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abstract: '\[abstract\] The lowest $v=0$ level of the [A$^1\Pi$]{}state of the $^{13}$C$^{16}$O isotopologue of carbon monoxide has been reinvestigated with a variety of high resolution spectroscopic techniques. The [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band has been studied by vacuum-ultraviolet Fourier-transform absorption spectroscopy, using the SOLEIL synchrotron as a radiation source. Spectra were obtained under quasi-static gas conditions at liquid-nitrogen temperature, room temperature and at an elevated temperature of 900 K, with absolute accuracies of 0.01$-$0.03 [cm$^{-1}$]{}. Two-photon Doppler-free laser spectroscopy has been applied to a limited number of transitions in the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band, under collision-free circumstances of a molecular beam, yielding an absolute accuracy of 0.002 [cm$^{-1}$]{}. The third technique is high-resolution Fourier-transform emission spectroscopy in the visible region applied to the [B$^1\Sigma^+-$A$^1\Pi$]{}$(0,0)$ band in a gas discharge, at an absolute accuracy of up to 0.003 [cm$^{-1}$]{}. With these methods rotational levels of [A$^1\Pi$]{}(0) could be studied in both parity components up to a rotational quantum number of $J=46$. The frequencies of 397 transitions were used to analyse the perturbations between the [A$^1\Pi$]{}$(0)$ level by vibrational levels of the [D$^1\!\Delta$]{}, [e$^3\Sigma^-$]{}, [d$^3\!\Delta$]{}, and [a$''^3\Sigma^+$]{} states.'
author:
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    M. L. Niu,$^{a}$ R. Hakalla,$^{b}$ T. Madhu Trivikram,$^{a}$ A. N. Heays,$^{a}$ N. de Oliveira,$^{c}$ E. 