Knowledge Bank

The levels of the bending vibration $(\nu 2=1-11)$ of $CCH(X~2\Sigma +)$ were determined by UV laser-induced fluorescence of the products of photolysis of acetylene at 193 nm. Large negative anharmonicity was found in the lowest five levels of the bending vibration. As the number of bending quanta increases, the anharmonicity decreases considerably. Levels $\nu 4=0-5$ of the trans-bending motion of the acetylene cation $(X~2\Pi u)$, isoelectronic with $CCH1$ and its deuterated isotopomer have been determined by 1 + 1 PFI-ZEKE spectroscopy via a single rovibronic level of $A~1Au$ as an intermediate. Spectral analysis shows that negative anharmonicity was also present in the latter case. Large negative anharmonicity of CCH is attributed to vibronic $interaction1$ between $A~2\Pi$ and $X~2\Sigma +$ as well as vibration-rotational interaction among the vibrational levels of the $X~$ $state.2$. The potential-energy curve of the bending vibration of CCH $(X~)$ and that of the trans-bending vibration of $C2H2$ calculated by an ab initio method will be presented to show that negative anharmonicities of these molecules may be due to the flatness of the (zero-order) potential-energy curves. The Renner-Teller effect of the trans-bending vibration of the acetylene cation will be discussed.