Fig. 3.

Renner–Teller excitation mechanism in neutral CS₂. (A) Potential energy curves (PECs) for the field-free (solid curves) neutral CS₂ in the ground electronic state along with the ${\stackrel{\sim }{\text{X}}}^1{\text{A}}_1$ (blue), the ${\stackrel{\sim }{\text{A}}}^1{\text{A}}_2$ (red), and the ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$ (green) excited electronic states are shown as a function of the S-C-S angle at fixed R_CS = 1.86 Å. The corresponding field-dressed (dashed curves) PECs are also shown. In the linear geometry (D_∞h), a transition from the ${\stackrel{\sim }{\text{X}}}^1{\Sigma }_{\text{g}}^{+}$ ground electronic state to the ¹Δ_u excited electronic state is dipole forbidden (gray vertical arrow) due to symmetry considerations. However, our calculations show that the molecule begins to bend by 10° (C_2v) in the presence of a strong field. At the same time, at bent geometries, the twofold degeneracy of ¹Δ_u is lifted and splits into two distinct bent excited electronic states: ${\stackrel{\sim }{\text{A}}}^1{\text{A}}_2$ and ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$. At these bent geometries, a transition from the ${\stackrel{\sim }{\text{X}}}^1{\text{A}}_1$ ground state to the ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$ excited state becomes dipole allowed (black vertical arrow). (B) Potential energy surfaces (PESs) of field-dressed (FD) CS₂ in the ${\stackrel{\sim }{\text{X}}}^1{\text{A}}_1$ ground electronic state and the ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$ excited state. Once the ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$ state is populated, the nuclear wave packet evolves toward the equilibrium position of the ${\stackrel{\sim }{\text{B}}}^1{\text{B}}_2$ state.