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Spectroscopy and perturbation analysis of the CO A$^1Π-$X$^1Σ^+$ (2,0), (3,0) and (4,0) bands

The (2,0) (3,0) and (4,0) bands of the A$^1Π-$X$^1Σ^+$ system of $^{12}$C$^{16}$O have been re-investigated by high-resolution vacuum ultraviolet absorption spectroscopy. A VUV Fourier-transform spectrometer, illuminated by synchrotron radiation, was applied to record a jet-cooled spectrum, a room temperature static gas spectrum and a high temperature (900 K) quasi-static gas spectrum, resulting in absolute accuracies of 0.01$-$0.02 cm$^{-1}$ for the rotational line frequencies. Precise laser-based data were included in the analysis allowing for a highly accurate determination of band origins. Rotational levels up to $J=52$ were observed. The data were used to perform an improved analysis of the perturbations in the A$^1Π$, $v=2$, $v=3$, and $v=4$ levels by vibrational levels of the D$^1Δ$, I$^1Σ^-$, e$^3Σ^-$, d$^3Δ$, and a$'^3Σ^+$ states.