Laser cooling of CH molecule: Insights from ab initio study

The feasibility of laser cooling a CH molecule is investigated theoretically by employing the ab initio method. The potential energy curves for the five Λ-S states and eight Ω states of CH are determined by the multi-reference configuration interaction with the Davidson corrections (MRCI+Q) level of theory. The results agree well with the available experimental data and other theoretical values. Also, the permanent dipole moments and transition dipole moments of the CH molecule are calculated at the multi-reference configuration interaction (MRCI) level. We find highly diagonally distributed Franck-Condon factors (f₀₀=0.9950 and 0.9998) and branching ratios (R₀₀=0.983 and 0.993) for the A²Δ→X²Π and C²Σ⁺→X²Π transitions. Moreover, the values of suitable radiative lifetime τ of the A²Δ and C²Σ⁺ states are evaluated to be 9.64×10^-7 s and 2.02×10^-7 s, respectively, for rapid laser cooling. A scheme for laser cooling the CH molecule is designed. In the proposed cooling scheme, three wavelengths for A²Δ→X²Π and C²Σ⁺→X²Π transitions are used, and the main pump lasers are λ₀₀=430.86 nm and 313.45 nm, respectively. The feasibility of laser cooling the CH molecules is demonstrated for each of these schemes, and this study offers a theoretical basis for experimental research into preparation of cold CH molecules.