\[fig:FTAXspec\](b) and (c). Intermediate $J''$ levels have higher populations at 295K and also provide a cross-check for low $J''$ lines. The 900K spectrum presented in Fig. \[fig:FTAXspec\](c) was measured at about 60 times higher column density than the room temperature spectrum, with the goal of obtaining as much information as possible on the highest rotational quantum states, in which case transitions from the low $J''$ were saturated. Ultimately, the rotational progression of [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ could be followed to $64\,000$ [cm$^{-1}$]{} and for transition with $J'$ as high as 46, whereupon a severe overlap with the [A$^1\Pi-$X$^1\Sigma^+$]{}$(1,1)$ band sets in.

Figure \[fig:FTAXspecdetail\] shows details of VUV-FT spectra in the bandhead region and demonstrates the complementarity of spectra recorded under different temperature and saturation conditions. Also appearing are forbidden transitions to the [e$^3\Sigma^-$]{}$(4)$ level due to intensity borrowing from the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band. The mutual perturbation of [e$^3\Sigma^-$]{}$(4)$ and [A$^1\Pi$]{}$(0)$ also leads to rotational line structure discontinuities as, for instance, the perturbation of $R(6)$ and $R(7)$ lines in Fig. \[fig:FTAXspecdetail\]. Further lines appear from the forbidden [d$^3\!\Delta-$X$^1\Sigma^+$]{}$(4,0)$ band in our spectra.

The frequency calibration of Fourier-transform spectra is inherently linear and requires only a single absolute standard. Here, the absolute calibration was made by 