THE RENNER-TELLER AND JAHN-TELLER EFFECTS IN PROTOTYPICAL MOLECULAR CATIONS SUBJECT TO A VERY LARGE SPIN-ORBIT COUPLING

Abstract

PFI-ZEKE photoelectron spectra of the $\mathrm{X}^+\;^2\Pi \leftarrow \mathrm{X}\;^1\Sigma^{+}$ transition of HC$_2$I and of the $\tilde{\mathrm{X}}^+\;^2\mathrm{E}_{3/2} \leftarrow \tilde{\mathrm{X}}\;^1\mathrm{A}_1$ transition of CH$_3$I have been recorded at a resolution sufficiently high to observe, at least partially, their rotational structure. The spin-rovibronic energy-level structures of HC$_2$I$^+$ and CH$_3$I$^+$ could be determined at low energies and enabled us to study the Renner-Teller and Jahn-Teller effects in molecular cations subject to a very large spin-orbit coupling with unprecedented details. In the case of HC$_2$I, the nominally forbidden 5$^1_0$ band has been observed in addition to the origin band and allowed us to determine a splitting of 2~cm$^{-1}$ between the two Renner-Teller components of the 5$^1$ vibrational level of the cation. In the case of CH$_3$I, the rotational structure of the origin and of the 2$^1_0$ and 3$^1_0$ bands are dominated by satellite bands of spin-rovibronic origin \textbf{134}, 054308 (2011).}. The 5$^1_0$ and 6$^1_0$ bands reveal an additional splitting corresponding to the separation between the two Jahn-Teller components of $j=1/2$ and $j=3/2$ symmetry of the 5$^1$ and 6$^1$ levels of the cation \textbf{17} (4), 435 (1998).}.