Negative ion formation in cerium triiodide

An apparatus is described which permits heating of solid samples to temperatures up to ∼1000 °K, and confinement of the resulting vapor in a collision chamber held at a suitably higher temperature. The collision chamber is traversed by an electron beam and the resulting ions are identified by mass analysis. Measurements are reported of the negative ions produced by electron impact on CeI3 molecules in the vapor phase. Fragment ions I−, I−2, CeI−, and CeI−2 are observed, and their appearance potentials are interpreted in the light of available knowledge of bond strengths (D) and affinities (A). Resulting derived quantities include D (CeI2–I) ?3.0 eV; A (CeI2) ?0.3 eV; D (CeI–I2) ?5.7 eV; D (Ce–I) ?3.8 eV, and D (Ce–2I) ?8.0 eV. CeI−3 is also observed, formed by direct attachment of very low energy electrons, and additionally by the charge transfer reaction CeI−2+CeI3→CeI−3+CeI2, suggesting that the affinity of CeI3 exceeds that of CeI2. In addition to the ions formed by electron impact, measurements of the background ion signals of I− and CeI−4 are reported and interpreted to give the heat of formation of gaseous CeI−4, ΔHf, 298 [CeI−4(g)]=−193 kcal/mole.