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 Δ S 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 Δ S 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 Δ Q k in normal coordinates, which are then transformed into the Δ S i by applying the L ‐matrix. The changes Δ R CX , Δ R CC and Δ R 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].