Ix. Physical Electronics and Surface Physics Academic and Research Staff

The quantity To is of the order of a lattice vibration period, that is, -10 1 2 sec. (Steiner 1 and Gyftopoulos have recently reported theoretical estimates of the desorption energies of halogen atoms on (110) tungsten, the values being 4. 49 eV for F, 3. 53 eV for Cl, 2.94 eV for Br, and 2.45 eV for I.) Equation 1 is plotted in Fig. IX-l for T = X 10 1 2 se and several values of E d . It can be seen that the temperature range corresponding to -4 10 sec < T 4 1 sec is easily attainable. The reasons for choosing the halogens follow. (i) Because of their low dissociation energies, both atomic and molecular beams of halogens are easily obtained. (ii) The halogens form a series of similar valence in the Periodic Table, and therefore allow comparison of parameters, such as ionization potential and dissociation energy, to correlate data. (iii) Halogens would be convenient in a future study of surface catalysis with alkali atoms.