Electron impact ionization of CHF2Cl:: Unusual ordering of ionization energies for parent and fragment ions

Electron impact ionization of the chlorodifluoromethane molecule is studied using crossed beams of high-resolution electrons and an effusive molecular beam of CHF2Cl. Ionization energies (IEs) for many positive ions from CHF2Cl (CHF2Cl+,CF2Cl+,CHFCl+,CFCl+,CHF2+,CF2+,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(CHF2Cl+/CHF2Cl)=12.50(+/-0.05) eV lies above that for the fragment ions CHF2+, CHFCl+, and CF+ [IE(CHF2+/CHF2Cl)=12.24(+/-0.03) eV, IE(CHFCl+/CHF2Cl)=12.3(+/-0.05) eV, and IE(CF+/CHF2Cl)=11.5(+/-0.1) eV]. Experiments using a three sector field BEE mass spectrometer provide evidence for the existence of two states of the parent CHF2Cl+ 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 (>20 mus) with an IE of 12.50 eV. (C) 2003 American Institute of Physics.