Fragmentation of aliphatic chlorocarbons under low-energy (≲10 eV) electron impact

Abstract

The fragmentation of ten aliphatic chlorocarbons (CHCl3, 1,1,1-C2H3Cl3, 1,1,2-C2H 3Cl3, 1,1-C2H4Cl2, 1,2-C2H4Cl2, C2Cl4, C2HCl3, 1,1-C2H2Cl2, cis-1,2-C2H2Cl2, and trans-1,2-C 2H2Cl2) under low-energy (0-10 eV) electron impact has been investigated with a time-of-flight mass spectrometer. Three types of fragment negative ions were observed: Cl-, Cl 2-, and (M-Cl)- (parent molecule, M, less one Cl atom)-; for C2Cl4 the parent ion, C 2Cl4-*, was also observed at ∼0.0 eV and found to be metastable with an autodetachment lifetime of 14 ± 3 μsec. The Cl- ion was by far the most abundant. The intensities of Cl2- and (M-Cl)- with respect to Cl - depended very strongly on the number and relative positions of the Cl atoms in the molecule. The yield of Cl2- was, as a rule, very much lower when the two Cl atoms in the Cl2- ion originated from the same C atom. The measured Cl- currents as a function of electron energy were corrected for the finite width of the electron pulse, and the resultant unfolded functions exhibited distinct resonances whose relative intensity depended strongly on molecular structure. They indicated the existence of at least five such resonances below ∼2 eV for both the chloroethanes and the chloroethylenes studied. The average positions of the observed resonances are 0.18, 0.54, 0.8, 1.06, and ∼1.45 (?) eV for the chloroethanes and 0.19, 0.53, 0.75, 0.95, and ∼1.35 eV for the chloroethylenes. The spacings and energy dependences of the intensities of the observed resonances seem to preclude the involvement of vibrational levels, and the similarity in their positions for the two groups of molecules (chloroethanes and chloroethylenes) seems to suggest that they are associated with orbitals dominated by the p-orbitals of the Cl atoms. The C = C π-orbital is clearly involved in the formation of the long-lived C2Cl4-* ion. The relative magnitudes and energy dependences of the Cl-, Cl2-, and (M-Cl)- ions have been partially rationalized on the basis of the energetics involved in the production of the respective ions. Comparison of the present results with existing electron-swarm data is satisfactory; based on the swarm data, absolute cross-section values for the Cl- ions of some of the compounds studied have been obtained. Copyright © 1977 American Institute of Physics.