CaSH分子高精度电子结构计算及用于激光制冷目标分子的理论分析

High-precision electron structure calculation of CaSH molecules and theoretical analysis of its application to laser-cooled target molecules

作为非对称多原子分子制冷的一个重要目标分子,CaSH的冷却有望打破双原子分子及线性三原子分子在激光冷却中的技术局限.本文使用高精度的EA-EOM-CCSD(electron attachment equation-of-motion coupled cluster singles and doubles)方法,通过cc-pVXZ/cc-pCVXZ(X=T,Q)系列基组外推至基组极限,得到了CaSH基态和3个最低激发态精确的几何结构及基态到激发态的跃迁能.其中,基态X^(2)A′几何结构参数分别为RCaS=2.564A;RSH=1.357A;∠CaSH=91.0°;从X^(2)A′到A^(2)A′,B^(2)A"和C^(2)A′的垂直激发能分别为1.898,1.945和1.966 eV,与已有实验符合得很好.进一步,在3ζ级别基组上,计算了该分子4个最低电子态的势能面,并通过求解核运动方程给出CaS键伸缩、CaSH弯曲两个振动模的频率.最后,理论计算给出的X^(2)A′(0,0,0)态到激发态A^(2)A′(0,0,0),B^(2)A"(0,0,0)和C^(2)A′(0,0,0)跃迁的Frank-Condon(FC)因子分别为0.9268,0.9958和0.9248.结合Frank-Condon因子和激发态寿命分析,本文给出了可能用于CaSH冷却的光学循环,为CaSH的激光冷却提供了理论参考.

The CaSH molecule is an important target in the field of laser cooling non-linear polyatomic molecules.Successful cooling of such molecules marks a breakthrough of the technical limitations of laser cooling diatomic and linear triatomic molecules.To identify the possible optical cycle in cooling CaSH,precise geometries of the CaSH ground state and the three lowest excited states,along with their excitation energy,are determined by utilizing the EA-EOM-CCSD(electron attachment equation-of-motion coupled cluster singles and doubles)method,in combination with energy extrapolation using cc-pVXZ/cc-pCVXZ(X=T,Q)serial basis sets Geometric parameters of the ground state 2A′are found to be Roas=2.564 A,Rop=1.357 A,and∠CaSH=91.0°.Additionally,the equilibrium geometries of three excited states are also obtained.The B^(2)A"state has a similar equilibrium structure to the ground state,while the A^(2)A′and C^(2)A′states exhibit aignificant conformer distortions.Specifcally,the CaS bond of the A^(2)A′state and C^(2)A′state tend to contract,and the CaSH angel bends by 5°relative to the ground state.The vertical excitation energy from the ground state to A^(2)A′,B^(2)A"and C^(2)A′are of 1898,1.945 and 1.966 eV,respectively,which are in good agreement with the previous experimental results.Moreover,the potential energy surfacee of the four lowest electronic states of CaSH are calculated by EA-EOM-COSD with 3ζlevel of basis sets.The nuclear equations of motion are solved to obtain the vibrational frequencies of the CaS bond stretching and CaSH bending.The vibrational frequencies of the(0,1,0)mode and the CaS stretching frequency of four states are 316 cm^(-1),315 cm^(-1),331 cm^(-1)and 325 cm^(-1),which are in close agreement with the available experimental results.The frequencies of the CaSH bending mode are presented for the first time,with the values of 357 cm^(-1),396 cm^(-1),384 cm^(-1),411 cm^(-1)for the X2A′,A^(2)A′,B^(2)A"and C^(2)A′states,respectively.Theoretical calculations give the Frank-Condon factors of 0.9268,0.9958 and 0.9248 for the X^(2)A′(0,0,0)to A^(2)A′(0,0,0),B^(2)A"(0,0,0)and C^(2)A′(0,0,0)transitions.All three excited states are the bright states with considerable oscillator strength relative to the ground state.Based on the Frank-Condon factor and lifetime of excited states,the X^(2)A′(0,0,0)→B^(2)A"(0,0,0)trancition is regarded as the main cooling cycle for the CaSH molecule.The corresponding pump light wavelength is 678 nm.By exciting the vibrational excited states(0,1,0)and(0,0,1)of the X2A′state to A^(2)A′(0,0,0)uaing lasers at 666 nm and 668 nm,respectively,the optical cooling branch ratio of CaSH is expected to exceed 0.9998.