Knowledge Bank

We present a comparison between experimental results (vibronic energies, absorption and LIDFS intensity ratios, rotational constants) and ab-initio calculations (diabatic and adiabatic energies, vibronic matrix elements). The three main inputs required in the analysis of the $X~2A1-A~2B2$ conical intersection are: i) the diabatic levels of the $X~2A1$ state (i. e., the complete set of approximatively 200 low lying levels of the $X~2A1$ observed by LIDFS), ii) the diabatic levels of the $A~2B2$ state (they are approximately predicted by the ab-initio calculations), iii)the matrix elements of the vibronic interaction, $V12$, between the $X~2A1$ and $A~2B2$ electronic states. The validity of the simplified form proposed for $V12$, namely ``$\lambda Q3\prime \prime$, will be discussed. The comparison allows to assign some observed vibronic levels, which in return can be used to improve some parameters of the initial ab-initio PESurfaces. Up to now, our analysis is limited to the four lowest polyads of the $A~2B2$ state ranging from $9700cm-1$ to $12300cm-1$. At higher energy the interactions are stronger, leading to vibronic chaos above $\sim 1700cm-1$.