Dissociation of multiply ionized alkanes from methane to n-butane due to electron impact

The dissociation of up to triply ionized alkanes from methane to n-butane has been studied through electron impact ionization at an electron energy of 200 eV. The ionic fragments are recorded by a covariance mapping technique using a focusing time-of-flight mass spectrometer. The absolute cross-sections for the numerous dissociation channels of all the alkane monocations and dications, as well as propane and n-butane trications have been obtained. All the alkane dications are unstable and dissociate mostly into ion pairs, in which the production of proton pairs is found to be a major dissociation channel. The ionic products from the dissociation of ethane, propane and n-butane dications have a similar distribution, in which the first few abundant ion species are H+, CH3+, C2H3+ and C2H2+. The total single ionization cross-sections agree well with the results of the bin ary-enco un ter-Bethe method. The distortion of the ion coincidence islands has been explained as due to the deviation from the space focusing condition of the mass spectrometer. Metastable decays, such as C3H52+ --> CH3+ + C2H2+ in propane and C4H10+ --> C3H7+ + CH3 in n-butane have been observed and identified using the initial and final angles of the decay traces. (C) 2002 Elsevier Science B.V. All rights reserved.