ROTATIONALLY RESOLVED SPECTRA OF JAHN-TELLER ACTIVE VIBRATIONS IN AN ELECTRONICALLY ALLOWED $A \leftarrow E$ TRANSITION

Abstract

Due to the Jahn-Teller effect, the fundamental band of a Jahn-Teller active vibration becomes allowed in an electronic $A \leftarrow E$ transition. The corresponding rotational selection rules differ from those of the origin band and the intensity gives a good indication of the magnitude for the Jahn-Teller interaction. We have recorded high resolution, rotationally resolved, LIF spectra of a number of different vibronic bands of jet-cooled $C_{5}H_{5} (\bar{A} ^{3}A_{2} \leftarrow \bar{X} {^{3}}E_{1}). C_{6}F_{6}H_{5}^{+}(\bar{B}{^{2}}A_{2}+ \bar{X}{^{2}}E)$and $C_{6}F_{6}{^{+}}(\bar{B}{^{2}}A_{2} \leftarrow \bar{X}{^{2}}E_{1})$. Several of these vibronic bands with rotational structures different from those of the corresponding origin bands are assigned to the fundamental transitions of Jahn-Teller active vibrations. Detailed rotational analyses yield, in addition to the normal molecular constants, the values of the angular moments, $\zeta_{\nu}$, for various Jahn-Teller active vibrations. For $C_{6}F_{3}H_{3}^{+}$ and $C_{6}F_{6}^{+}$, the previous assignment of the Jahn-Teller active vibrations is confirmed. For $C_{4}F_{4}$, the first definitive identification of the Jahn-Teller active vibrations is made.