Knowledge Bank

The $\nu 9$ (methyl rock) fundamental band of ethane occurs in the 12 $\mu $m region. It is the strongest band of ethane in a terrestrial window and is commonly used for the identification of ethane in the Jovian planets. The $\nu 9+\nu 4-\nu 4$ band occurs in the same region; neither can be analysed as an isolated band, since both are embedded in the torsional bath of the ground vibrational state. We report here a global fit model including data from the $\nu 3$ (C-C stretch) fundamental, the $\nu 4$, $2\nu 4-\nu 4$, and $3\nu 4$ torsional transitions, the $\nu 9$ fundamental, and its $\nu 9+\nu 4-\nu 4$ hot band. The combined data set of more than 4000 lines has been fitted to within experimental uncertainty using a model Hamiltonian which includes many terms which were important in analyzing the corresponding hot band of CD$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$CD$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$. We have therefore identified Coriolis interactions with the ground vibrational state (gs) for both the $\nu 9$ and $\nu 9+\nu 4-\nu 4$ bands, as well as gs/$\nu 3$ Fermi coupling, and have determined several barrier-dependence terms which were not previously accessible. More remarkably, we have also identified a new, resonant interaction with the $\nu 12$ fundamental which -- unlike terms included in previous models -- is off-diagonal in the torsional quantum number $\sigma$.