Abstract
The vibrational and rotational analysis of the absorption spectrum of the C2H2, C2D2, 13C2H2 and C2HD molecules, photographied at high resolution in the region of 1250-1050 Å has been performed. It brings new information on the Rydberg states of the acetylene molecule. The presence of three Rydberg series leading to the first ionisation potential is confirmed and tentative assignements are given for a fourth expected series. The rotational analysis of several bands involving states belonging to two different series (nR and nR'') definitely establishes that these states imply a linear geometry and have 1Πu symmetry. Some properties of the 2Πu state of the acetylene ion are extrapolated from the present results.

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