Knowledge Bank

Vibrational relaxation rates for gaseous mixtures $M-HC\ell$, with $M=CO2$ or $N2O$, in which vibrational energy transfer can occur from the $(00\circ 1)$ level of M to the v = 1 level of $HC\ell$, has been measured as a function of the temperature using the laser-induced vibrational fluorescence technique. The relaxation processes which must be considered are: - the V-V transfer process: \begin{eqnarray*} &&M(00^{\circ }1)+ HC (v=0)\begin{array}{c}^{k}M-HC\ell\ \rightleftharpoons\ ^{k}HC\ell-M\end{array}M(00^{\circ}0)+ HC\ell(v=1)+ \Delta E=he\Delta\nu\ &&with\ \Delta\nu=-537, cm^{-1} for\ CO_{2}, -663, cm ^{-1}\ for\ N_{2}O \end{eqnarray*} - the V-TR de-excitation processes: \begin{eqnarray*} M(00^{\circ}1)+ HC\ell(or M)\stackrel{k^{HC\ell}{M}}{(o\vec{r}; k{M})}M(mn^{\ell}0)+HC\ell (or ; M)\ HC\ell(v=1)+ M(or; HC\ell)\stackrel{k^{M}{HC\ell}}{(o\vec{r}; k{HC\ell})}HC\ell(v=0)+M(or ; HC\ell) \end{eqnarray*} For most of the systems in which near-resonant V-V transfers occur, the V-TR de-excitation rates are negligible compared to the V-V transfer rates. But this is not the case for the M-HC$\ell$ systems considered in this work. The de-excitation rates $kHC\ell M$ and $kMHC\ell$ are of the same order of magnitude as the V-V transfer rates $kHC\ell -M$ and $kM-HC\ell$ respectively. In order to determine separately all these rates, relaxation measurements have been performed by exciting either H to the $(00\circ 1)$ level or $HC\ell$ to the v = 1 level, and measuring the relaxation rates versus the molar fraction of the gas excited by laser. The results are discussed and compared with the values of the rates calculated by using a Morse potential as the intermolecular potential, and according to a semi-classical method in which a vibration-rotation exchange is assumed.