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

Abstract

Electronic spectra of C${}_6$H are measured in the $18950-21100$ 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${}^2\Pi$ 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 ${\nu }_6$ mode associated with the C-C bond vibration and the ${\nu }_4$ and ${\nu }_3$ modes associated with C$\equiv$C triple bonds. For the two lowest ${\nu }_{11}$ and ${\nu }_{10}$ bending modes, a Renner-Teller analysis is performed identifying the ${\mu }^2\Sigma$(${\nu }_{11}$) and both ${\mu }^2\Sigma$(${\nu }_{10}$) and ${\kappa }^2\Sigma$(${\nu }_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as ${\mu }^2\Sigma$(${\nu }_9$) and ${\mu }^2\Sigma$(${\nu }_8$) levels. In the excitation region below the first non-degenerate vibration (${\nu }_6$), some ${}^2\Pi {-}^2\Pi$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a ${}^2\Pi {-}^2\Pi$ transition found above the ${\nu }_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B${}^2\Pi$ state of C${}_6$H is derived and presented.

Accepted for publication in The Journal of Physical Chemistry A (26 July 2016)

References in corpus (2)

• Detection of C5N- and vibrationally excited C6H in IRC +10216
• The B${}^2\Pi -$X${}^2\Pi$ electronic origin band of ${}^{13}$C${}_6$H

Cited by in corpus (1)

• Cavity Ring-down UV spectroscopy of the C${}^2{\Sigma }^{+}$-X${}^2\Pi$ electronic transition of CH