Valence-Shell Ionization Potentials of Halomethanes by Photoelectron Spectroscopy. I CH3Cl, CH3Br, CH3I. Vibrational Frequencies and Vibronic Interaction in CH3Br+ and CH3Cl+

Photoelectron spectra of methyl chloride, methyl bromide, and methyl iodide obtained with 584‐Å radiation yield in each case four of the five valence‐shell ionization potentials. Values of 11.29, 10.53, and 9.50 eV are obtained for the first ionization potentials, respectively, corresponding to ionization from halogen “lone‐pair” orbitals. Resolved vibrational structure allows determination of frequencies of the corresponding normal vibrations of the positive ions in several cases. Relative intensities of vibrational components in the first ionization potential are dictated by vibronic interaction. Approximate values of the Jahn–Teller parameters $D$ for ${\nu }_4{\mathrm{,\; \nu }}_5{\mathrm{,\; \nu }}_6$ in CH₃Br⁺ may be obtained from these intensities. Small values of $\text{D (D\hspace{0.17em}≤\hspace{0.17em}0.1)}$ suffice to produce sizeable modifications of intensity for the intermediate spin–orbit splittings observed in these ions.