The dissociative photoionization onset energy of the CH3I → CH3+ + I reaction was studied at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (SLS) using a new imaging photoelectron photoion coincidence (iPEPICO) apparatus operating with a photon resolution of 2 meV and a threshold electron kinetic energy resolution of about 1 meV. Three previous attempts at establishing this value accurately, namely a pulsed field ionization (PFI)-PEPICO measurement, ab initio calculations and a mass-analyzed threshold ionization (MATI) experiment, in which the onset energy was bracketed by state-selected excitation to vibrationally excited 2A1 A states of the parent ion, have yielded contradictory results. It is shown that dimers and adducts formed in the supersonic molecular beam affected the PFI-PEPICO onset energy. The room temperature iPEPICO experiment yields an accurate 0 K onset of 12.248 ± 0.003 eV, from which we derive a ΔfHo298 K(CH3I) = 15.23 ± 0.3 kJ mol−1, and the C–I bond energy in CH3I is 232.4 ± 0.4 kJ mol−1. The room temperature breakdown diagram shows a fine structure that corresponds to the threshold photoelectron spectrum (TPES) of the A state. Low internal energy neutrals seem to be preferentially ionized in the A state when compared with the X state, and A state peaks in the TPES are Stark-shifted as a function of the DC field, whereas the dissociative photoionization of X state ions is not affected. This suggests that there are different competing mechanisms at play to produce ions in the A state vs. ions in the X state. The competition between field ionization and autoionization in CH3I is compared with that in Ar, N2 and in the H-atom loss energy region in CH4+. The binding energies of the neutral and ionic Ar–CH3I clusters were found to be 26 and 66 meV, respectively.
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