Violet/blue photoluminescence was observed from epitaxial cerium oxide films on silicon substrates. The films were deposited on silicon (111) substrates under ultrahigh vacuum conditions using pulsed laser ablation of a cerium oxide target and treated by rapid thermal annealing in argon. High resolution transmission electron microscopy and x-ray diffraction measurements indicated the formation of a single crystal cerium oxide phase Ce6O11 different from CeO2 in the annealed films. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide.

This contribution explores the relationship between teacher-centred and student-centred learning environments from a student's perspective. Three different views with respect to this relationship can be retrieved. The balance view suggests that the more teacher-centred a learning environment is, the less student-centred it is and vice versa. The transactional view stresses the continuous renegotiation of teacher-and student-roles. The independent view argues that teacher-and studentcentredness are independent features of learning environments. Results from three survey studies of higher education students' conceptions of quality education are discussed. While the practiceoriented literature regularly seems to adopt a balance view, factor analyses did not reveal evidence for the balance view in any of these studies. In students' minds student-centredness and teachercentredness seem to be mutually reinforcing features of high quality education. From a curricular point of view, and especially with regard to teacher training, the results warrant to argue for the development of so-called powerful learning environments rather than for the transition from teacher-centred towards student-centred learning environments.

The watershed of Ward Creek, a tributary to oligotrophic Lake Tahoe in the Sierra Nevada, has been investigated since 1971 with the objective of improving our knowledge of processes of nutrient and sediment release and transport to the lake. Quantitative data on selected stream water parameters were collected for 3 yr (1972—1975) at three stations on Ward Creek, two on the main upper tributaries and one near the stream mouth. Comparative data were collected at a stream mouth station on adjacent Blackwood Creek in the 3rd yr. The parameters were selected on the basis of their significance to eutrophication of Lake Tahoe. Precipitation in a normal year is over 90% snow but annual patterns vary widely and rainfall at any time of year can be highly important in sediment and nutrient transport. Water discharge and the flux of suspended sediments, NO3—N, phosphorus, iron and trace metals were dominated by the spring snowmelt runoff from mid—April to mid—June. However, in 1974 heavy fall and summer rains accounted for a large percentage of the annual flux of sediments and nutrients in a total of only 14 d. The spring runoff was characterized by distinct diel water discharge patterns. Similar but not coincident patterns were found to exist for sediments and nutrients, including NO3—N, but not for soluble phosphorus. The Ward watershed has 87% the area of Blackwood but discharge proportionately much lower quantities of sediment and nutrients in comparable water yields per hectare in water year 1975. This contrast in fluxes was probably accounted for in the history of greater disturbance by man in Blackwood Canyon. The principal source of suspended sediments in Ward Creek was streambank erosion in the lower reaches of the channel. The dominant form of inorganic nitrogen in Ward was NO3—N derived from precipitation, symbiotic nitrogen fixation and nitrification of organic nitrogen in forest soil. Phosphorus and iron were almost entirely in particulate form and thus their periods of flux occurred during high flows and sediment transport. Sediment and nutrient loading of Lake Tahoe from the Ward and Blackwood watersheds reflects a history of soil disturbance and vegetation removal. Logging, fire and stream channel diversion have been the dominant perturbations. Conservative extrapolation of annual loading data from this study to the entire basin indicates that algal nutrient levels in the lake probably have increased sufficiently in the century of man's intensive disturbance of the basin watersheds to account for increased phytoplankton and periphyton production that have been measured and observed since 1958.

There is considerable uncertainty in the estimates of indirect N 2 O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N 2 O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N 2 O budget.The objectives of this research were to measure the N 2 O fluxes from a spring-fed river, relate these fluxes to the dissolved N 2 O concentrations and NO 3 -N loading of the river, and to try and define the indirect emission factor (EF5-r) for the river.Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL .6% of that calculated using the IPCC methodology, and this itself was considered to be an over-estimate due to the degassing of antecedent dissolved N 2 O present in the groundwater that fed the river.2

lnorg~nic ~itrogen supply is an important factor limiting primary productivity ~~ Lake ~ahoe, California-Nevada. To estimate the influence of a watershed ecosystem on mtrogen mput to the lake, samples of precipitation, soil water and stream water were collected and a~alyze.d !'?r ~Oz-~, NH,-N and, with less intensity, dissolved organic nitrogen. Factors con.trol!mg mtr.Ification m se.le~ted plots were studied using a soil perfusion technique. . Inorgam.c mtrogen mput by precipitation to the watershed is 1-2 kg· ha-' · yr-'. Output of morgan~c m.trogen f!om the watershed to the lake is 10 to 33% of the precipitation input. Ino~gamc mtrogen IS re~'?ved from snowmelt water almost completely by well-developed comfer stands, but less efficiently by poorly developed or disturbed stands. Alder stands release NO,-N to soil water especially in fall and early winter. Nitrification does occur in some coniferous forest soils in the watershed and is closely associated with NO,-N release. The occurrence of significant .nitrification in the watershed is related more to the C:N ratio and stand basal area than to either pH or release of toxic substances by decaying litter.