Knowledge Bank

Transitions with $\Delta l=\pm 2$ are forbidden by purely vibrational selection rules but become weakly allowed when the rotational resonance interaction is taken into $account.1$ Such transitions have been identified, for the first time in vibration-rotation bands, in the Connes' interferometric spectra of $Venus2$ and Mars. The observed lines have $J\prime \prime$ values from 16 to 56 in R-and P- branches of $5\Delta -\Sigma$ bands of $CO2$, the $\{1221,0421\}-0$ Fermi doublet and the $\{2221,1421,0621\}-0$ Fermi triplet. The line positions check with the observed $\Delta -\Delta$ transitions to the same upper states. The expected intensity has been calculated from the molecular constants. It may be expressed by the formula $Sm(\Delta -\Sigma )/Sm(\Sigma -\Sigma )=rvJ2(J+1)2$, where $Sm(\Sigma -\Sigma )$ is the strength of the corresponding line in the strongest neighboring $\Sigma -\Sigma$ band (201 II and 301 III respectively), and $rv$ is a band constant. The results are: $$\begin{array}{lrrrrr}v &\multicolumn{1}{l} {121,II}&\multicolumn{1}{l} {121,I}&\multicolumn{1}{l}{221,III}&\multicolumn{1}{l} {221,II}&\multicolumn{1}{l} {221,I}\\nu_{0} &4887.97&5061.78&6103.67&6288.49&6474.53\10^ {10}r_{v}-&2.03&2.24&1.89&5.33&1.77\end{array}$$ The observed lines are of the expected strength, being comparable to lines of the same m in the isotopic form $C16O17O$ (relative abundance $3.7.10-4)$ when $m\sim 30$.