Knowledge Bank

In low resolution ($30cm-1$) Dispersed Fluorescence (DF) spectra of acetylene from $2v3\prime$ and $3v3\prime$ levels of $A¯$ $Au$, the expected Franck-Condon (FC) bright states are progressions in $v2\prime \prime$ (CC stretch) and $V_{4}^{\prime\prime}$ (Irans-bend). It was hypothesized that two additional progressions prominent in the DF spectrum were either built on excited $v_{1}$ (sym, CH"""" stretch)$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$, possibly FC bright due to the slight elongation of the CH bond in the A state, or on even quanta of $v5\prime \prime$ $(cis-bend)2$ appearing via a strong but previously undetected $\backslash DeltaV4\prime \prime =V5\prime \prime =\pm 2$ Darling-Dennison (DD) resonance. Either assignment was plausible because $V1\prime \prime V2\prime \prime +2v5\prime$ In high resolution ($0.1cm-l$) Stimulated Emission Pumping (SEP) spectra of acetylene from the 2$v3$^{\prime} level of the $A~$ state to the $7000cm-1$ region of the $X~$ state, the observation of two more $t=2$ levels than $t=0$ levels proves that not only is the DD assignment correct but, a well characterized $\backslash DeltaV3\prime \prime$ (antisym ``CH” stretch)=-$\Delta v2-\Delta v4\prime \prime =-Av5\prime \prime \pm 1$ Fermi $resonance3$ hasáallowed ($\nu 1,\nu 2,\nu 3,\nu 4,\nu 5)=(0,0,1,5,1)$ to borrow intensity from (0,1,0,6,2) which in turn borrowed its intensity from the FC bright (0,1,0,8,0).""