Laser-induced fluorescence spectroscopy of the C4H and C4D radicals in a supersonic jet

Laser-induced fluorescence (LIF) spectra of the ${\mathrm{C}}_{\mathrm{4}}\mathrm{H}$ radical and its isotopomer ${\mathrm{C}}_{\mathrm{4}}\mathrm{D}$ have been observed in a supersonic free jet expansion for the first time. The jet cooled free radicals have been generated in an electric discharge of 0.5% ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{2}}$ or ${\mathrm{C}}_{\mathrm{2}}{\mathrm{D}}_{\mathrm{2}}$ in Ar using a pulsed discharge nozzle (PDN). Twenty and eighteen vibronic bands of ${\mathrm{C}}_{\mathrm{4}}\mathrm{H}$ and ${\mathrm{C}}_{\mathrm{4}}\mathrm{D},$ respectively, have been observed in the $\text{24\hspace{0.17em}000–25\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ region. Most of the observed bands have been assigned as either ${}^2{\mathrm{\Pi –}}^2\Sigma$ or two types of ${}^2{\mathrm{\Sigma –}}^2\mathrm{\Sigma .}$ Effective spin–orbit interaction constants were determined by analyses of the ${}^2{\mathrm{\Pi –}}^2\Sigma$ bands to be $\text{−14.7644(80)}$ and $\text{−10.9926(35)\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ for ${\mathrm{C}}_{\mathrm{4}}\mathrm{H}$ and ${\mathrm{C}}_{\mathrm{4}}\mathrm{D},$ respectively, leading to conclusion that the upper electronic state of the observed band system is ${\mathrm{B̃\hspace{0.17em}}}^2{\Pi }_i,$ in agreement with a recent ab initio calculation by Sobolewski and Adamowicz [J. Chem. Phys. 102, 394 (1995)]. Observation of two types of ${}^2{\mathrm{\Sigma –}}^2\Sigma$ bands, ${}^2{\Sigma }^{+}{–}^2{\Sigma }^{+}$ and ${}^2{\Sigma }^{\mathrm{(\pm )}}{–}^2{\Sigma }^{+},$ is explained by the difference of the magnitudes of Renner–Teller interactions for the bending vibrational modes involved. We were able to assign the ${\nu }_5$ (CCH bending) and ${\nu }_6$ (CCC bending) progressions of the ${\mathrm{B̃\hspace{0.17em}}}^2{\Pi }_i$ state, where the Renner–Teller interaction is large for ${\nu }_5,$ and small for ${\nu }_6.$ The vibrational frequencies and the Renner parameters were determined to be ${\omega }_5\text{=344\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ ${\epsilon }_5\text{=−0.657,}$ ${\omega }_6\text{=189.3\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ and ${\epsilon }_6\text{=−0.0158}$ for ${\mathrm{C}}_{\mathrm{4}}\mathrm{H},$ and ${\omega }_5\text{=295\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ ${\epsilon }_5\text{=−0.692,}$ ${\omega }_6\text{=183.4\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ and ${\epsilon }_6\text{=−0.0188}$ for ${\mathrm{C}}_{\mathrm{4}}\mathrm{D},$ respectively. Intensities of the symmetry forbidden ${}^2{\mathrm{\Sigma –}}^2\Sigma$ bands were larger than those of the symmetry allowed ${}^2{\mathrm{\Pi –}}^2\Sigma$ bands, suggesting a very small energy gap and strong vibronic mixing between the ${\mathrm{Ã\hspace{0.17em}}}^2{\Pi }_i$ and ${\mathrm{X̃\hspace{0.17em}}}^2{\Sigma }^{+}$ states. Fluorescence lifetime profiles exhibited fast decay (10–20 ns) followed by a very weak and slow decay (3–4 μs) component with complicated beat structures. The implication is that ${\mathrm{C}}_{\mathrm{4}}\mathrm{H}$ in the ${\mathrm{B̃\hspace{0.17em}}}^2{\Pi }_i$ state has a fast relaxation path to nonradiative states through internal conversion, and therefore does not dissociate under near uv radiation at 400–417 nm.