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A Spectroscopic Survey of Electronic Transitions of C$_6$H, $^{13}$C$_6$H, and C$_6$D

Electronic spectra of C$_6$H are measured in the $18\,950-21\,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2Π$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $ν_6$ mode associated with the C-C bond vibration and the $ν_4$ and $ν_3$ modes associated with C$\equiv$C triple bonds. For the two lowest $ν_{11}$ and $ν_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $μ^2Σ$($ν_{11}$) and both $μ^2Σ$($ν_{10}$) and $κ^2Σ$($ν_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $μ^2Σ$($ν_9$) and $μ^2Σ$($ν_8$) levels. In the excitation region below the first non-degenerate vibration ($ν_6$), some $^2Π-^{2}Π$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2Π-^{2}Π$ transition found above the $ν_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2Π$ state of C$_6$H is derived and presented.