Using an ultracold gas of 87Rb 133Cs molecules, we perform hyperfine-resolved spectroscopy of transitions from the vibronic ground state to the lowest rovibrational states of the electronic state b 3II0 as a function of magnetic field. These transitions are spin forbidden, resulting in narrow linewidths, and feature near-diagonal Franck-Condon factors. We develop a model of the hyperfine and Zeeman structure that includes coupling between the 0+ and 0-components of b 3II0. We fit the spectra to obtain rotational and hyperfine coupling constants. We measure transition dipole moments associated with specific transitions by directly observing Rabi oscillations as a function of a resonant laser pulse duration. Using resonant n pulses, we prepare molecules in the electronically excited state and directly measure the spontaneous emission rate.

High-resolution laser spectroscopy of the ^85Rb_2, ^85Rb^87Rb, and ^87Rb_2 isotopologues has been performed in a supersonic molecular beam using a continuous-wave (cw) tunable diode laser. A total of 958 rovibronic transitions were recorded up to 7 cm^-1 below the vibrational band heads of the X^1Σ_g^+(v”=0)→ B^1Π_u(v'=1,2) and X^1Σ_g^+(v”=1)→ B^1Π_u(v'=1) bands, with a spectral resolution of 3.3×10^-4 cm^-1. Although restricted to the v'=1 and v'=2 vibrational levels of the excited B^1Π_u state, the measurements extend previous work by Amiot and Vergès [Chemical Physics Letters 274, 91 (1997)] through substantially higher resolution and dense low-J' rotational data for all three isotopologues. A global least-squares analysis combining the new and published data, yields improved Dunham coefficients for the excited B^1Π_u state, significantly refines rotational and rovibrational coupling constants. In addition, the Λ-doubling constants of B^1Π_u state were determined for the three isotopologues.

Molecular hydrogen (H_2) and hydrogen deuteride (HD) are key coolants in primordial gas and regulate the formation of the first stars and proto-galaxies. Recent results from the James Webb Space Telescope provide striking insights into galaxies detected at high redshifts, which are found to be significantly more abundant and luminous than expected from galaxy formation models, thus suggesting a gap in our understanding of the early Universe. Standard pathways for H_2 formation in the early Universe proceed through the H^- and H_2^+ intermediates, both of which are strongly suppressed at high redshift by the cosmic microwave background. We propose an additional pathway for H2 and HD formation that could be active as early as the end of the epoch of recombination and could enable the formation of the first stars earlier than the current prediction at redshift z 30 - 20. The proposed pathway relies on the manifestation of Jahn-Teller dynamical coupling between electronic states of H_3^+. This coupling induces transient three-body recombination in H^+, H and H, and charge exchange within the charged atom-dimer complex that directly creates ground-state H_2 (and HD), bypassing the fragile intermediates that limit the standard primordial pathways. Our analysis shows that this mechanism could occur under the thermodynamic conditions of the post-recombination epoch, also suggesting that it might be playing a role in the active galactic nuclei feedback processes, regulating the formation rates of the first stars and the accretion rates of the first black holes. Though the global impact on galaxy formation and black-hole growth is not yet determined and will require quantitative assessment in future modeling, the mechanism offers an additional chemical route for H_2 and HD formation, with substantial cosmological relevance for primordial chemistry and early structure formation.

The quantum dynamics of ultracold collisions between rubidium atoms and excited metastable strontium ions is treated in the laboratory frame, enlightening the importance of the coupling between internal angular momenta of the particles and their mutual rotation. The study reveals a subtle competition between electronic excitation exchange and fine structure quenching, with no charge exchange, which is found to be very sensitive to the details of ion-atom interactions. The rate constant for electronic excitation exchange is found in agreement with the experimental results of Ben-Shlomi et al. (Phys. Rev. A 102, 031301(R) (2020)), while the rate for fine structure quenching is predicted to strongly depend on the initial polarization of the reactants.

We have performed hyperfine spectroscopy of two transitions in ground-state deuterium and searched for violations of CPT and Lorentz symmetry that would manifest as sidereal variations of the observed transition frequencies. Several nonrelativistic proton coefficients of the Standard-Model extension framework have been addressed. The spin-independent coefficients with momentum power k=2, 4 are constrained for the first time. Bounds on spin-dependent coefficients are improved by exploiting a sensitivity enhancement originating from the relative momenta of the nucleons in the deuteron. The best previous constraints by hydrogen maser measurements are surpassed by 4 and 14 orders of magnitude for coefficients with k=2 and 4, respectively.

We study the rovibronic transitions in NaK 2 between its electronic ground state 1 2 A ′ and its second excited state 3 2 A ′ , to identify possible pathways for the creation of ultracold ground-state triatomic molecules. Our methodology relies on the computation of potential energy surfaces and transition dipole moment surfaces for the relevant electronic states using methods. Rovibrational energy levels and wave functions are determined using the discrete variable representation approach. A double-well structure of the potential energy surface is identified for both states, and the related transition strengths between the rovibrational levels are derived. Our calculations show that the formation of ultracold ground-state NaK 2 molecules is expected when starting from an excited electronic state of NaK 2 , which can be created by photoassociation of NaK and K, as observed by optical means by Cao, [].

Using an ultracold gas of ^{87}Rb^{133}Cs molecules, we perform hyperfine-resolved spectroscopy of transitions from the vibronic ground state to the lowest rovibrational states of the electronic state b^{3}Π_{0} as a function of magnetic field. These transitions are spin forbidden, resulting in narrow linewidths, and feature near-diagonal Franck-Condon factors. We develop a model of the hyperfine and Zeeman structure that includes coupling between the 0^{+} and 0^{−} components of b^{3}Π_{0}. We fit the spectra to obtain rotational and hyperfine coupling constants. We measure transition dipole moments associated with specific transitions by directly observing Rabi oscillations as a function of a resonant laser pulse duration. Using resonant π pulses, we prepare molecules in the electronically excited state and directly measure the spontaneous emission rate.

We theoretically investigate the collisions between ultracold polar molecules in the presence of two lasers ensuring a Raman resonant transition on individual molecules to suppress photon scattering, taking the example of bosonic ^23Na^39K molecules. By varying laser detunings and intensities, we enable a repulsive long-range interaction potential between molecules. After solving a set of coupled Schrödinger equations with the Hamiltonian written in the basis of laser-dressed states of the molecule pair at infinite distance, we identify quasi-resonant conditions under which elastic collisions are favored over inelastic and reactive ones, by a factor of about 2, thus demonstrating a promising pathway for efficient two-photon optical shielding of ultracold molecular collisions. The results are analyzed in terms of scattering length of the colliding laser-dressed molecules, which exhibit prominent resonances assigned to the interaction of the entrance channel with other specific channels, consistent with the existence of a quasi-bound level of the long-range molecular pair induced by the lasers.

We present a theoretical formalism to treat the ultracold dynamics of a pair of colliding polar molecules submitted to two laser fields. We express the dressed Hamiltonian including the dipole-dipole interaction of the colliding molecular pair, both in their ground and electronic excited states, as well as their interaction with the two laser fields. We apply adiabatic elimination of the electronic excited state to reduce the size of the dressed-state basis in which the dressed Hamiltonian is expressed. In an application, we investigate the feasibility of two-photon collisional shielding between two 23Na39K molecules, which could be favored by the Raman resonance condition suppressing unwanted spontaneous emission and photon scattering. We demonstrate the influence of laser Rabi frequencies on the dynamics through the computation of elastic, inelastic, and reactive collision rates, which is still insufficient to ensure noticeable shielding.

We study the rovibronic transitions in NaK_2 between its electronic ground state 1^2A' and its second excited state 3^2A', to identify possible pathways for the creation of ultracold ground-state triatomic molecules. Our methodology relies on the computation of potential energy surfaces and transition dipole moment surfaces for the relevant electronic states using ab initio methods. Rovibrational energy levels and wave functions are determined using the discrete variable representation approach. A double-well structure of the potential energy surface is identified for both states, and the related transition strengths between the rovibrational levels are derived. Our calculations show that the formation of ultracold ground-state NaK_2 molecules is expected when starting from an excited electronic state of NaK_2, which can be created by photoassociation of NaK and K observed by optical means by Cao et al. (Phys. Rev. Lett. 2024, 132, 093403).

We propose a platform for observing and controlling the interactions between atomic ions and a quantum gas of polar molecules in the ultracold regime. This approach is based on the combination of several recently developed methods in two so-far complementary research domains: ion-atom collisions and studies of ultracold polar molecules. In contrast to collisions between ions and ground-state atoms, which are dominated by losses due to three-body recombination (TBR) already at densities far below those typical for quantum degenerate ensembles, our proposal makes use of polar molecules, their rich level structure, and sensitivity to electric fields to design effective interaction potentials where ion-neutral TBR losses and molecule-molecule losses due to sticky collisions could be strongly suppressed. This may open a broad range of applications including precise control of collisional properties in molecular ensembles using ions, quantum simulations, and cold quantum chemistry between polyatomic molecules.

A theoretical model is proposed for the formation of ultracold ground-state triatomic molecules in weakly bound energy levels. The process is driven by the electric component of a microwave field, which induces the association of an ultracold atom colliding with an ultracold diatomic molecule. This model is exemplified using ^39K atoms and ^23Na^39K molecules, both in their ground states, a scenario of experimental relevance. The model assumes that the dynamics of the association are dominated by the long-range van der Waals interaction between ^39K and ^23Na^39K. The electric microwave association mechanism relies on the intrinsic electric dipole moment of ^23Na^39K, which drives transitions between its lowest rotational levels ( j=0 and j=1). The energies of the uppermost triatomic energy levels are computed by numerically solving coupled Schrödinger equations using the Mapped Fourier Grid Hamiltonian method. Measurable association rates are derived within the framework of a perturbative approach. This method of electric microwave association provides an alternative to atom-molecule association via magnetic Feshbach resonances for forming ultracold, deeply bound triatomic molecules, and is applicable to a wide range of polar diatomic molecules.

We report the first spectroscopic investigation of the NaSr molecule. Spectra related to the B(2)2Σ+ → X(1)2Σ+ transition were observed with partial rotational resolution by thermoluminescence and laser-induced fluorescence techniques. Simultaneously, potential energy curves of the lowest electronic states of NaSr and transition dipole moments were calculated by using two different theoretical approaches. Comparison with theoretical results allowed to interpret the experimental spectra and deduce the salient molecular constants of the X(1)2Σ+ and B(2)2Σ+ states. Reliability of the employed theoretical methods was tested.

We report the measurement of the ionization energy of the Rb-85(2) molecule through resonantly enhanced two-photon ionization in a supersonic beam. The first photon excites the X-1 Sigma(+)(g)(v(X) = 0) -> B-1 Pi(u)(v(B) = 2) transition, while the second photon wave number is scanned over the 16 730-16 755 cm(-1) range, thus yielding a structured spectrum of Rb-2(+) ions extracted by an electric field and recorded by mass spectrometry. We modeled the onset of the ionization signal as a function of the electric field strength between 18 and 180 V/cm, leading to the Rb-2 ionization energy E-i = 31497.3 +/- 0.6 cm(-1) and to the dissociation energy of the Rb-2(+) ground state D-0 = 6158.2 +/- 0.6 cm(-1). Our measured value E-i is found to be 149.3 cm(-1) larger than the one reported in the experiment by Bellos et al. [Phys. Rev. A 87, 012508 (2013)]. Our value of D-0 agrees with our theoretical determination using a quantum chemistry approach. Using a simple theoretical model, we assign unevenly spaced structures of the ionization spectrum to molecular Rydberg levels belonging to several series that converge to the lowest vibrational levels of Rb-2(+).

The formation of Li_2^+ and subsequently Li^+ ions, during the excitation of ^7Li atoms to the 3S_1/2 state in a ^7Li magneto optical trap (MOT), is probed in an ion-atom hybrid trap. Associative ionization occurs during the collision of Li(2P_3/2) and Li(3S_1/2) ultracold atoms, creating Li_2^+ ions. Photodissociation of Li_2^+ by the MOT lasers is an active channel for the conversion of Li_2^+ to Li^+. A fraction of the Li_2^+ ions is long lived even in the presence of MOT light. Additionally, rapid formation of Li^+ from Li_2^+ in the absence of MOT light is observed. Resonant excitation of ultracold atoms, resulting in intricate molecular dynamics, reveals important processes in ultracold dilute gases.

Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, asthey feature both excellent control over the electronic state preparation and readout as well as aversatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single138Ba+ion prepared in the5d2D3/2,5/2metastable states with a ground state6Li gas near quantum degeneracy. We showthat in contrast to previously reported atom-ion mixtures, several non-radiative processes,including charge exchange, excitation exchange and quenching, compete with each other due tothe inherent complexity of the ion-atom molecular structure. We present a full quantum modelbased on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internalangular momenta and the mechanical rotation of the colliding pair, which is relevant in anyother hybrid system composed of an alkali-metal atom and an alkaline-earth ion

Based on an accurate determination of the potential energy surfaces of Rb+ 3 correlated to its first asymptotic limit Rb+ + Rb(5s) + Rb(5s), we identify the presence of intersections of a pair of singlet and triplet surfaces over all interparticle distances, leading to Jahn-Teller (JT) couplings. We elaborate scenarios for charge exchange between ultracold charged atom-dimer complex (Rb + Rb+ 2 or Rb+ + Rb2), predicting a strong selectivity on the preparation of the initial state of the dimer. We also demonstrate that the JT couplings must drive the threebody recombination (TBR) of Rb+, Rb, and Rb at ultracold energies. Using the current analysis, we provide a consistent picture of the TBR experiments performed in ion-atom hybrid Rb samples. We also demonstrate the presence of JT coupling as a general phenomenon in the singly charged homonuclear alkali triatomic systems.

For a homonuclear ion-atom system, we show that the exchange symmetry leads to special outcomes for ion transport that manifest themselves in ultracold experiments. We compute the two body charge hopping probabilities and rates, which are used to model charge hopping in the dynamics of an ultracold 6/7 Li+ ion immersed within an ultracold gas of 6/7 Li atoms at micro-Kelvin temperatures. We show that the charge hopping and collisional diffusion compete, giving unique results leading to charge trapping in regions of high atomic density gradient. These investigations in ion-atom systems open a new approach to probe quantum phenomena in various systems with exchange symmetry.