Magnetic shielding studies on the alkali molecules Na2 and Cs2 by NMR

NMR in the alkali molecules Na₂ and Cs₂ is performed by the atom-molecule exchange optical pumping method. The shielding differences σ(Na)?σ(Na₂)=(29±16)·10^?6 and σ(Cs)?σ(Cs₂)=(221±12)·10^?6 are obtained. The investigation of the contribution of the valence electron to the magnetic shielding is supported by a NMR experiment in free Cs⁺ ions, which yields the shielding difference σ(Cs)?σ(Cs⁺)=(14±12)·10^?6. These measurements allow an estimation of the spin rotation interaction constant in these molecules.     

Thermal averaging of the spin-rotation coupling in small molecules leads to an isotropic NMR shielding

It has been known for over 70 years that nuclear spins couple to molecular rotation via a Zeeman interaction. This spin–rotation coupling can be observed as a discrete splitting in molecular beam magnetic resonance experiments, but is quenched by molecular collisions at higher pressures. We show that because of differential thermal population of M_J levels at high magnetic fields, the spin rotation coupling retains a small isotropic component at high field. For all but the smallest molecules at very low temperature, the residual coupling is temperature independent and linear in the magnetic field; it therefore closely mimics the chemical shift. The ‘super spin rotation’ shift may in the future be a necessary correction to ultra – high precision computations of the NMR chemical shielding of small molecules in gases and liquids.     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

Anisotropic rotational diffusion model calculations of spin-rotation interaction in fluids of asymmetric-top molecules

The spin-lattice relaxation time, T ₁, due to the spin-rotation interaction in fluids of asymmetric-top molecules are calculated by using the rotational diffusion equation of Favro. In the rotational diffusion model, T ₁ is expressed in terms of three principal rotational diffusional constants. The results of the present calculation should aid interpretations of the experimental T ₁ data of fluids of asymmetric-top molecules.     

Prospects for the formation of ultracold ground state polar molecules from mixed alkali atom pairs

Photoassociation rates of mixed cold alkali atom pairs and formation rates of cold heteronuclear alkali dimers in their ground state are computed within an approach where the wave functions are calculated exactly, and the laser field is treated as a perturbation. These rates are predicted to have the same magnitude for all heteronuclear species involving either Rb or Cs atoms. Moreover, these rates are found slightly smaller than, or similar to, the same rates for Cs₂ molecules, which is encouraging for future experiments. We studied the specific case of the photoassociation into excited molecular states coupled with spin-orbit interaction, emphasizing the role of the so-called resonant coupling in the formation of stable ultracold molecules with low vibrational levels. The comparison with recent experimental results on RbCs photoassociation and cold molecule formation show their reasonable agreement with our calculations.Received: 2 July 2004, Published online: 23 November 2004PACS: 32.80.Pj Optical cooling of atoms; trapping - 33.20.Tp Vibrational analysis - 33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors     

Spin-orbit and spin-rotation coupling in doublet states of diatomic molecules

The spin-rotation interaction and the centrifugal correction to the spin-orbit coupling make indistinguishable contributions to the energy levels of a diatomic molecule in a multiplet state with Λ ≠ 0. A previous method of separating these two contributions, based on the use of the vibrational dependence of the spin-orbit coupling constant, is unreliable. It is suggested here that a better procedure is one based on the isotope dependences of the spin-rotation coupling constant γ and the centrifugal correction to the spin-orbit coupling constant A_D. Both γ_e and A_De are shown to be inversely proportional to μ, the reduced mass of the molecule, but their contributions to the energy have different isotope effects. The method is used to determine values of γ_e and A_De for the X²Π state of HCl⁺, and the form of the spin-orbit coupling function A(r) in the vicinity of the equilibrium bond length is derived. The implications for RKR calculations are considered briefly.     

Coupled cluster study of NMR shielding constants and spin-rotation constants in SiH4, PH3 and H2S molecules

Ab initio methods are applied to analyse the NMR shielding constants and spin-rotation constants in SiH₄, PH₃ and H₂S molecules. The electron correlation effects are studied applying the MP2 and coupled cluster perturbation approaches. The basis set convergence is examined at the same time, and the final results for the equilibrium geometries are obtained at the CCSD(T)/cc-pCVQZ level. Zero-point vibrational and temperature contributions are computed at the SCF, MP2 and CCSD level of approximation. In addition, for the shielding constants we also estimate the relativistic effects, to determine total values of the shielding of the third-row nuclei in the studied molecules. Our final results for the shielding constants at 300?K are σ (²⁹Si in SiH₄)?=?482.35?ppm, σ (³¹P in PH₃)?=?611.64?ppm and σ (³³S in H₂S)?=?736.13?ppm. These values, together with estimated corrections and error bars, can be used to determine absolute NMR shielding scales for the heavy nuclei.     

Ultracold triplet molecules in the rovibrational ground state

We report here on the production of an ultracold gas of tightly bound Rb2 triplet molecules in the rovibrational ground state, close to quantum degeneracy. This is achieved by optically transferring weakly bound Rb2 molecules to the absolute lowest level of the ground triplet potential with a transfer efficiency of about 90%. The transfer takes place in a 3D optical lattice which traps a sizeable fraction of the tightly bound molecules with a lifetime exceeding 200 ms.     

Magnetoelectrostatic trapping of ground state OH molecules

We report magnetic confinement of neutral, ground state OH at a density of approximately 3 x 10(3) cm(-3) and temperature of approximately 30 mK. An adjustable electric field sufficiently large to polarize the OH is superimposed on the trap in various geometries, making an overall potential arising from both Zeeman and Stark effects. An effective molecular Hamiltonian is constructed, with Monte Carlo simulations accurately modeling the observed single-molecule dynamics in various trap configurations. Magnetic trapping of cold polar molecules under adjustable electric fields may enable study of low energy dipolar interactions.     

Sternheimer shielding in molecules

The effect of perturbing electric fields and electric-field gradients on the nuclear quadrupole coupling in molecules is investigated. The theory for the first-order effects is developed and it is shown that not only a perturbing field gradient but also a perturbing field, and higher terms, induce a finite field gradient at a nucleus in a molecule. The so-called Sternheimer (anti)shielding is thus considerably more complex in molecular than in atomic systems. The different shielding components have been calculated for HCl, HCN and NH₄ ⁺ using a finite perturbation ab initio molecular orbital method. The calculated values are then used to rationalize experimentally determined ¹⁴N quadrupole coupling constants in condensed media for HCN and NH₄ ⁺.     

Selecting molecules in the vibrational and rotational ground state by deflection

A beam of diatomic molecules scattered off a standing wave laser mode splits according to the rovibrational quantum state of the molecules. Our numerical calculation shows that single state resolution can be achieved by properly tuned, monochromatic light. The proposed scheme allows for selecting non-vibrating and non-rotating molecules from a thermal beam, implementing a laser Maxwell's demon to prepare a rovibrationally cold molecular ensemble. Received 23 August 2000 and Received in final form 17 November 2000     

Formation of ultracold polar molecules in the rovibrational ground state

Ultracold LiCs molecules in the absolute ground state X1Sigma+, v' = 0, J' = 0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of v' = 0, J' = 2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5 x 10(3) molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.     

Magnetic ground state of pure and doped CeFe2

A combination of neutron elastic and inelastic, resonant x-ray scattering, and 57Fe M?ssbauer experiments are used to determine the unusual magnetic ground state of CeFe2. The complementarities between different time-scale techniques may allow one to understand the dynamic features of the ground state in CeFe2 and its pseudobinary compounds, and how the frustration of Fe tetrahedra leads the appearance of antiferromagnetic fluctuations in the presence of ferrimagnetism. The resulting model can be used to rationalize many of the unusual and conflicting experimental results reported for this material in the literature.     

Modified firsov interaction potential for neutrals in ground state

An analytic expression is obtained for atomic-interaction repulsive potential. The theoretical data agree well with experimental data and quantum-mechanical calculations for small and intermediate interatomic distance and can be used in studying problems on the motion of heavy charged ions in matter.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 70–74, April, 1973.In conclusion, the author thanks G. A. Gumanskii for active discussion of the results.     

Magnetic dipolar interaction between two isotropically rotating molecules

The average value of all the components of the magnetic dipolar interaction tensor is calculated for two isotropically rotating molecules. The result is equivalent to the one obtained by assuming the spins to be located at the centres of the spheres.     

Rotational diffusion of complex molecules in nonviscous solutions in the ground state and the excited state

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 50, No. 5, pp. 790–795, May, 1989.     

Formation of cold Cs ground state molecules through photoassociation in the pure long-range state

We report on the formation of translationally cold Cs₂ ground state molecules through photoassociation in the 1_u attractive molecular state below the 6 s _1/2 +6 p _3/2 dissociation limit. The cold molecules are obtained after spontaneous decay of photoassociated molecules in a MOT and in a dark SPOT. We also used polarized atoms, in the f =3, m f =+3Zeeman ground state. Purely asymptotic and adiabatic calculations including hyperfine interaction and rotation are in excellent agreement with observed structures. As expected, the 1_u state is actually a pure long-range state, consisting of paired atoms, uniquely linked by the first terms of the multipole expansion of the electrostatic interaction. A temperature of 20 K has been measured for the molecular cloud. Received 19 July 1999     

Creating ground state molecules with optical feshbach resonances in tight traps

We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by adiabatic crossing of an optical Feshbach resonance. We envision a scheme where the laser intensity and possibly also frequency are linearly ramped over the resonance. Our calculations for (87)Rb show that for sufficiently tight traps it is possible to avoid spontaneous emission while retaining adiabaticity, and conversion efficiencies of up to 50% can be expected.