We report on a cycling scheme for Doppler cooling of trapped OH + ions using transitions between the electronic ground state X 3 Σ – and the first excited triplet state A 3 Π. We have identified relevant transitions for photon cycling and repumping, have found that coupling into other electronic states is strongly suppressed, and have calculated the number of photon scatterings required to cool OH + to a temperature where Raman sideband cooling can take over. In contrast to the standard approach, where molecular ions are sympathetically cooled, our scheme does not require co-trapping of another species and opens the door to the creation of pure samples of cold molecular ions with potential applications in quantum information, quantum chemistry, and astrochemistry. The laser cooling scheme identified for OH + is efficient despite the absence of near-diagonal Franck–Condon factors, suggesting that broader classes of molecules and molecular ions are amenable to laser cooling than commonly assumed.
We report on a cycling scheme for Doppler cooling of trapped OH + ions using transitions between the electronic ground state X 3 Σ – and the first excited triplet state A 3 Π. We have identified relevant transitions for photon cycling and repumping, have found that coupling into other electronic states is strongly suppressed, and have calculated the number of photon scatterings required to cool OH + to a temperature where Raman sideband cooling can take over. In contrast to the standard approach, where molecular ions are sympathetically cooled, our scheme does not require co-trapping of another species and opens the door to the creation of pure samples of cold molecular ions with potential applications in quantum information, quantum chemistry, and astrochemistry. The laser cooling scheme identified for OH + is efficient despite the absence of near-diagonal Franck–Condon factors, suggesting that broader classes of molecules and molecular ions are amenable to laser cooling than commonly assumed.
Abstract We investigate an optical cycling scheme for Doppler cooling cold trapped 11B14N− ions using transitions between the X 2Σ+ ground state and the B 2Σ+ excited state, and analyze here the relevant transitions for photon cycling and repumping. Our results show that slow population decay via the first excited electronic state A 2Π cannot be neglected. To improve the optical cycling efficiency, we consider additional transitions beyond what would be expected from the highly diagonal FranckCondon factor involving the B 2Σ+(v = 0) ← X 2Σ+(v = 0) transition. We estimate that the number of cycled photons alone is not likely to be sufficient to bring buffer-gas-cooled 11B14N− to temperatures near the Doppler cooling limit. Hence, pre-cooling, e.g., using a combination of cryogenic buffer gas and photodetachment cooling, will be essential to maximize the optical cycling efficiency and to reach a regime where Coulomb crystallization occurs. To explore pre-cooling with He or Ar buffer gases, we therefore also performed extensive quantum calculations of potential energy curves, transition moments and radiative rate coefficients for the BN−–buffer gas systems, to be implemented in a later study. Our results provide key insights for generating cold negative ions. These anions have, in fact, promising applications in various fields, ranging from quantum science and technology to fundamental physics and chemistry.
Although ion-ion sympathetic cooling is now commonly used to reduce speeds of trapped molecular ions to milliKelvin temperatures, cooling of the decoupled internal molecular modes remains a challenge. Here, we demonstrate that a single spectrally filtered femtosecond laser, tuned to the electronic A-X transition of trapped AlH+, can efficiently cool rotations from room temperature to the ground state. Since this single-laser technique simultaneously drives rotationally de-exciting electronic excitations from all initially populated states, each parity can be cooled as quickly as 100 μs. In our current implementation, full population collection into the N = 0 positive-parity level relies on vibrational relaxation on the timescale of 100 ms. Since this technique cools by step-wise rotational pumping, it is most readily applicable to “alkali-like” molecular species, such as AlH+, with semi-closed cycling transitions. Although we currently use a destructive two-photon state readout technique, these species are also promising candidates for single-molecule fluorescence detection. The ability to quickly cool molecular rotations is expected to be important for precision spectroscopy experiments, quantum information processing applications, and for ultracold chemistry studies.
The potential energy curves and transition dipole moments of seven Λ-S states and six Ω states of LuH+ cation are calculated by using the multireference configuration interaction method plus Davidson correction with the pseudopotential electron basis set, and the spectroscopic parameters of the bound states are obtained. The spin–orbit coupling effect is considered in the calculations. The Franck–Condon factors (FCFs) and the Einstein spontaneous emission coefficients are obtained. The results indicate that the A2Δ3/2(ν′ = 0) ↔ X2Σ+1/2(ν″ = 0) and B2Π1/2(ν′ = 0) ↔ X2Σ+1/2 (ν″ = 0) transitions provide highly diagonally distributed FCFs. Spontaneous radiative lifetimes of A2Δ3/2 (ν′ = 0) and B2Π1/2 (ν′ = 0) excited states are both obtained. The cooling schemes of LuH+ cation are obtained using the ab initio method. For the B2Π1/2 ↔ X2Σ+1/2 transition, the influence of the intermediate state on the construction of quasi-closed cyclic transitions can be neglected. A main pump laser and two repumping lasers are required to construct the A2Δ3/2 ↔ X2Σ+1/2 and B2Π1/2 ↔ X2Σ+1/2 quasi-cycling transitions for laser cooling of LuH+ molecular cation. The results in this work provide reliable theoretical guidance for further experiments of laser cooling of LuH+ cation.
Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple-repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling of particles without closed transitions, identify conditions necessary to achieve efficient cooling, and suggest solutions given experimental constraints. Specifically, it is shown that cooperativities much greater than unity are required for cooling without loss, and that this could be achieved via the superradiant scattering associated with intracavity self-localization of the molecules. Particular emphasis is given to the polar hydroxyl radical (OH), cold samples of which are readily obtained from Stark deceleration.
We demonstrate sympathetic Doppler cooling and our progress towards sideband cooling of a CaH+ ion co-trapped with a Ca+ atomic ion in a linear Paul trap. Molecular ions are generally difficult to laser cool due to a lack of closed electronic transitions as a result of vibrational and rotational states. Despite this challenge, they can be cooled indirectly through their Coulombic interaction with a fluorescent atomic ion that is being directly laser cooled. Ions are firstly Doppler cooled to get to the Lamb-Dicke regime, where the ion motion is small relative to the excitation wavelength and then sideband cooled reaching temperatures below 1 μK. All the ions' axial modes (center of mass and breathing mode) and radial modes ( two center of mass and two tilt modes) are addressed, and the temperature is determined by examining the ratio of sidebands.
Abstract Aiming at a laser cooling investigation, we have used ab initio complete active space self consistent field (CASSCF)/(MRCI +Q) calculations to study the electronic structure of the diatomic molecule CaK and its molecular ions CaK+ and CaK−. The potential energy curves and the static dipole moment curves have been investigated for the considered molecules along with the spectroscopic constants T e, ω e, B e, and R e, in addition to the values of dipole moment μ e and dissociation energy D e. Overall, 19 and 14 electronic states have been studied respectively for CaK, CaK+, from which 12 for CaK and six for CaK+ have been investigated here for the first time. Our obtained results agree well with data related to states that have been previously examined. Nineteen electronic states have been explored for CaK−, which up to our knowledge have not been previously calculated. The transition dipole moments have been calculated for the lowest Σ+–Σ+ and Σ+–Π transitions along with the Franck–Condon factor, Einstein coefficient, the spontaneous radiative lifetime, and the emission oscillator strength corresponding to the investigated transitions. A ro-vibrational analysis has been done via the canonical function approach, where the vibrational parameters E v, B v, D v, and the turning points R min and R max have been determined. These calculations showed that the molecule CaK is a suitable candidate for Doppler laser cooling, and we propose a laser cooling scheme to this end. The Doppler limit temperature T D and recoil temperature T r have values as low as T D = 51 μK and T r = 156 nK. The results should provide a useful reference for experimental spectroscopic and ultra-cold molecular physics studies.