Thermal desorption of bromine and iodine from an ionizer surface made of cold pressed and sintered LaB6 powder was studied at 800 to 1300C. A technique, wherein the extraction field was accelerating only during short intervals, was developed to monitor separately the neutral desorption of readily ionized elements. The technique was combined with the modulated beam and the modulated voltage methods for measurements of residence times and ionization efficiencies. It was also combined with the temperature programmed desorption method used for determination of the Arrhenius parameters of desorption. The following values were obtained for l- and l0, the activation energies of ionic and neutral desorption, and for the corresponding pre-exponential factors C and D (D = 4C) for halogens): Bromine: l- = 3.8eV, l0 = 4.3eV, C = 2.0 x 1013/s; Iodine: l- = 3.4eV, l0 = 3.7eV, C = 1.1 x 1013/s. The ionization efficiencies measured at 1100C, 0.95 for bromine and 0.7 for iodine, corresponded well to those given by the Saha-Langmuir equation using a work function of 2.7eV. All of the measurements were performed with the number of adsorbed particles well below 1017 atoms/m2. For higher coverages l- was found to increase linearly by about 0.15eV for an adsorption of 1018 atoms/m2.

Thermal Desorption and Surface Lifetimes of Bromine and Iodine Adsorbed on an Ionizer of LaB6. Mårtenson, B.M., Wilhelmsson, S.O.: Surface Science, 1985, 161[1], 181-201