Electron impact ionization of CHF_2Cl:Unusual ordering of ionization energies for parent and fragment ions

摘要

Electron impact ionization of thechlorodifluoromethane molecule is studied using crossed beams of high-resolution electrons and an effusive molecular beam of CHF_2Cl.Ionization energies(IEs) for many positive ions from CHF_2Cl(CHF_2Cl~+,CF_2Cl~+,CHFCl~+,CFCl~+,CHF~+_2,CF~+_2,HCl~+,Cl~+,CF~+,CH~+,F~+C~+) are determined from a careful examination of the threshold behavior of the ionization cross sections.Reaction pathways for the dissociative ionization products are suggested using known thermodynamic quantities.Surprisingly,it is observed that the ionization threshold for the parent positive ion IE(CHF_2Cl/CHF_2Cl)=12.5(+-0.05)eVlies above that for the fragment ions CHF~+_2,CHFCl~+,and CF~+[IE(CHF~+_2/CHF_2Cl)=12.24(+-0.03) eV,IE(CHFCl~+/CHF_2Cl)=12.3(+-0.05)eV,and IE(CF~+/CHF_2Cl)=11.5(+-0.1)eV].Experiments usign a three sector field BEE mass spectrometr provide evidence for the existence of two states of the parent CHF_2Cl~+ ion,one exhibiting a short lifetime of about 2 mus and another parent ion state which appears to be stable on the time scale of the experiment(>20mus) with an IE of 12.50 eV.