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The formation and decay of parent negative ions have been studied with electron-swarm and electron-beam techniques. The rate of attachment at thermal energies under electron-swarm conditions for SF6 and C6H5NO2 was determined to be 1.24×109 and 2.1×107 sec−1·torr−1, respectively. Autoionization lifetimes for SF6−, C6H5NO2−, and (CH3CO)2− are reported to be 25, 40, and 12 μsec, respectively. The absolute rate of electron attachment in SF6 was found to be independent of the temperature (298°≤T≤418°K), and is thus consistent with an electron capture cross section for SF6 which varies inversely with the speed of the electron. The attachment rates and lifetimes were combined through the principle of detailed balance to calculate the ratio of density of states of the negative ion to that of the neutral plus electron. A simple theoretical treatment of long-lived negative ions is presented, and the possibility of estimating electron affinities from measurements of attachment cross sections and negative-ion lifetimes is pointed out.

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We have studied dissociative electron capture in a series of benzene derivatives using the combined swarm—beam method. The benzene derivatives C6H5Cl, o-C6H4Cl2, o-C6H4CH3Cl, C6H5Br, C6D5Br, and o-C6H4CH3Br, capture low-energy (including thermal) electrons dissociatively in a capture process which peaks at about 1 eV and yields only Cl− or Br− ions. The energies (in electron volts) at which the capture cross sections σc(ε) for the ∼1-eV processes peak and the values of σc(ε) (in cm2×1017) at peak energy for the above compounds are 0.86, 1.4; 0.36, 43; 1.1, 2.2; 0.84, 9.6; 0.8, 10.4; 0.95, 6 for C6H5Cl through o-C6H4CH3Br, respectively. The rates of capture at thermal electron energies are also given for each compound (but because of impurities are in question). For comparison with the benzene derivatives, C2H5Cl was also studied with the swarm—beam method. Beam data are also presented for dissociative electron capture by C6H5I, C6H5NO2, and o- and m-C6H4CH3NO2.