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The response of TiO2 and SrTiO3 photoelectrodes to subband gap light has been explored as a function of numerous variables, including intensity and wavelength of light, applied potential, temperature, and electrolyte composition. The photoresponse is shown to be linear with light intensity, to depend on the square root of the applied potential, to occur only for hv > 2.2 eV, and to be independent of temperature and electrolyte composition. These results suggest a bulk excitation process involving impurity and defect-related states in the gaps of these semiconductors. A qualitative model is presented to account for the main features of this excitation process and to provide a basis for suggesting future experiments.

resist and backscattering from the substrate are negligible. Therefore, the proximity effect which puts severe limitations on electron beam fabrication of closely packed narrowlinewidth pattern is expected not to be a serious problem with ion-beam lithography. Encouraging results in this direction have also been quoted by Tarui. 5 These preliminary results are very encouraging and indicate that ion beams can be employed in lithography for the production and/or projection transfer of very-narrowlinewidth patterns. In the meantime, the development of high-intensity field-emission sources which are needed for the production of fine ion beams is progressing rapidly. One such source is under development here, at the department of Applied Physics of Cornell University by B. Siegel and G. Hanson 9 and similar work has been reported by Levi-Setti.lo In conclusion, we would like to thank Professor e. Tang and Professor J. Ballantyne for kindly making available to us the Ar laser and Professor R. Burhman for use of the ion miller. The microphotopgraphs were taken using the Cambridge S 150 SEM available at the Submicron Facility in

Data are presented on the second order Raman spectrum of rutile TiOt for various scattering geometries, at room and liquid Nz temperatures. The results of a standard critical point analysis (i) allow the most important addition processes at low energies to be identified, (ii) indicate the importance of matrix element effects, and (iii) cannot account for a peak at 172 cm-' for the Y ( x y ) X geometry.The Raman and infrared fundamental modes of rutile at the r point of the Brillouin zone have been assigned in many papers published previously' and extensive neutron scattering data are available for the r, X , M and 2 points' with symmetry assignment. To our knowledge, however, no one has assigned the second order Raman spectra of this crystal, which are very intense. In this work we present data on and a preliminary analysis of the second order Raman scattering in rutile TiOz.The sample was purchased from MRC and it was grown by the Verneuil technique, all impurities were stated to be less than 100 ppm, and the possible effects of vacancies were minimized by oxidizing the crystal in a furnace for several hours. The crystal was oriented by X-rays, cut and polished, and mounted in a liquid nitrogen cryostat described elsewhere.' The data were taken with a conventional Raman scattering system, using an Ar laser operating at 4880 A with a power of about 400 mW. A pair of Glan polarizers with extinction ratio of 1 : 1@ were used as a laser polarizer and scattering light analyzer in order to eliminate 'leak through' of signals in the wrong polarization. The double spectrometer was set on a resolution better than 0.8 cm-' and a typical counting rate of 10' countss-' was achieved in the region of 250 cm-' of the spectrum. In Fig. 1 are shown the room temperature spectra in the region of 250 cm-' whereas Fig. 2 shows the region of the spectra most important for multi-phonon study. The latter spectra were taken at liquid nitrogen temperature.Our aim is to provide a preliminary analysis of the second order Raman spectra of rutile Ti02 presented above. Thus we limit ourselves to a consideration of the overtone and combination two-phonon processes taking into account only the special points r, X , M and Z of the Brillouin zone.Rutile TiOz has the space group Dtt with two molecules, i.e. six ions per primitive cell. This means that we have a very large number of phonons (18 per Brillouin zone point). The lattice dynamics have been studied both by the rigid ion and shell model appro ache^.'.^ The latter, not surprisingly, yields better fits to the neutron data. In ' Table 1 we show the relevant phonon symmetries and energies. To study the two-phonon processes we perfoim the standard group-theoretical calculations, obtaining the irreducible representations of the full space group 0:; at the special points I', X , M and Z? The Kronecker products (combinations) or symmetrized Kronecker products (overtones) of these 300 200 I00 cm-1 Figure 1. Raman room temperature spectrum of Ti02 near the strong two phonon region: (a) x y componen...