The $\nu_{7} + \nu_{8} - \nu_{9}$ Hot Band in Ethane

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Ohio State University

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Applying an IRIR double resonance technique in a seeded molecular jet, a rotationally cooled hot band spectrum of ethane has been recorded in the region of the C-H stretching fundamental $\nu_{7}$ near $3000 cm^{-1}$. A mixture of $C_{2}H_{6}$ and $SF_{5}$ in Ar is expanded through a slit nozzle. A 10 Watts cw $CO_{2}$ laser excites a single rotational level in the $\nu_{3}$ manifold of $SF_{6}$. Collisional relaxation redistributes the excited state population over many rovibrational levels of $C_{2}H_{6}$. A continuously tunable F-center laser (FCL) the probes the changes in $C_{2}H_{6}$ population induced by the $CO_{2}$ laser. No evidence has been found for hot bands starting from excited torsional states indicating that these levels are quickly deactivated by V-T transfer to the ground state in the collisional environment of the jet. Significant population of high rotational levels in the ground state demonstrate rapid conversion of internal rotation into end-over-end rotation of the molecule. The $v_{9}$ state collects substantial population suggesting that this state is collisionally rather stable. The observed hot bands are assigned to the two perpendicular components of the $\nu_{7} + \nu_{9}$ band. The spectrum appears to be heavily perturbed by dark state interactions in the upper level forcing us to analyse each K-stack separately. Rotational levels in the $\nu_{7} + \nu_{9}$ state up to $J = 15$ and $K = 5$ have been least squares litted to standard symmetric rotor expressions with an average standard deviation of $0.0014 cm^{-1}$.


Author Institution: Department of Molecular and Laser Physics, Catholic University of Nijmegen