Knowledge Bank

A new triplet band system has been discovered near $6500cm-1$ in absorption using distributed-feedback (DFB) diode lasers as radiation sources. The band system is also observed in emission from a microwave discharge of $CH4$ in He using a Fourier-transform spectrometer. $C3$ was generated in a hollow cathode discharge in $C2H2$, or $C2H4$, or $C2H6$ with He buffer. The band has exhibited a characteristic triplet pattern with a slight but significant staggering in the rotational line spacing. Numerous perturbations have been found in the upper state. According to ab initio calculations, the lowest triplet state, $a¯3\Pi u$, is located at about 2 eV above the ground state, $X¯1\sigma 9+$ and Weltner and coworkers idenftified a weak emission band of $C3$ in Ar or Ne matrices at around 590 nm to be from this triplet state to the ground state. The next triplet state is located about 0.77 eV above this lowest lying triplet state according to an ab initio calculation. The band found in this work has been assigned to be the transition between these triplet states $b¯3\Pi g-a¯3\Pi u$. The vibrational assignments were not made uniquely, but it is very likely that the band is the $v=0-0$ band. A least-squares analysis has been carried out to determine the spectroscopic constants with an effective Hamiltonian derived by Brown and Merer.