On the ionization and dissociative photoionization of iodomethane: a definitive experimental enthalpy of formation of CH3I

Abstract

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 Δ_fH^o_{298 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.