J. Salumbides,$^{a}$ and W. Ubachs$^{a}$\
    $^{a}$[*[Department of Physics and Astronomy, and LaserLaB, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands]{}*]{}\
    $^{b}$[*[Materials Spectroscopy Laboratory, Department of Experimental Physics, Faculty of Mathematics and Natural Science, University of Rzeszów, ul. Prof. S. Pigonia 1, 35-959 Rzeszów, Poland.]{}*]{}\
    $^{c}$[*[Synchrotron SOLEIL, Orme de Merisiers, St. Aubin, BP 48, F-91192 Gif sur Yvette Cedex, France.]{}*]{}\
title: 'Spectroscopy and perturbation analysis of the $\bm{{\rm A}{}^1\Pi(v=0)}$ state of $\bm{{}^{13}{\rm C}{}^{16}{\rm O}}$'
---

Ultraviolet spectra; FT-spectroscopy; Doppler-free laser; Perturbation analysis; Carbon monoxide

Introduction
============

Spectroscopy of the carbon monoxide molecule remains of interest, in large part, because of its role as the prime tracer of astrophysical molecular gas in interstellar clouds [@Kong2015], circumstellar material [@Flaherty2015], entire galaxies [@Bolatto2013], at very high redshifts [@Levshakov2012] and in gamma-ray bursts [@Prochaska2009]. Astronomical CO has also been observed in the ultraviolet, through absorption or emission of the fourth positive system ([A$^1\Pi-$X$^1\Sigma^+$]{}) [@Federman2003; @Mcjunkin2013]. In addition, the CO molecule is of interest from a molecular physics perspective, since its spectrum exhibits a wealth of perturbations arising from spin-orbit and centrifugal effects, as well as Rydberg-valence mixing [@krupenie1966; @Tilford1972; @eidelsberg2004a].

Perturbations affecting the [A$^1\Pi$]{} state were extensively investigated by Simmons and Tilford [@Simmons1966] 