BH~+离子基态及激发态的势能曲线和跃迁性质的研究

利用高精度的多组态相互作用及Davidson修正方法 (MRCI+Q),采用ACV5Z-DK全电子基组计算了BH~+离子的前4个离解通道B~+(~1S_g)+H(~2S_g),B~+(~3Pu)+H(~2S_g),B(~2P_u)+H~+(~1Sg)和B~+(~1Pu)+H(~2Sg)的9个Λ—S态的势能曲线.X~2Σ~+,A~2Π和B~2Σ~+态的光谱常数和已有实验值符合得很好,其中b~4Σ~+,3~2Σ~+,3~2Π和4~2Σ~+态的光谱常数为首次报道,3~2Π和4~2Σ~+态具有双势阱结构.预测了A~2Π和B~2Σ~+态的辐射寿命:τ(A~2Π)=239.2 ns和τ(B~2Σ~+)=431.2 ns.最后在考虑自旋轨道耦合效应下讨论了B~2Σ~+与A~2Π态的势能曲线的相交对激光冷却BH~+离子的影响.

Potential energy curves and transition properties for the ground and excited states of BH+ cation

BH⁺ cation is one of the candidates for laser cooling. The potential energy curves (PECs) for nine electronic states (X²Σ⁺, A²Π, B²Σ⁺, a⁴Π, b⁴Σ⁺, 3²Σ⁺, 2²Π, 3²Π, 4²Σ⁺) relating to the B⁺(¹S_g)+H(²S_g), B⁺(³P_u)+H(²S_g), B(²P_u)+H⁺(¹S_g), and B⁺(¹P_u)+H(²S_g) dissociation channels of BH⁺ cation are obtained using highly accurate multi-reference configuration interaction (MRCI) plus Davidson correction. All-electron basis sets AV5Z-DK for H and ACV5Z-DK for B are used in PEC calculations for the Λ-i-S states of BH⁺ cation, respectively. In complete active space self-consistent field (CASSCF) calculation, H(1s2s2p3s3p) and B(2s2p) are chosen as active orbitals, B(1s) is the closed shell; in the MRCI calculation, the core-valence (CV) correction is considered, i.e., B(1s) shell is used for CV correlation. Spin-orbit coupling effects are considered with Breit-Pauli operators. Spectroscopic constants are fitted using the Murrell-Sorbie function. Spectroscopic constants for the X²Σ⁺, A²Π, and B²Σ⁺ states are in excellent agreement with the available experimental data; spectroscopic constants for the b⁴Σ⁺, 3²Σ⁺, 3²Π, and 4²Σ⁺ states are reported. Two potential wells for the 3²Π and 4²Σ⁺ states are found. The maximum fitting error of all electronic states is only 3.407 cm^-1. In addition, PECs for the A²Π and B²Σ⁺ states are crossed at about 2.7 Å. Then, the transition dipole moments (TDMs) for the A²Π→ X²Σ⁺, B²Σ⁺→X²Σ⁺, 3²Σ⁺→X²Σ⁺, B²Σ⁺→ A²Π, 3²Π→ X²Σ⁺ and b⁴Σ⁺→ a⁴Π transitions are also obtained. The strength for the B²Σ⁺→ A²Π transition is very weak. Based on the accurate PECs and TDMs, the Franck-Condon factors and spontaneous radiative lifetimes are calculated. A strongly diagonal Franck-Condon factor (f₀₀) for the A²Π→X²Σ⁺ transition is obtained, which equals 0.9414. Spontaneous radiative lifetime for the A²Π and B²Σ⁺ states is also predicted. i.e., τ (A²Π)=239.2 ns and τ(B²Σ⁺)=431.2 ns. When SOC effect is considered, the A²Π_1/2 and B²Σ_1/2⁺ states avoid crossing in the Franck-Condon region (R is about 2.7 Å). Calculated f₀₀ for the A²Π_1/2→ X²Σ_1/2⁺ transition is 0.9430; spontaneous radiative lifetime for the A²Π_1/2 is 239.0 ns. Our calculated results indicate that the influence for laser cooling BH⁺ cation via the crossing between B²Σ⁺ and A²Π states can be ignored.