Knowledge Bank

Rotationally resolved LIF spectra of cyclopentadienyl radical associated with the $A¯\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A2\leftarrow X¯\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1$ electronic transition are recorded in a free-jet expansion $C6H5$ and $C6D5$ are produced in the jet by photolyzing cyclopentadine ($-h0$ or $-d0$) with a KrF excimer laser. The presence of vibrational bands with different rotational structures from that of the origin band indicates the excitation of Jahn-Teller active vibrations. The analysis of the origin bands of both $C5H5$ and $C4D5$ allows for an accurate determination of the radical's structure in both the $X¯$ and the $A¯$ states. Parameters directly related to the Jahn-Teller effect in the $X¯\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1$ state are obtained. Using the isotopic relationships, the implications for the Jahn-Teller distortion and vibronic coupling are discussed.