Electronically excited-state properties and predissociation mechanisms of phosphorus monofluoride: a theoretical study including spin-orbit coupling

A theoretical study on excited-state dynamical properties and laser cooling of zinc monohydride including spin-orbit couplings

By means of highly accurate ab initio and dynamical calculations, we identify a suitable laser cooling candidate that contains a transition metal element, namely zinc monohydride (ZnH). The internally contracted multireference configuration interaction method is employed to compute the five lowest-lying Λ-S states of ZnH with the spin-orbit coupling effects included, and very good agreement is obtained between our calculated and experimental spectroscopic data. Our findings show that the position of crossing point of the A2Π and B2Σ+ states of ZnH is above the v' = 2 vibrational level of the A2Π state indicating that the crossings with higher electronic states will have no effect on laser cooling. Hence, we construct a feasible laser-cooling scheme for ZnH using five lasers based on the A2Π1/2 → X2Σ+ 1/2 transition, which features a large vibrational branching ratio R 00 (0.8458), a large number of scattered photons (9.8 × 103) and an extremely short radiative lifetime (64 ns). The present work demonstrates the importance of electronic state crossings and spin-orbit couplings in the study of molecular laser cooling.

Production of ultracold polyatomic molecules with strong polarity by laser cooling: A detailed theoretical study on CaNC and SrNC

Laser cooling molecules to the ultracold regime is the prerequisite for many novel science and technologies. It is desirable to take advantage of theoretical approaches to explore polyatomic molecular candidates, which are capable of being cooled to the ultracold regime. In this work, we explore two polyatomic candidates, CaNC and SrNC, which are suitable for laser cooling. These molecules possess impressively large permanent dipole moments (∼6 Debye), which is preferred for applications using an external electric field. High-level ab initio calculations are carried out to reveal electronic structures of these molecules, and the calculated spectroscopic constants agree very well with the available experimental data. For each molecule, the Franck-Condon factor matrix is calculated and shows a diagonal distribution. The radiative lifetimes for CaNC and SrNC are estimated to be 15.5 and 15.8 ns, respectively. Based upon the features of various electronic states and by choosing suitable spin-orbit states, we construct two feasible laser cooling schemes for the two molecules, each of which allows scattering nearly 10000 photons for direct laser cooling. These indicate that CaNC and SrNC are excellent ultracold polyatomic candidates with strong polarity.

A theoretical study on laser cooling feasibility of XH (X = As, Sb and Bi): effects of intersystem crossings and spin-orbit couplings

We investigate the low-lying electronic states and feasibility of direct laser cooling of AsH, SbH and BiH by means of the highly accurate ab initio and dynamical methods with the inclusion of the spin-orbit coupling effects. Twelve low-lying Ω states for each of them are computed using the internally contracted multireference configuration interaction method. Our computed spectroscopic constants are in excellent agreement with the available experimental data. The calculated spin-orbit matrix elements are large enough, and thus the intersystem crossings from the A³Π state and the transitions to the a¹Δ₂ state should be considered in laser cooling. We find that, from AsH to BiH, the location of the crossing point between the A³Π and ⁵Σ^- states moves down towards the ground vibrational level of A³Π along with enhanced spin-orbit coupling effects, which increases the difficulty of laser cooling heavier hydrides. An empirical law of "crossing point shifting down" down a group in the periodic table is generalized, which may become a helpful caveat when cooling diatomic molecules containing heavier elements. By choosing specific spin-orbit states, we construct feasible laser cooling schemes for AsH and SbH based on the A³Π₂ → X³Σ-1 transitions, which feature very large vibrational branching ratios R₀₀ (AsH: 0.9662; SbH: 0.9248) and short radiative lifetimes (AsH: 914 ns; SbH: 883 ns). In particular, a constructed laser cooling scheme for AsH is able to scatter 1.24 × 10⁴ photons, whereas that for SbH can scatter 8.60 × 10³ photons, which are enough to cool AsH and SbH to the ultracold regime. The present work demonstrates the importance of intersystem crossings and spin-orbit couplings in molecular laser cooling.

Excellent Ultracold Molecular Candidates From Group VA Hydrides: Whether Do Nearby Electronic States Interfere?

By means of highly accurate ab initio calculations, we identify two excellent ultracold molecular candidates from group VA hydrides. We find that NH and PH are suitable for the production of ultracold molecules, and the feasibility and advantage of two laser cooling schemes are demonstrated, which involve different spin-orbit states (A 3 Π 2 andX 3 Σ 1 - ). The internally contracted multireference configuration interaction method is applied in calculations of the six low-lying Λ-S states of NH and PH with the spin-orbit coupling effects included, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the locations of crossing point between theA 3 Π andΣ - 5 states of NH and PH are higher than the corresponding v' = 2 vibrational levels of theA 3 Π state indicating that the crossings with higher electronic states would not affect laser cooling. Meanwhile, the extremely small vibrational branching loss ratios of theA 3 Π 2 →a 1 Δ 2 transition for NH and PH (NH: 1.81 × 10-8; PH: 1.08 × 10-6) indicate that thea 1 Δ 2 intermediate electronic state will not interfere with the laser cooling. Consequently, we construct feasible laser-cooling schemes for NH and PH using three lasers based on theA 3 Π 2 →X 3 Σ 1 - transition, which feature highly diagonal vibrational branching ratioR 00 (NH: 0.9952; PH: 0.9977), the large number of scattered photons (NH: 1.04×105; PH: 8.32×106) and very short radiative lifetimes (NH: 474 ns; PH: 526 ns). Our work suggests that feasible laser-cooling schemes could be established for a molecular system with extra electronic states close to those chosen for laser-cooling.

Production of ultracold CaCCH and SrCCH molecules by direct laser cooling: A theoretical study based on accurate ab initio calculations

Laser cooling of polyatomic molecules to the ultracold regime may enable some new science and technology applications; however, the related study is still at its very early stage. Here, by means of accurate ab initio and dynamical calculations, we identify two new candidate tetratomic molecules that are suitable for laser cooling and demonstrate the feasibility and advantage of two laser cooling schemes that are able to produce ultracold CaCCH and SrCCH molecules. The internally contracted multiconfiguration reference configuration interaction method is applied, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the X²Σ_1/2 ⁺→A²Π_1/2 transitions for both candidates feature diagonal Franck-Condon factors, short radiative lifetimes, and no interference from intermediate electronic states. In addition, the crossings with higher electronic states do not interfere. We further construct feasible laser cooling schemes for CaCCH and SrCCH, each of which allows scattering 10⁴ photons for direct laser cooling. The estimated Doppler temperatures for both CaCCH and SrCCH are on the order of μK.

Spectroscopic properties and spin-orbit coupling of electronic excited states of the germanium dimer

The electronic structure and spectroscopic properties of the germanium dimer have been investigated by high-level ab initio calculations with consideration of spin-orbit coupling (SOC). The potential energy curves (PECs) of 19 Λ-S states, as well as those of 52 Ω states generated from the Λ-S states, are calculated by the multireference configuration interaction plus Davidson correction (MRCI + Q) method. The properties of the F3Σ+u1-X3Σ-g1 and H3Σ-u1-X3Σ-g1 transitions as well as the interactions of the F3Σ+u and H3Σ-u states with other excited states induced by SOC are investigated based on the calculated SO matrix and the PECs of the Ω states. Our results indicate that the previously observed spectra of Ge2 in the range 20 500-22 000 cm-1 should be assigned as the transition between the Ω = 1g component of the X3Σ-g state and Ω = 1u of the F3Σ+u state. Moreover, owing to the strong SOC with the repulsive 25Πu state, the H3Σ-u state is predissociative, leading to the Ge(3P2) + Ge(3P1) channel at vibrational levels higher than v' = 6. Our theoretical study would provide comprehensive information and shed light on understanding the spectroscopy and dynamics of the electronic excited states of Ge2.

A Theoretical Study on Laser Cooling Feasibility of Group IVA Hydrides XH (X = Si, Ge, Sn, and Pb): The Role of Electronic State Crossing

The feasibility of direct laser cooling of SiH, GeH, SnH, and PbH is investigated and assessed based upon first principles. The internally contracted multi-reference configuration interaction method with the Davidson correction is applied. Very good agreement is obtained between our computed spectroscopic constants and the available experimental data. We find that the locations of crossing point between the B²Σ^- and A²Δ states have the tendency of moving downwards from CH to SnH relative to the bottom of the corresponding A²Δ potential, which precludes the laser cooling of GeH, SnH, and PbH. By including the spin-orbit coupling effects and on the basis of the A 2 Δ 5 / 2 → X 2 Π 3 / 2 transition, we propose a feasible laser cooling scheme for SiH using three lasers with wavelengths varying from 400 to 500 nm, which features a very large vibrational branching ratio (0.9954) and a very short radiative lifetime (575 ns). Moreover, similar studies are extended to carbon monosulfide (CS) with a feasible laser cooling scheme proposed. The importance of electronic state crossing in molecular laser cooling is underscored, and our work suggests useful caveats to the choice of promising candidates for producing ultracold molecules.

Laser cooling of CaBr molecules and production of ultracold Br atoms: A theoretical study including spin-orbit coupling

Owing to the exciting potential applications of ultracold atoms and molecules in many fields, developing new cooling schemes has attracted great interests in recent years. Here, we investigate laser cooling of CaBr molecules and design a photonic scheme for the production of ultracold Br atoms using the highly accurate ab initio and dynamical methods. We find that the AΠ1/22(ν^'=0)→X²Σ_1/2⁺(ν=0) transition for CaBr features a large vibrational branching ratio, a significant photon-scattering rate, and no intermediate electronic-state interference, indicating that the ultracold CaBr could be produced through a three-laser cooling scheme. Moreover, an efficient four-pulse excitation scheme from the ground rovibrational level of the cooled CaBr molecules is proposed to yield ultracold Br atoms, in which a few spin-orbit excited states are utilized as the intermediate states. The importance of the spin-orbit coupling is underscored in this work.

Multireference configuration interaction study of the 27 low-lying states of the PF+ cation

This paper studied the spectroscopic parameters and vibrational properties of 27 Λ-S and 60Ω states of PF⁺ cation. The 27 Λ-S states were the X²Π, A²Σ⁺, B²Π, C²Σ^-, D²Δ, a⁴Σ^-, b⁴Π, c⁴Σ⁺, d⁴Δ, 2²Σ⁺, 3²Σ⁺, 4²Σ⁺, 2²Σ^-, 3²Σ^-, 3²Π, 4²Π, 5²Π, 6²Π, 2²Δ, 3²Δ, 1²Φ, 2⁴Σ^-, 3⁴Σ^-, 2⁴Π, 3⁴Π, 1⁶Σ^-, and 1⁶Π, which were generated from the first four dissociation limits. The 60Ω states were produced from the 27 Λ-S states. All the potential energy curves were calculated with the CASSCF method, which was followed by the icMRCI+Q approach. The a⁴Σ^-, b⁴Π, and D²Δ states were inverted with the spin-orbit coupling effect accounted for. The 2⁴Π, 2⁴Σ^-, 2²Δ, 3²Δ, 3²Σ⁺, 4²Σ⁺, 1²Φ, and 2²Σ^- states were repulsive whether the spin-orbit coupling effect was included or not, but the 5²Π and D²Δ states became repulsive only with the spin-orbit coupling effect included. The C²Σ^- state was very weakly bound. The a⁴Σ^- state had one barrier. The avoided crossings existed between the a⁴Σ^- and 2⁴Σ^- states, the 2²Σ⁺ and 3²Σ⁺ states as well as the D²Δ and 2²Δ states. Core-valence correlation and scalar relativistic corrections were taken into account. The extrapolation to the complete basis set limit was done. The spectroscopic parameters and vibrational properties were determined. The transition dipole moments were calculated and the Franck-Condon factors of some electric dipole transitions were evaluated. The spin-orbit coupling effect on the spectroscopic and vibrational properties was discussed.

Extensive theoretical study on electronically excited states of calcium monochloride: Molecular laser cooling and production of ultracold chlorine atoms

Nine doublet Λ-S states of calcium monochloride (CaCl) are calculated using the internally contracted multireference configuration interaction method with the Davidson correction. Both the core subvalence and spin-orbit coupling effects are taken into account. Laser cooling of CaCl and production of ultracold chlorine atoms are investigated and assessed. Our computed spectroscopic constants and radiative lifetimes match the available experimental data very well. The determined Franck-Condon factors and vibrational branching ratios of the A(2)Π1/2(ν('))←X(2)Σ1/2 (+)(ν) transition are highly diagonally distributed and the evaluated radiative lifetime for the A(2)Π1/2(ν' = 0) state is 28.2 ns, which is short enough for rapid laser cooling. Subsequently, detection of cold molecules via resonance enhanced multiphoton ionization to determine the final quantum state populations is discussed and the ionization energy calculated. A multi-pulse excitation scheme is proposed for producing ultracold chlorine atoms from zero-energy photodissociation of the cooled CaCl. Our results demonstrate the possibility of producing ultracold CaCl molecules and Cl atoms.

Laser cooling of copper monofluoride: a theoretical study including spin–orbit coupling

A laser cooling scheme is proposed for CuF by including the spin–orbit coupling effects, and based on our calculated radiative lifetimes and vibrational branching ratios.