Knowledge Bank

Laser-induced-fluorescence spectra of the $C4H$ and $C4D$ radicals have been observed for the first time. The molecules were generated in a supersonic jet by an electric discharge of $C2H2$ or $C2D2$ diluted in Ar. Fourteen and thirteen vibronic bands were observed for $C4H$ and $C4D$ respectively, in the near UV region (24000 - 25000 $cm-1$). The bands were assigned to the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma -2\Sigma$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi -2\Sigma$ type and several $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma -2\Sigma$ bands were subjected to rotational analyses. The determined rotational constants for the lower state agree with those for $C4H/C4D$ in the ground vibrational level in the $X2\Sigma$ state, which were reported in a previous microwave study . The spin-orbit splittings in the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi -2\Sigma$ bands are 16 and $12cm-1$ for $C4H$ and $C4D$, respectively, leading a conclusion that the excited electronic state for the observed band system is . The present observation is consistent with a recent CASSCF $studyc$, which predicted the $B2\Pi$ state with the excitation energy of $24500cm-1$.