were performed using 99% enriched samples of $^{13}$C$^{16}$O gas. The sampling rate and maximum path difference of the FT spectrometer was set to provide resolutions of 0.075, 0.075, and 0.27[cm$^{-1}$]{} FWHM for the 90, 295, and 900K measurements, respectively. The respective Doppler broadening of these measurements are 0.08, 0.14, and 0.25[cm$^{-1}$]{}FWHM. Further experimental and analytical details of the VUV-FT spectra are provided in Refs. [@Niu2013; @Niu2015; @Niu2016].

![Spectra of the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band of $^{13}$C$^{16}$O recorded with the VUV-FT at the SOLEIL synchrotron under three different temperature regimes: (a) cooling of the cell with liquid nitrogen; (b) at room temperature; (c) cell heated to $\sim 900$ K. The A$^1\Pi-$X$^1\Sigma^+(1,1)$ absorption band is also evident at 64100[cm$^{-1}$]{} in the latter case.[]{data-label="fig:FTAXspec"}](FTspectraAX5.eps){width="\linewidth"}

![image](FTspectraAXdetail1.eps){width="\linewidth"}

Overview spectra of the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band, recorded at the three different temperatures, are displayed in Fig. \[fig:FTAXspec\], covering $64\,000-64\,800$ [cm$^{-1}$]{}. These spectra enable good line-position determinations for transitions from different ranges of ground state $J''$. The lowest $J''$ states are populated at 90K and lead to transitions with the narrowest Doppler widths, resulting in the least-congested spectra (i.e., in Fig. \[fig:FTAXspec\](a)), thus facilitating straightforward line assignment. Analysing the 90K spectrum first aided the assignment of higher temperature spectra in Fig. 