Threshold Photoelectron Spectroscopy of the CH₂I, CHI, and CI Radicals

Abstract

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 Sztáray 28688391 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.

An ab initio study of the photodissociation of CH2I and CH2I+

“…Ionization energies for the 0–0 vibronic transitions in the C–I stretching mode for the eight first electronic states of CH 2 I + from the ground state of CH 2 I, were calculated and compared with the experimental values in ref. 8. All the absolute values of the computed energies were lower than the experimental ones by a nearly constant amount of about 0.5 eV.…”

“…A large separation of 8.334 eV, which would correspond to the adiabatic ionization potential of CH 2 I (accounting for both the radical and cation ZPEs), is found between the ground state of CH 2 I and the ground state of CH 2 I + , which implies high ionization energies of more than 8 and 10 eV to reach the ground and first excited electronic states of CH 2 I + , respectively. 8 The energy separation between the asymptotes of the curves of the neutral and cationic species is even larger.…”

“…7 More recently, potential-energy curves (PEC) along the C–I bond distance were reported for the ground electronic state of CH 2 I, and for the ground and first excited electronic states of CH 2 I + . 8…”

“…16 In recent photoion-photoelectron coincidence spectra obtained with synchroton radiation, the VUV photoionozation of CH 2 I was investigated. 8 The kinetics of the reaction of CH 2 I with O 2 was also explored by measuring the reaction rate constant at different atmospheric temperatures. 17 In the present work the structure of the electronic states of CH 2 I and CH 2 I + is investigated in order to understand their photodissociation dynamics along some of their fragmentation pathways.…”

“…As already mentioned, pathways (4) and (5) were previously investigated in ref. 8 by computing the PECs of the ground electronic state of pathway (4), and of the ground and first excited states of pathway (5), along a range of distances R C–I ≤ 3.5 Å, that is, essentially restricted to the interaction region of the curves. In the present work, the study of pathways (4) and (5) is extended to a lager number of excited states, and also to larger R C–I distances in the asymptotic region.…”