Linear polyatomic molecules with Π ground state: Sensitivity to variation of the fundamental constants

In polyatomic molecules with a Π electronic ground state, the rovibrational spectrum can be strongly modified by the Renner-Teller effect (the coupling between bending vibrational mode and electrons in the degenerate Π state). The linear form of the C₃H molecule has particularly strong Renner-Teller interactions and a very-low-lying vibronic Σ1/2⁺ level, which corresponds to the excited bending vibrational mode. This leads to the increased sensitivities of the microwave and submillimeter transition frequencies to the possible variation of the fine structure constant α and electron-to-proton mass ratio μ.