Formation of association ions in the photoionization of alkyl halides

The methyl and ethyl chlorides and bromides, as well as methyl iodide, were photoionized in the vacuum ultraviolet at 300 K in a mass spectrometer over the pressure range 0.5 to approximately 100 millitorr. Under these conditions, stabilized parent ion dimers are found in CH 3 Br, CH 3 I, and C 2 H 5 Br, but not in the chlorides. Lower limits for the dissociative lifetimes of the ion–molecule collision complexes were estimated and are as follows: (CH 3 Br) 2 + , 1.6 μs; (CH 3 I) 2 + , 1.9 μs; and (C 2 H 5 Br) 2 + , 5.4 μs. An increase in photon energy (internal energy content of the reactant ion) decreases the dissociative lifetime of the collision complex in CH 3 I.