Rovibrational Ground State Research Articles

Optical spectroscopy of tungsten carbide for uncertainty analysis in electron electric-dipole-moment search

We perform laser-induced fluorescence spectroscopy on a pulsed supersonic beam of tungsten carbide (WC) molecules, which has been proposed as a candidate molecular system for a permanent electric dipole moment (EDM) search of the electron in its rovibrational ground state of the $X\phantom{\rule{0.16em}{0ex}}{}^{3}{\ensuremath{\Delta}}_{1}$ state. In particular, $[20.6]\ensuremath{\Omega}=2,{v}^{\ensuremath{'}}=4\ensuremath{\leftarrow}X\phantom{\rule{0.16em}{0ex}}{}^{3}{\ensuremath{\Delta}}_{1},{v}^{\ensuremath{'}\ensuremath{'}}=0$ transition at 485 nm was used for the detection. The hyperfine structure and the $\ensuremath{\Omega}$ doublet of the transition is measured, which are essential for estimating the size of the potential systematic uncertainties for electron EDM measurement. For further suppression of the systematic uncertainty, an alternative electron EDM measurement scheme utilizing the $g$ factor crossing point of the $\ensuremath{\Omega}$-doublet levels is discussed. On the other hand, flux and internal temperature of the molecular beam are characterized, which sets the limit on the statistical uncertainty of the electron EDM experiment. With the given results, the prospect of electron EDM experiment with the $X\phantom{\rule{0.16em}{0ex}}{}^{3}{\ensuremath{\Delta}}_{1}$ state of WC molecule is discussed.

Blackbody-mediated rotational laser cooling schemes in MgH+, DCl+, HCl+, LiH and CsH

Ensembles of ultra-cold atoms, molecules and ions (both atomic and molecular) can be held in traps for increasingly long periods of time. While these trapped species remain translationally cold, for molecules the absorption of ambient black-body radiation can result in rapid thermalisation of the rotational (and vibrational) degrees of freedom. At 300 K, internal state purity is lost typically on the order of tens of seconds, inhibiting the study of quantum state selected reactions. In this paper a theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several (1)Σ and (2)Π diatomic species. The rotational cooling is particularly effective for DCl(+) and HCl(+), for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source (simultaneously exciting up to four transitions) is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% over that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the rotational cooling achievable is also considered. An extension to polyatomic species is discussed and a combination of cold trap environments (at 77 K) and optical pumping schemes is proposed.

Full-dimensional quantum dynamics study of exchange processes for the D + H2O and D + HOD reactions

The exchange processes of D + H(2)O and D + HOD reactions are studied using initial state-selected time-dependent wave packet approach in full dimension. The total reaction probabilities for different partial waves, together with the integral cross sections, are obtained both by the centrifugal sudden (CS) approximation and exact coupled-channel (CC) calculations, for the H(2)O(HOD) reactant initially in the ground rovibrational state. In the CC calculations, small resonance peaks in the reaction probabilities and quick diminishing of the resonance peaks with the increase of total angular momenta J do not lead to clear step-like features just above the threshold in the cross sections for the title reactions, which are different in other isotopically substituted reactions where the hydrogen atom was included as the reactant instead of the deuterium atom [B. Fu, Y. Zhou, and D. H. Zhang, Chem. Sci. 3, 270 (2012); B. Fu and D. H. Zhang, J. Phys. Chem. A 116, 820 (2012)]. It is interesting that the shape resonance-induced features resulting from the reaction tunneling are significantly diminished accordingly in the reactions of the deuterium atom and H(2)O or HOD, owing to the weaker tunneling capability of the reagent deuterium atom in the title reactions than the reagent hydrogen atom in other reactions. In the CS calculations, the resonance peaks persist in many partial waves but cannot survive the partial-wave summations. The cross sections for the D(') + H(2)O → D(')OH + H and D(') + HOD → D(')OD + H reactions are substantially larger than those for the D(') + HOD → HOD(') + D reaction, indicating that the D(')/H exchange reactions are much more favored than the D(')/D exchange.

Full-Dimensional Quantum Dynamics Study of the H + H2O and H + HOD Exchange Reactions

The initial state-selected time-dependent wave packet approach is employed to study the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H, HOH' + D exchange reactions with both OH bonds in the H(2)O reactant and OH(D) bond in the HOD reactant treated as reactive bonds. The total reaction probabilities for different partial waves, as well as the integral cross sections, which are the exact CC (coupled-channel) results, are first obtained in this study for the H(2)O(HOD) reactant initially in the ground rovibrational state. Because of the shallow C(3v) minimum along the reaction path, the reaction probabilities for the three reactions present several resonance peaks, with one dominant resonance peak just above the threshold. The cross sections for the H' + HOD → HOH' + D reaction are substantially smaller than those for the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H reactions, indicating that the H'/H exchange reactions are much more favored. In the CC calculations, the resonance peaks in the reaction probabilities diminish quickly with the increase in total angular momenta J, resulting in the existence of a clear step-like feature just above the threshold in the cross sections for the title reactions, which manifests the signature of shape resonances in these reactions. In the CS calculations, the resonance peaks on reaction probabilities persist in many partial waves, and thus the resonance structures can no longer survive the partial-wave summation and are washed out completely in the CS cross sections for the title reactions.

Differential reaction cross sections from rotationally resolved quantum scattering calculations: application to gas-phase SN2 reactions

Differential reaction cross sections have been computed based on previous rotationally resolved time-independent quantum-mechanical scattering calculations for the complex-forming S(N)2 reaction Cl(-) + CH(3)Br → ClCH(3) + Br(-). The results show almost isotropic cross sections for reactant molecules with high rotational quantum numbers. Backward scattering is disfavoured for reaction out of states with small rotational excitation, in particular the rovibrational ground state. This is a quantum-mechanical effect (interference of partial waves) that can partly be rationalized by simple classical arguments. In particular for higher vibrational excitations, an umbrella effect can be observed that favours the backward direction. It can be explained by the strong enhancement of the reactivity by opening a direct mechanism. The ion-dipole interaction exerts a torque onto the molecule which carries out a rotation by about 90° and then completes the reaction.

Communication: State-to-state quantum dynamics study of the OH + CO → H + CO2 reaction in full dimensions (J = 0)

A full dimensional state-to-state quantum dynamics study is carried out for the prototypical complex-formation OH + CO → H + CO(2) reaction in the ground rovibrational initial state on the Lakin-Troya-Schatz-Harding potential energy surface by using the reactant-product decoupling method. With three heavy atoms and deep wells on the reaction path, the reaction represents a huge challenge for accurate quantum dynamics study. This state-to-state calculation is the first such a study on a four-atom reaction other than the H(2) + OH ↔ H(2)O + H and its isotope analogies. The product CO(2) vibrational and rotational state distributions, and product energy partitioning information are presented for ground initial rovibrational state with the total angular momentum J = 0.

An atom–molecule platform for quantum computing

We propose a combined atom---molecule system for quantum information processing in individual traps, such as provided by optical lattices. In this platform, different species of atoms--one atom carrying a qubit and the other enabling the interaction--are used to store and process quantum information via intermediate molecular states. We show how gates, initialization, and readout operations could be implemented using this approach. In particular, we describe in some detail the implementation of a two-qubit phase gate in which a pair of atoms is transferred into the ground rovibrational state of a polar molecule with a large dipole moment, thus allowing atoms transferred into molecules to interact via their dipole-dipole interaction. We also discuss how the reverse process could be used as a non-destructive readout tool of molecular qubit states. Finally, we generalize these ideas to use a decoherence-free subspace for qubit encoding to minimize the decoherence due to magnetic field fluctuations. In this case, qubits will be encoded into field-insensitive states of two identical atoms, while a third atom of a different species will be used to realize a phase gate.

Significant nonadiabatic effects in the C + CH reaction dynamics

Rigorous quantum nonadiabatic calculations are carried out on the two coupled electronic states (1(2)A' and 2(2)A') for the C + CH reaction. For all calculations, the initial wave packet was started from the entrance channel of the 1(2)A' state and the initial state of the CH reactant was kept in its ground rovibrational state. Reaction probabilities for total angular momenta J from 0 to 160 are calculated to obtain the integral cross section over an energy range from 0.005 to 0.8 eV collision energy. Significant nonadiabatic effects are found in the reaction dynamics. The branching ratio of the ground state and excited state of C(2) produced is around 0.6, varying slightly with the collision energy. Also, a value of 2.52 × 10(-11) cm(3) molecule(-1) s(-1) for the state selected rate constant k (v = 0, j = 0) at 300 K is obtained, which may be seen as a reference in the future chemical models of interstellar clouds.

Coherent Transfer of Photoassociated Molecules into the Rovibrational Ground State

We report on the direct conversion of laser-cooled 41K and 87Rb atoms into ultracold 41K87Rb molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

Feshbach-to-ultracold molecular state Raman transitions via a femtosecond optical frequency comb

We report on a theoretical study of implementation of a single femtosecond optical frequency comb to perform an efficient transfer of Feshbach molecules to the ground electronic and rovibrational state. A standard optical frequency comb is considered with zero offset frequency and a sinusoidally phase modulated frequency comb, having modulation frequency in the IR region. A stepwise population transfer to the ultracold molecular state is observed, with a negligible population of the excited state in the case of the sinusoidally modulated frequency comb.

External field control of collective spin excitations in an optical lattice of 2Σ molecules

We show that an ensemble of 2Σ molecules in the ro-vibrational ground state trapped on an optical lattice exhibits collective spin excitations that can be controlled by applying superimposed electric and magnetic fields. In particular, we show that the lowest energy excitation of the molecular ensemble at certain combinations of electric and magnetic fields leads to the formation of a magnetic Frenkel exciton. The exciton bandwidth can be tuned by varying the electric or magnetic fields. We show that the exciton states can be localized by creating vacancies in the optical lattice. The localization patterns of the magnetic exciton states are sensitive to the number and distribution of vacancies, which can be exploited for engineering many-body entangled spin states. We consider the dynamics of magnetic exciton wavepackets and show that the spin excitation transfer between molecules in an optical lattice can be accelerated or slowed down by tuning an external magnetic or electric field.

Sympathetic Cooling of Molecular Ions in Selected Rotational and Vibrational States Produced by Threshold Photoionization

We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N₂(+) ions as a test system, we achieve >90% selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.

Permanent dipole moment of LiCs in the ground state

Recently we demonstrated the formation of ultracold polar LiCs molecules in deeply bound levels of the X1S+ ground state, including the rovibrational ground state [J. Deiglmayr et al., Phys. Rev. Lett. 101, 133004 (2008)]. Here we report on the first experimental determination of the permanent electric dipole moment of deeply bound LiCs molecules. For X1S+,v"=2 and v"=3 we measure values of mu=5.5(2) Debye and 5.3(2) Debye respectively.

All-optical preparation of molecular ions in the rovibrational ground state

Molecular targets prepared in well-defined quantum states play an essential part in a wide range of fields, from metrology to astrochemistry. Now, HD+ ions have been prepared in their lowest vibrational and rotational level using a laser-cooling scheme. This provides a fresh approach for exploring such phenomena and applications experimentally.

Phase gate and readout with an atom-molecule hybrid platform

We suggest a combined atomic/molecular system for quantum computation, which takes advantage of highly developed techniques to control atoms and recent experimental progress in manipulation of ultracold molecules. We show that two atoms of different species in a given site, {\it e.g.}, in an optical lattice, could be used for qubit encoding, initialization and readout, with one atom carrying the qubit, the other enabling a gate. In particular, we describe how a two-qubit phase gate can be realized by transferring a pair of atoms into the ground rovibrational state of a polar molecule with a large dipole moment, and allowing two molecules to interact via their dipole-dipole interaction. We also discuss how the reverse process of coherently transferring a molecule into a pair of atoms could be used as a readout tool for molecular quantum computers.

Collective motion of polarized dipolar Fermi gases in the hydrodynamic regime

Recently, a seminal STIRAP experiment allowed the creation of 40K-87Rb molecules in the rovibrational ground state [K.-K. Ni et al., Science 322, 231 (2008)]. In order to describe such a polarized dipolar Fermi gas in the hydrodynamic regime, we work out a variational time-dependent Hartree-Fock approach. With this we calculate dynamical properties of such a system as, for instance, the frequencies of the low-lying excitations and the time-of-flight expansion. We find that the dipole-dipole interaction induces anisotropic breathing oscillations in momentum space. In addition, after release from the trap, the momentum distribution becomes asymptotically isotropic, while the particle density becomes anisotropic.

A five-dimensional quantum dynamics study of the F(P2)+CH4 reaction

By applying the semirigid vibrating rotor target (SVRT) model to the title reaction, five-dimensional wave packet quantum dynamics calculations have been carried out on the new potential energy surface PES-2006 [Espinosa-Garcia et al., J. Phys. Chem. A 111, 2761 (2007)]. The reaction probabilities have been calculated for total angular momentum J up to 105 to obtain the converged integral cross sections over a collision energy range of 0.01-0.345 eV. With the polyatomic system initially in its ground rovibrational state |000>, no obvious resonance signature has been observed in the integral cross sections although it appears in the reaction probabilities for J < or = 40. However, when the umbrella mode of the collision system is initially excited to nu(alpha0) = 1 level, |001> state, there is resonance signature in both the reaction probabilities with J < or = 55 and the integral cross sections. In addition, rate constants are calculated by Boltzmann averaging of the |000> integral cross sections over collision energy and compared with both the previous kinetic calculations and the experimental measurements. A reasonable agreement has been achieved over the investigated temperature range of 180-400 K.

Effects of the rotational excitation of D2 and of the potential energy surface on the H++D2→HD+D+ reaction

The H(+) + D(2) --> HD + D(+) reaction has been theoretically investigated by means of an exact quantum mechanical approach, a quasiclassical trajectory method, and two statistical methods based in the propagation of either wave functions or trajectories. The study addresses the possible changes on the overall dynamics of the title reaction when the D(2) diatom is rotationally excited to its v = 0, j = 1 state. In addition, the reactivity for the ground rotational state on two different potential energy surfaces (PESs), namely, the surface by Aguado et al. [J. Chem. Phys. 112, 1240 (2000)] and the PES by Kamisaka et al. [J. Chem. Phys. 116, 654 (2002)], is examined. Reaction probabilities and cross sections at 0.524 and 0.1 eV collision energies are calculated. The major differences with respect to the reaction initiated with D(2) in its ground rovibrational state are observed for the lowest collision energy E(c) = 0.1 eV. Differential cross sections have been found to depend to some extend on the PES employed. In addition, at E(c) = 0.1 eV further discrepancies in the total and rotational cross sections are noticeable.

Relativistic hybrid density functional calculations of indirect nuclear spin–spin coupling tensors — Comparison with experiment for diatomic alkali metal halides,

The accurate calculation of the isotropic (Jiso) and anisotropic (ΔJ) parts of indirect nuclear spin–spin coupling tensors is a stringent test for quantum chemistry, particularly for couplings involving heavy isotopes where relativistic effects and relativity – electron correlation cross terms are expected to play an important role. Experimental measurements on diatomic molecules in the gas phase offer ideal data for testing the success of computational approaches, since the data are essentially free from intermolecular effects, and precise coupling anisotropies may be reliably extracted in favourable cases. On the basis of available experimental molecular-beam coupling-tensor parameters for diatomic alkali metal halides, we tabulate known values of Jiso and, taking rotational–vibrational corrections to the direct dipolar coupling constant into account, precise values of ΔJ are determined for the ground rovibrational state. First-principles calculations of the coupling tensors were performed using a recently developed program based on hybrid density functional theory using the two-component relativistic zeroth-order regular approximation (ZORA). Experimental trends in Jiso and ΔJ are reproduced with correlation coefficients of 0.993 and 0.977, respectively. Periodic trends in the coupling constants and their dependence on the product of the atomic numbers of the coupled nuclei are discussed. Finally, the hybrid functional method is also successfully tested against experimental data for a series of polyatomic xenon fluorides and group-17 fluorides.

Dynamics of thermal Casimir-Polder forces on polar molecules

We study the influence of thermal Casimir-Polder forces on the near-surface trapping of cold polar molecules, with emphasis on LiH and YbF near an Au surface at room temperature. We show that for a molecule initially prepared in its electronic and rovibrational ground state, the Casimir-Polder force oscillates with the molecule-wall separation. The non-resonant force contribution and the evanescent part of the resonant force contribution almost exactly cancel at high temperature which results in a saturation of the (attractive) force in this limit. A dynamical calculation reveals how the spatial oscillations die out as thermalisation of the molecule with its environment sets in.