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Vacuum ultraviolet ͑VUV͒ spectroscopy is used to determine the valence-band structure and location of defect states within the bandgap of porous organosilicate ͑SiCOH͒ dielectrics both before and after VUV and UV irradiation. SiCOH dielectrics have bandgap energies of about 9 eV. In addition, positive charge is trapped by defect states located 1 eV above the top of the SiCOH valence-band edge. These defect states can be populated or depopulated with electrons during UV and VUV irradiation, respectively. This is verified by measuring the magnitude and polarity of the trapped charge after VUV irradiation using two techniques: ͑i͒ capacitance vs voltage characteristics obtained with a mercury probe and ͑ii͒ surface-potential measurements obtained with a Kelvin probe. Both techniques show that the defect states are uncharged when occupied with electrons and positively charged when depleted of electrons.

Articles you may be interested inNonlinearities in composition dependence of structure parameters and magnetic properties of nanocrystalline fcc/bcc-mixed Co-Ni-Fe thin films Thin iron films (~18A thick and 90% enriched in Fe S7 ) were prepared on (001) Cu single-crystal substrates. The fcc structure was verified by electron microscopy. By Mossbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80 ± 10K with hyperfine fields of about 16-20 kOe at 4.2 K. Additional proof for the existence of antiferromagnetism has been obtained by measuring the films in a longitudinal (parallel to the ')'-ray direction) external magnetic field. The apparent discrepancy in the literature of ferromagnetic and antiferromagnetic ordering in fcc films might be resolved by taking into account the differences concerning the film orientations.

The ^^Fe Mossbauer effect on 16.8-at.% Fe-Au displays the magnetic double transition from paramagnetism to ferromagnetism at Tc == 165 and from ferromagnetism to spinglass-like behavior at Tf^ 45 K. Below T/ the magnetic state is characterized byfreezing of Fe spin components v^^hich are transverse with respect to a strong applied dc field.PACS numbers: 75.30. Kz, 76.80.+y, 75.25. + Z One of the most interesting phenomena of Az^Fe alloys is the suggested magnetic double transition for alloys near the percolation concentration (-16 at.% Fe): a transition from paramagnetism to ferromagnetism at the Curie temperature T^ followed by a transition at some lower temperature Tf which has been interpreted as a disappearance of ferromagnetism and the appearance of a spinglass-like state/"^ This suggestion is based principally on the anomalous temperature behavior of the low-field ac and dc susceptibilities. However, magnetic measurements in these alloys are difficult to interpret.'* Thus the magnetic double transition in Au-Fe alloys has been questioned recently.^ Moreover it has been found in a neutron diffraction study^ that ferromagnetism persists even down to the lowest temperatures.In this Letter we report ^'^Fe Mossbauer-effect results on AwFe which give new information on Fe spin correlations and hyperfine interactions in this alloy. Our 16.8-at.% Fe-Au samples were foils of ~ 6 /im thickness, solution-annealed at 820 °C in vacuum for 49 h, and rapidly quenched in water.The transition from paramagnetism to ferromagnetism on decrease of temperature is indicated by a sudden spectral broadening near T^ due to the development of magnetic hyperfine splitting (Fig. 1). The paramagnetic spectra obtained above 165 K are in agreement with earlier Mossbauer investigations.'^'^ Taking the onset of spectral line broadening for the transition yields T^ = 165± 5 K (see inset in Fig. 1). At 77 K our alloy shows typicsil ferromagnetic Fe spin alignment in an applied (longitudinal) field //ex t of 20 kOe. The corresponding spectrum (Fig. 2, top) indicates that essentially all of the Fe spins at 77 K were aligned parallel to i/ex t, since the absorption lines No. 2 and No. 5 (i.e., the Am=0 nuclear transitions) disappear as compared to the zero-field spectrum at 77 K (Fig. 1).The spectra shown in Fig. 2 were obtained by field cooling the sample from 77 to 4.2 K in a (longitudinal) field of H^^ , =20 kOe. The striking effect is the reappearance of the Am=0 lines at ~ 45 K and the gradual increase in their relative intensity as the temperature is lowered to 4.2 K. This means that the Fe spins (starting at ~ 45 K) gradually rotate away from the applied field direction as the temperature decreases, the ensemble of Fe spins now forming an average angle 6 =5^0 relative to the applied field. Thus below 45 K the magnetic state is characterized by a freezing of Fe spin components which are transverse with respect to the applied field and which increase in magnitude if the temperature is lowered. When 100 CO 1.001 < cr LU a95( > LU cr n \ \ r ...

a 51-year-old seven-story building within our hospital complex was demolished by explosives. The concern that this event might release large numbers of thermotolerant fungi (TF), potentially hazardous to immunosuppressed patients, led us to seal hospital windows and doors. The air-handling systems were also manipulated. Concentrations of airborne TF, especially Aspergillus fumigatus, were determined before and after demolition, using Andersen and Cassella air samplers with inhibitory mold agar plates. Two outdoor and two hospital ward locations were sampled. The plates were incubated at 37°C; the CFU per cubic meter were counted at 72 h. The outdoor concentration of TF increased at one site by an average of 1.8 logl0 (102 to 105) over the predemolition level. A. fumigatus increased 3.3 log10 (100 to 104) at the other outdoor site. The indoor TF concentrations increased about 1 logl0 (101 to 102) after demolition. Counts on the hospital wards were not remarkable when compared with previous surveillance air sampling. Protective measures apparently minimized the infiltration of TF during explosive demolition.