Threshold Photoelectron Spectroscopy of the CH 2 I, CHI, and CI Radicals.

VUV photoionization of the CH _n I radicals (with n = 0, 1, and 2) is investigated by means of synchrotron radiation coupled with a double imaging photoion-photoelectron coincidence spectrometer. Photoionization efficiencies and threshold photoelectron spectra (TPES) for photon energies ranging between 9.2 and 12.0 eV are reported. An adiabatic ionization energy (AIE) of 8.334 ± 0.005 eV is obtained for CH ₂ I, which is in good agreement with previous results [8.333 ± 0.015 eV, Sztáray J. Chem. Phys. 2017, 147, 013944], while for CI an AIE of 8.374 ± 0.005 eV is measured for the first time and a value of ∼8.8 eV is estimated for CHI. Ab initio calculations have been carried out for the ground state of the CH ₂ I radical and for the ground state and excited states of the radical cation CH ₂ I ⁺ , including potential energy curves along the C-I coordinate. Franck-Condon factors are calculated for transitions from the CH ₂ I(X̃ ² B ₁ ) ground state of the neutral radical to the ground state and excited states of the radical cation. The TPES measured for the CH ₂ I radical shows several structures that correspond to the photoionization into excited states of the radical cation and are fully assigned on the basis of the calculations. The TPES obtained for the CHI is characterized by a broad structure peaking at 9.335 eV, which could be due to the photoionization from both the singlet and the triplet states and into one or more electronic states of the cation. A vibrational progression is clearly observed in the TPES for the CI radical and a frequency for the C-I stretching mode of 760 ± 60 cm ^-1 characterizing the CI ⁺ electronic ground state has been extracted.