An Estimate of the Interatomic Distances in Monohaloacetylene Radical Cations from Photoelectron-spectroscopic Data

A convenient method is given for the semiquantitative determination of those changes ΔS_i in internal coordinates, which describe the structure of the radical cation M⁺(Ψ) relative to the neutral molecule M in its electronic ground state. The changes ΔS_i correspond to those associated with the relaxation process which follows the ionisation M + hv → M⁺(Ψ) + e, M⁺ being in the electronic state Ψ. The method, based on the procedure originally developped by Smith & Warsop 3], uses the spacing and relative intensity of the vibrational fine-structure components of the bands in the photoelectron spectrum of M. From these data are calculated the changes ΔQ_k in normal coordinates, which are then transformed into the ΔS_i by applying the L -matrix. The changes ΔR_CX, ΔR_CC and ΔR_CH which describe the structure of the radical cations of the four monohaloacetylenes I(X) (X = F, Cl, Br, I) in their ² II _Ω(1) and ² II _Ω(2) states have been determined. It is shown that the results are in agreement with theoretical expectation and that they confirm the band assignement proposed in a previous communication 2].