Knowledge Bank

Employing techniques of resonance enhanced two photon ionization and threshhold electron field ionization, we have measured both the $S1\leftarrow S0$ and cation $\leftarrow S1$ spectra of 2,6-difluorotoluene (DFT)$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ DFT’s $S1\leftarrow S0$ origin lies at $37948cm-1(\pm 0.3cm-1)$, and DFT shows an adiabatic ionization potential of $73673cm-1(\pm 7.0cm-1)$. Slightly to higher energy of the $S1$ and cation origins. We observe a number of band which we assign to internal rotation of the DFT methyl group. Fitting these features to a sixfold symmetric potential $(\nu 42(1-\cos 6\alpha ))$ suggests a very low barrier to methyl rotation in both the $S1$ $(|V6(S1)|\sim 20 cm-1)$ state and cation ground state $(|V6(Do)|\sim 10cm-1)$. These measured barriers are consistent with ab initio calculations predicting a small $V6$ in the ground state of DFT $(V6(So)=17.4cm-1)$. This talk will focus on the origin of these small $V6$ terms in the context of $V6$ potentials found for other six-fold symmetric molecules. Additionally, a coupling mechanism invoked to explain several nominally forbidden transitions observed in the spectra will be discussed.