1664
COMMUNICATION TO THE EDITOR
of irradiated ammonium perchlorate below 300’ involves C102 radical as an intermediate, then the role of C103- ion may principally be that of a source of radicals. The presence of C103may also, in part, account for the seemingly uniform reaction
~
~
Vol. 65
throughout the irradated crystal when heated a t 200’. Acknowledgment.-The authors wish to thank Dr. James Hyde of Varian Associates for conducting the electron spin resonance experiments.
~
COMMUNICATION TO THE EDITOR EVIDENCE FOR ELECTRONlC INTERACTION BETWEEN IODINE AND A SOLID SURFACE Sir:
The presence of an electric field a t the surface of a solid has been inferred from certain phenomena associated with the solid-gas interface : namely, surface potentials’ and a significant reduction in the intermolecular attraction within the adsorbed film,2J which has been traced to the effect of parallel-oriented induced dipoles. The surface fields for various solids have been estimated to lie in the range lo7 to los volts em.; a field of this order of magnitude would be expected to affect the electronic absorption spectrum of an adsorbed molecule in one or more of the following ways: shifts in the electronic states due to a highly polarized condition of the molecule; splitting of the electronic energy levels, L e . , a molecular Stark effect; ionization of the adsorbate or partial charge transfer with the conduction bands of the substrate. Since iodine is known to enter into charge-transfer complex formation readily and since its major absorption band is in the visible range, its use as an adsorbate seemed likely to offer a method whereby perturbations of its electronic spectrum by the surface could be observed easily. Preliminary experiments with adsorbed iodine have shown that the expected interaction does take place, with marked changes in the visible spectrum and in some systems with the production of strong absorption in the ultraviolet region. Spectral measurements from 300 to 700 mp were made a t room temperature by reflectance from powdered solids containing adsorbed iodine; in all cases the adsorption was reversible, the iodine being desorbed readily by evacuating the sample a t 100”. The quantity of iodine adsorbed was known only approximately, but was always less than a complete monomolecular layer; nevertheless, the colored adsorbed film was clearly visible and its departure from the violet color of iodine vapor in many cases could be detected a t once by the eye. The material that displayed the least effect among those observed is silica (when freed of adsorbed water). The principal absorption band (1) J. C. P. Mignolet, “Chemisorption.” ed. W. E. Garner, Butterworth, London, 1957, p. 118. (2) J. H. de Boer, “The Dynamioal Character of Adsorption,” Clerendon Press,Oxford, 1953,pp. 168-169. (3) E.Clark and 8.Ross,J. A n . C b m . Soc., 76,6081 (1953).
of iodine vapor, centered about, 520 mp, is shifted to about 480 mp. One specimen of silica in the form of a transparent, porous slab was suitable for transmission measurements down to 220 mp: adsorbed iodine on dry silica showed no ultraviolet absorption. Silica-alumina cracking catalysts also show an absorption maximum in the visible region a t around 450-480 mp, but have developed an increasing absorption below 350 mp. We could not investigate the absorption a t shorter wave lengths with our present apparatus. The samples mentioned above were violet to pink in color. Samples of pure alumina and boron nitride showed a gradual increase of absorption with decreasing wave length below 700 mp; they did not, however, display any absorption maximum in the visible region. The wsible color of these systems is bright yellow to brownish. A slight difference between y- and 11-alumina is apparent to the eye: the slightly redder tint obtained with y-alumina plus iodine can be traced to its greater absorptioii in the green portion of the spectrum. On a third group of solids the adsorbed iodine film is characterized by having only a slight absorption in the visible spectrum, thus appearing white or faintly yellow in color. These substances include the alkali halides, calcium fluoride, and zirconium and titanium oxides. The reflectance spectra show sharply increasing absorption below 350 rnk. The colors formed by the interaction of iodine with various solid surfaces is reminiscent of thc colors of iodine in various solvents. The greatest changes in the absorption spectra of iodine solutions are found with the more polar solvents, and presumably the same rough correlation exists with adsorbed iodine on surfaces of differing polarity. Furthermore, the charge-transfer complex mechanism postulated to account for the spectra of certain iodine solutions is also a reasonable explanation of the spectra of the adsorption complexes: if this is the case, the study of the spectra of adsorbed iodine should yield important information about the electronic nature of solid surfaces, which mould be pertinent to their action in heterogeneous catalysis. A more detailed study of such systems is now in progress. This work forms part of a program sponsored a t Rensselaer Polytechnic Institute by Esso ReResearch and Engineering Company. RENSSELAER POLYTECHNIC INSTITUTE SYDNEY Ross TROY,N. Y. JAMES P. OLIVXER JUNE 20, 1961 RECEIVED
