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

Abstract

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 +, CH 3 +, C 2H 3 + and C 2H 2 +. The total single ionization cross-sections agree well with the results of the binary-encounter-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 C 3H 5 2+→CH 3 ++C 2H 2 + in propane and C 4H 10 +→C 3H 7 ++CH 3 in n-butane have been observed and identified using the initial and final angles of the decay traces.