Superexcited states of carbon monoxide studied by fast-electron impact

Absolute optical oscillator strength density and double differential cross section spectra of CO below 120\,eV are determined by fast electron impact. Some peaks above the first ionization threshold stand out as the momentum transfer square $K^2$ increases. The doubly excited Rydberg states converging to $C~^2\Sigma^+$, $D~^2\Pi$, and $F~^2\Pi$ states of CO$^+$, respectively, are confirmed in our spectra. Another peak at around 32\,eV is assigned to the transition of $(3\sigma)^{-1}(2\pi)^1~^1\Pi\leftarrow~X^1\Sigma^+ $.