Negative Ion Formation in Iodine Vapor by Electron Impacts

Abstract

Using an apparatus similar to the one used by Lozier, the negative ion formation in I₂ vapor was investigated as a function of the electron velocity (0–7 ev). The following reaction is responsible for negative ion formation at these velocities: I₂+e→I^—+I. Measurements of the kinetic energy of the ions for different electron energies show that all the ions are formed by the same reaction. The variation of the yield as a function of the velocity of the electrons can be understood with the help of a potential‐curve diagram, by considering transitions from the ground state of the I₂ molecule to the ground state of the I_2^— molecule. The probability that an electron colliding with an I₂ molecule forms a negative ion varies between 10^—4 and 10^—3. With the aid of an energy cycle one can calculate the electron‐affinity of the iodine atom, if one knows the dissociation energy of the neutral molecule, the kinetic energy of the incoming electron, and the kinetic energy of the ions. The value of the electron affinity obtained by this method is 3.0±0.2 ev, which is in agreement with other determinations.