First Observation of an Identity-Forbidden Transition in Two-Photon Absorption Spectroscopy:I1Σ−−X1Σ+Transition in CO

An identity-forbidden two-photon transition has been observed for the first time, between the $I^1{\Sigma }^{-}$ and $X^1{\Sigma }^{+}$ states in carbon monoxide. Such transitions are dipole allowed only for the absorption of two photons with different frequencies and polarization states. Only $P$ and $R$ rotational branches of the (7-0) band are observed, and the line intensities are maximized for perpendicular photon polarizations. This is the first experimental demonstration of the two-photon electric-dipole selection rule ${\Sigma }^{+}\to {\Sigma }^{-}$.