Unimolecular, collision-induced and photoinduced dissociation of CH 3I +− and CD 3+−

Abstract

The properties of CH 3I +− and CD 3I +− ions have been studied using a mass spectrometer having facilities for collisional excitation and photoexcitation of mass-selected ions. Ion-kinetic-energy spectra for unimolecular dissociation of the metastable ions were measured and compared with those obtained following excitation of stable ions using the two techniques. For CH 3 + or CD 3 + fragment-ion formation, two distinct dissociation processes appear to occur for the metastable ions, but only one for ions excited in the spectrometer. One of the two processes is shown to depend on the energy of the electrons used to ionise the molecules in the source. Measurements of the translational energy released following photoexcitation shows it to decrease as the photoexcitation energy increases. This is explained in terms of the increasing importance at higher energies of the dissociation reaction giving I + in an excited state. Similar data obtained for I + fragment ions are described and discussed. An unusual feature of the I + fragmentation peaks resulting from collision-induced dissociation is that they are composite, with a very broad component which relates to large releases of translational energy, of several electronvolts. In this broad component marked asymmetry is observed which resembles that found in the fragment-ion peaks resulting from collision-induced dissociation of smaller ions. This effect has been explained by Russek, and his prediction that the degree of asymmetry is dependent on the polarisability of the collision gas is verified in the present work.