Spectroscopy and perturbation analysis of the A$^1\Pi$(v=0) state of   $^{13}$C$^{16}$O
  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. 
M. L. Niu and R. Hakalla and T. Madhu Trivikram and A. N. Heays and N.   de Oliveira and E. J. Salumbides and and W. Ubachs
physics.atom-ph