Knowledge Bank

Employing techniques of fluorescence excitation and dispersed fluorescence, we have recorded the $S1\leftarrow S0$ electronic spectra of 2PI in a supersonic jet. All spectra exhibit extended low frequency progressions which we assign to torsional motion around the bond joining the benzene and indole moieties. The $S0$ and $S1$ torsional potentials have been determined from a vibrational analysis of the spectra by fitting the torsional vibrational spacings to a one-dimensional potential of the form $V(\varphi )=\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/2\Sigma Vn(1-\cos n\varphi )$. The potentials provide a detailed picture of the conformations of 2PI in $S0$ and $S1$, yielding accurate torsional barriers. Additional features displayed in the spectra provide interesting information about vibronic coupling effects involving the $S1$ and $S2$ states of 2PI.