Using the inelastic x-ray scattering with a high resolution of 70 meV, squared form factors of molecular carbon monoxide have been determined from the ground state $X{}^1{\Sigma }^{+}$ to the vibronic states of $A{}^1\Pi$ and $B{}^1{\Sigma }^{+}$. Since the first Born approximation is satisfied in inelastic x-ray scattering, the inelastic squared form factors of the valence-shell excitations of carbon monoxide measured by this work not only provide the experimental benchmark data but also serve as the high-energy limit for the electron impact method. Based on the present experimental results, the validity conditions of the previous electron impact works are discussed. It is found that for the excitations of $A{}^1\Pi ({\nu }^{\prime }=0\text{–}6)$ the first Born approximation is reached in the region of $q^2<0.7$ a.u. at an impact electron energy of 1.5 keV, while for $B{}^1{\Sigma }^{+}({\nu }^{\prime }=0)$ the first Born approximation does not fully hold at the impact electron energy of 1.5 keV except for the $q^2<0.15$ a.u. The large discrepancies between the present inelastic x-ray scattering results and the theoretical calculations for most transitions suggest that the formerly reported calculations are not accurate enough.

• Received 12 February 2015

DOI:https://doi.org/10.1103/PhysRevA.91.042501