar2-HCl, there are substantial discrepancies between the experimental results and calculations employing a pairwise-additive potential. To explain the discrepancy it is necessary to include a non-additive term that arises from the interaction of the permanent multipoles of the DCl monomer with an overlap-induced quadrupole on Ar2. The new spectra should prove very valuable in a future determination of the non-additive contribution to the potential.

The theory of the vibration‐rotation states of atom‐spherical top van der Waals complexes is developed. The exact close‐coupled equations are closely analogous to those encountered in atom+spherical top scattering. The structure of the coupled equations is investigated, and close‐coupling calculations of the bound states of Ar–CH4 are presented for two different intermolecular potentials. The role of symmetry in the complex is discussed, and the energy levels are interpreted using a model in which the CH4 molecule undergoes hindered rotation in the field of the Ar atom. Correlation diagrams are presented, showing how the free‐rotor levels are converted into near‐rigid vibrational energy levels as the anisotropy of the intermolecular potential increases. The effect of higher‐order anisotropic terms is investigated, and correlation diagrams are given for complexes of tetrahedral, octahedral, and icosahedral molecules. The role of monomer vibrational angular momentum is investigated.

An interaction potential for Ar–HF, which was recently determined from extensive spectroscopic data for the van der Waals dimer and which includes dependence on the HF vibrational state, has been used with accurate close‐coupling molecular scattering calculations to predict line shape parameters for the pure rotational and the fundamental and first overtone vibrational bands of HF in a bath of Ar. Agreement with experiment is good; in fact, considering inconsistencies among the experimental values, the theoretical values may be the more reliable. This confirms the accuracy of the Ar–HF interaction potential, including the dependence on the HF vibrational level which is sensitively probed by the line shift cross sections.

Calculations on the vibration–rotation energy levels of the Van der Waals trimers Ar2–HF and Ar2–HCl are carried out, in order to investigate the role of three-body (non-pairwise-additive) forces. The present calculations focus on the lowest Van der Waals vibrational state formed from HX molecules in different intramolecular vibrational states, v. The calculations use both pairwise-additive potentials, derived from the accurately known Ar–Ar, Ar–HF and Ar–HCl potentials, and various different contributions to the three-body forces. All five intermolecular degrees of freedom are included. The red shift observed in the Ar2–HF fundamental band is fairly well predicted (within 0.5 cm–1) by the pairwise-additive potential, but the agreement with experiment is improved when non-additive forces are included. The rotational constants are found to be very sensitive to non-additive terms, and especially to the ‘exchange quadrupole’ term. Non-additive forces of the type usually used in studies of atomic forces, such as triple dipole forces, are found to be inadequate to reproduce the observed spectra.

We investigate the effect of hyperfine interactions on ultracold molecular collisions in magnetic fields, using ${}^{24}$Mg(${}^{1}S){+}^{14}$NH(${}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$) as a prototype system. We explore the energy and magnetic-field dependence of the cross sections, comparing the results with previous calculations that neglected hyperfine interactions [A.O.G. Wallis and J. M. Hutson, Phys. Rev. Lett. 103, 183201 (2009)]. The main effect of hyperfine interactions for spin relaxation cross sections is that the kinetic energy release of the dominant outgoing channels does not reduce to zero at low fields. This results in reduced centrifugal suppression of the cross sections and increased inelastic cross sections at low energy and low field. We also analyze state-to-state cross sections, for various initial states, and show that hyperfine interactions introduce additional mechanisms for spin relaxation. In particular, there are hyperfine-mediated collisions to outgoing channels that are not centrifugally suppressed. However, for $\mathrm{Mg}+\mathit{NH}$ these unsuppressed channels make only small contributions to the total cross sections. We consider the implications of our results for sympathetic cooling of NH by Mg and conclude that the ratio of elastic to inelastic cross sections remains high enough for sympathetic cooling to proceed.

We consider how trapped molecules can be sympathetically cooled by ultracold atoms. As a prototypical system, we study LiH molecules co-trapped with ultracold Li atoms. We calculate the elastic and inelastic collision cross sections of LiH + Li with the molecules initially in the ground state and in the first rotationally excited state. We then use these cross sections to simulate sympathetic cooling in a static electric trap, an ac electric trap, and a microwave trap. In the static trap we find that inelastic losses are too great for cooling to be feasible for this system. The ac and microwave traps confine ground-state molecules, and so inelastic losses are suppressed. However, collisions in the ac trap can take molecules from stable trajectories to unstable ones and so sympathetic cooling is accompanied by trap loss. In the microwave trap there are no such losses and sympathetic cooling should be possible., 9 pages, 10 figures, minor revisions following referee suggestions

We report on the observation of triatomic Efimov resonances in an ultracold gas of cesium atoms. Exploiting the wide tunability of interactions resulting from three broad Feshbach resonances in the same spin channel, we measure magnetic-field dependent three-body recombination loss. The positions of the loss resonances yield corresponding values for the three-body parameter, which in universal few-body physics is required to describe three-body phenomena and in particular to fix the spectrum of Efimov states. Our observations show a robust universal behavior with a three-body parameter that stays essentially constant., Comment: 4 pages, 3 figures

We show that conical intersections can be created in laboratory coordinates by dressing a parabolic trap for ultracold atoms or molecules with a combination of optical and static magnetic fields. The resulting ring trap can support single-particle states with half-integer rotational quantization and many-particle states with persistent flow. Two well-separated atomic or molecular states are brought into near resonance by an optical field and tuned across each other with an inhomogeneous magnetic field. Conical intersections occur at the nodes in the optical field.

Controlling interactions between cold molecules using external fields can elucidate the role of quantum mechanics in molecular collisions. We create a new experimental platform in which ultracold rubidium atoms and cold ammonia molecules are separately trapped by magnetic and electric fields and then combined to study collisions. We observe inelastic processes that are faster than expected from earlier field-free calculations. We use quantum scattering calculations to show that electric fields can have a major effect on collision outcomes, even in the absence of dipole-dipole interactions., 5 pages, 4 figures

Multichannel Quantum Defect Theory (MQDT) is shown to be capable of producing quantitatively accurate results for low-energy atom-molecule scattering calculations. With a suitable choice of reference potential and short-range matching distance, it is possible to define a matrix that encapsulates the short-range collision dynamics and is only weakly dependent on energy and magnetic field. Once this has been produced, calculations at additional energies and fields can be performed at a computational cost that is proportional to the number of channels N and not to N^3. MQDT thus provides a promising method for carrying out low-energy molecular scattering calculations on systems where full exploration of the energy- and field-dependence is currently impractical., Comment: 9 pages, 9 figures. Final version, incorporating responses to referees' comments

We present quantum-theoretical studies of collisions between an open-shell S-state atom and a ^2Pi-state molecule in the presence of a magnetic field. We analyze the collisional Hamiltonian and discuss possible mechanisms for inelastic collisions in such systems. The theory is applied to the collisions of the nitrogen atom (^4S) with the OH molecule, with both collision partners initially in fully spin-stretched (magnetically trappable) states, assuming that the interaction takes place exclusively on the two high-spin (quintet) potential energy surfaces. The surfaces for the quintet states are obtained from spin-unrestricted coupled-cluster calculations with single, double, and noniterative triple excitations. We find substantial inelasticity, arising from strong couplings due to the anisotropy of the interaction potential and the anisotropic spin-spin dipolar interaction. The mechanism involving the dipolar interaction dominates for small magnetic field strengths and ultralow collision energies, while the mechanism involving the potential anisotropy prevails when the field strength is larger (above 100 G) or the collision energy is higher (above 1 mK). The numerical results suggest that sympathetic cooling of magnetically trapped OH by collisions with ultracold N atoms will not be successful at higher temperatures., Comment: 13 pages, 10 figures

Elastic and spin-changing inelastic collision cross sections are presented for cold and ultracold magnetically trapped NH. The cross sections are obtained from coupled-channel scattering calculations as a function of energy and magnetic field. We specifically investigate the influence of the intramolecular spin-spin, spin-rotation, and intermolecular magnetic dipole coupling on the collision dynamics. It is shown that $^{15}$NH is a very suitable candidate for evaporative cooling experiments. The dominant trap-loss mechanism in the ultracold regime originates from the intermolecular dipolar coupling term. At higher energies and fields, intramolecular spin-spin coupling becomes increasingly important. Our qualitative results and conclusions are fairly independent of the exact form of the potential and of the size of the channel basis set., Comment: Submitted to Phys. Rev. A

We calculate the interaction potential between N atoms and NH molecules and use it to investigate cold and ultracold collisions important for sympathetic cooling. The ratio of elastic to inelastic cross sections is large over a wide range of collision energy and magnetic field for most isotopic combinations, so that sympathetic cooling of NH molecules by N atoms is a good prospect. However, there are important effects due to a p-wave shape resonance that may inhibit cooling in some cases. We show that scaling the reduced mass used in the collision is approximately equivalent to scaling the interaction potential. We then explore the dependence of the scattering properties on the reduced mass and explain the resonant effects observed using angular-momentum-insensitive quantum defect theory.

The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser‐induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher‐order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, an...

Empirical potential energy surfaces for Ar interacting with OH (X 2Π) are obtained by fitting to experimental results from microwave and stimulated‐emission pumping (SEP) spectra of the Ar–OH complex. The sum VΠ and difference V2 of the potentials for A’ and A‘ symmetry are determined. The sum potential is 126 cm−1 deep, which is about 24 cm−1 deeper than the ab initio CEPA potential of Degli Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)]. The equilibrium geometry is near‐linear, Ar—H–O, with a barrier to internal rotation of 44 cm−1. The potential satisfactorily reproduces all the parameters obtained from the SEP experiments, but not the parity doubling obtained from the microwave spectrum.

The spectra of van der Waals trimers formed from two atoms and a diatomic molecule are investigated. A computational method for calculating vibrational energies, rotational constants and angular expectation values for such complexes is developed and applied to the Ar2–HCl complex. All five low‐frequency modes of the trimer are included. The pair potentials for Ar–Ar and Ar–HCl interactions are well known, and calculations are performed on Ar2–HCl potentials obtained from them assuming pairwise additivity. The calculations are compared with experimental results obtained from microwave and far‐infrared spectroscopy. Substantial discrepancies between experiment and theory are found, and are attributed to the effects of nonadditive interactions. Several different contributions to the nonadditive interactions are investigated: dispersion forces, electrostatic induction forces, and exchange overlap forces are all found to be significant, but not large enough to explain the discrepancies. Exchange multipole forces, arising from the interaction between overlap‐induced multipoles on the Ar atoms and the permanent multipoles on the HCl molecule, are found to be important, and to improve the agreement between experiment and theory substantially. It seems likely that it will be possible to obtain detailed information on nonadditive interactions from the spectroscopy of van der Waals trimers.