Knowledge Bank

Rotationally resolved LIF spectra of the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A2\leftarrow 2E1$ band $(\nu 00=29.572.166cm-1)$ of cyclopentadienyl radical $(C5H5)$ were taken in a supersonic free-jet expansion. An effective Hamiltonian appropriate for $C4H5$ in $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1$ vibronic state is constructed. A least square fit of the $O00$ band yields molecular constants for both lower and upper states. Intensity distributions of the spectra at different degrees of coldness are reproduced satisfactorily by an intensity simulation based on transition moments, nuclear spin statistics and variable rotational temperatures (0.6 - 10K). Strong (2.2) interactions observed in the vibrationless level of the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1$ state may indicate a large Jahn-Teller effect. The spin effects (spin-orbit and spin-rotation interactions) in C5H5 are also discussed.