High Resolution Spectroscopic Study of the Rydberg Series of the Acetylene Isotopic Molecules

The vibrational and rotational analysis of the absorption spectrum of the C 2 H 2 , C 2 D 2 , 13 C 2 H 2 and C 2 HD 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 1 Π u symmetry. Some properties of the 2 Π u state of the acetylene ion are extrapolated from the present results.