Measurement of Negative Ions Formed by Electron Impact. VI. Negative Ion Mass Spectra and Ionization Efficiency Curves of Negative Ion of m/e 35 from Alkyl Chlorides

The negative ion mass spectra of alkyl (methyl, ethyl, n- and iso-propyl) chlorides, with emphasis on their relative abundance to the positive ions under 80, 40, 20 and 15 eV electron energies, and the ionization efficiency (IE) curves of Cl− ions to the extent of 30 eV electron energies were measured. In every sample, the electron impacts of 80 eV and 40 eV gave almost the same distribution of m/e for negative ion mass spectra, in which, besides the intense peaks of m/e 35 and 37 (Cl−), a relatively strong peak of m/e 25 (C2H−) in ethyl chloride and n- and iso-propyl chlorides was characteristic. For 20 eV and 15 eV, Cl− ions predominated other negative ions. The ratio of the abundance of negative ions to positive ions was the order 102–103 for CH3Cl+/Cl−, C2H5Cl+/Cl−, C3H6+/Cl− and C3H7+/Cl− throughout the range of electron energies. Each onset value observed in IE curves was interpreted to correspond to the appearance of the following reactions, the reaction scheme for second process being tentative. For the first process RCl+e−→R+Cl−, for the second process CH3Cl+e−→H+CH2+Cl−, C2H5Cl+e−→H2+H+C2H2+ Cl−, C3H7Cl(n- or iso-)+e−→H+C2H4+CH2+Cl−. For the third process the possibility of ion pair formation was pointed out. For the fourth process in ethyl chloride a brief discussion was made in view of the overlapping phenomena of a complicated dissociative electron capture process with an ion pair formation.