Spectroscopic Properties And Laser Cooling Schemes Of The Excited States Of SrBr,SrI,SOH And SeOH Molecules

Ultra-cold atoms and molecules with their applications in science and technology are the most active fields in atomic and molecular physics.For example,Bose-Einstein condensation,atomic clock,precision measurements,ultra-cold chemistry,precision measurement,high-resolution spectroscopy,quantum computing and quantum simulation depend on the cooled atom and molecule.There are many studies on the feasibility of laser cooling of atoms in experiment and theory,but few on molecules.Researches are interested in the laser cooling of molecules although it is much more difficult than that of atom.Based on the high-level ab initio theory,the laser cooling of the diatomic and triatomic molecules is studied in this paper.1.For diatomic molecules SrBr and SrI,according to the laser cooling of a molecule with heavy nuclei is often complicated because of the density distribution of the electronic states.Here,we evaluate the feasibility of the laser cooling of the SrBr and SrI molecules by calculating the potential energy curves(PECs)and transition dipole moments(TDMs)of the ground and low-lying excited states using the multi-reference configuration interaction plus Davidson corrections(MRCI+Q)and the all-electron basis sets.The relativistic effect is included,because Br,Sr and I are heavy atoms.Moreover,the spin-orbit coupling splits of the A²?state are considered.Based on the obtained PECs,the rotational and vibrational energy levels of each electronic state are determined by solving the nuclear motion equation of the molecule.And the spectroscopic parameters of each electronic states are fitted from the obtained energy levels by using Dunham expression.The Franck-Condon(FC)factors,radiation lifetimes,radiation widths,the Doppler and recoil temperatures between the X²?⁺and the ²?_1/2/²?_3/2/B²?⁺states of the two molecules are also calculated.The laser cooling schemes for the molecules are proposed,which can lead to the total effective FC factors of each transition can reach more than 0.999.All the obtained results suggest that the Sr B and SrI molecules are the feasible candidates for laser cooling.2.Based on ab initio calculation,it is found that the triatomic molecules SOH and SeOH are suitable for laser cooling,and the corresponding cooling scheme is established.Compared with diatomic molecules,the triatomic molecules have more degrees of freedom and more complex vibrational and rotational energy levels,which are very difficult to find molecules with satisfactory electronic and optical properties that meet the requirements of laser cooling.However,by analyzing the electronic structure and spectroscopic characteristics of molecules that have been found to be suitable for laser cooling,a new criteria for the qualified molecules is proposed,then a method to identify polyatomic molecules suitable for direct laser cooling is developed.The new criteria from the equilibrium geometrical structures and fundamental frequencies of the ground and low-lying excited states are used to replace the past ones based on FC factors.The new method can achieve to rapidly identify the preferable one among many candidate polyatomic molecules based on ab initio calculations because the new criteria are free of the construction of potential energy surfaces and solving the Schr(?)dinger equation of nuclear motion.In order to verify the reliability of this method,the electronic structure characteristics of the three lowest electronic states of the first two multiplicities of each 28 triatomic molecules containing OH were calculated and analyzed.All the reported triatomic molecules Ca OH,Sr OH,Ba OH and Ra OH as well as two new molecules SOH and SeOH suitable for direct laser cooling are identified by comparing 168 electronic states with the new criteria.Based on the constructed potential energy surface and the related vibrational energy levels of1²A'(?)X²A"transition,the calculated FC factor confirm that the newfound molecules can be used for laser cooling.Finally,the optical schemes for the direct laser cooling of the SOH and SeOH molecules are established,which confirm the credibility of the new criterion and the reliability of the screening method.