accuracy. For this purpose, high resolution Doppler-free laser measurements of the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band are combined with VUV-FT measurements under various temperature conditions that permit access to higher rotational levels, as well as with high-resolution visible Fourier-transform (VIS-FT) emission spectra obtained from a discharge. These data are combined in a comprehensive deperturbation analysis with the purpose of quantitatively describing the [A$^1\Pi$]{}(0) level and various vibronic states with which it interacts.

Experiments
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Independent high-resolution studies of the [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band of $^{13}$C$^{16}$O were performed by laser and VUV-FT spectroscopy, while the [B$^1\Sigma^+-$A$^1\Pi$]{}$(0,0)$ emission band was investigated with VIS-FT spectroscopy from a discharge. The methods used and the results obtained in the three different experiments are described below.

Laser experiments {#sec:laser experiments}
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Figure \[fig:PDAspec\] shows a typical spectrum of the S(0) transition of [A$^1\Pi-$X$^1\Sigma^+$]{}$(0,0)$ band of $^{13}$C$^{16}$O obtained in the laser experiment. Here, a narrow-band pulsed dye amplifier (PDA) laser system is used [@Ubachs1997], seeded by the output of a cw ring dye laser which is optimized for operation at 618 nm, and pumped by a Nd:YAG laser with a repetition rate of 10 Hz. The pulsed output of the PDA is frequency doubled and then employed in a two photon experiment 