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In a recent article in this journal, we discussed the application of a 3D-QSAR technique to the prediction of the toxicity of dibenzodioxins, dibenzofurans, and biphenyls (Vedani et al., 1999a). The use of such methods is legitimate because there is strong evidence that the toxicity is mediated by the Aryl hydrocarbon (Ah) receptor, a regulatory element involved in the mammalian metabolism of xenobiotics. In an extention to a concept developed at our laboratory (Vedani et al., 1998), we now show that safer predictions are possible if instead of a single - and, therefore, necessarily biased - assumption about the mutual orientation of the toxins, an ensemble of possible orientations is used for model construction. The contribution of a single entity within this ensemble to the toxin-receptor interaction energy is determined by a Boltzmann criterion. While in the single-orientation model two of the 26 toxins of an external test set were predicted false positive or false negative, all test substances are correctly predicted in the multiple-orientation model - including up to four different orientations per molecule - within a factor 10 of the experimental binding affinity. These results demonstrate that 3D-QSAR techniques based on a genetic algorithm can be used to predict the toxicity of chemical and pharmacological substances "in computo" if a receptor-mediated mechanism can be assumed. Consequently, the method can be used in toxicological screening assays, thereby replacing stressful tests on animals.

Irradiation effects on iron disilicide layers were induced by 3.5 keV argon ion bombardment and studied by XPS, AES and the Mössbauer spectroscopy. The use of these electron spectroscopies enables the investigation of different regions of the altered layer.

Si+ ions of 50 keV in energy were implanted into a-Fe (95% 57Fe) with a nominal dose of 5x10E17 cmE-2.

Mittels DCEMS wurde eine tiefenselektive Phasenanalyse der Fe-Disilizid-Bildung in Fe-ionenimplantiertem Si durchgeführt.

Iron was implanted with 50 keV Si ions. The depth distribution of the formed Fe-Si phases was investigated by depth selective conversion electron Mossbauer spectroscopy.

Durch Ionenstrahlverfahren hergestellte Eisensilizidproben wurden mittels XPS und AES untersucht.

Doses from 2 x 1015cm-2 to 2 x 1017cm-2 of 57Fe ions were implanted with 40 keV into n-type Si(111) at 350°C. The iron concentration profiles reach peak maximum concentrations from 0.3 at.% for the lowest dose up to about 34 at.%. For the highest dose a plateau-like profile is formed due to the effect of sputtering. The phase formation of the iron disilicides was studied by 57Fe conversion electron Mössbauer spectroscopy. Depending on the implanted dose two different regions of phase formation are found. For doses £ 1 x 1016cm-2 only the metastable g-FeSi2 is present whereas at values ³ 2 x 1016cm-2 a mixture of a- and b-FeSi2 is formed. The phase composition in this second region is dose dependent and with increasing iron content an enlarged fraction of the b-phase is found. At the highest dose of 2 x 1017cm-2 mostly b-FeSi2 and no a-FeSi2 is present. But additionally to the b-FeSi2 a second component occurs in the Mössbauer spectrum which is observed for the first time. With respect to the iron concentration this subspectrum could be attributed to a new metastable state of the mono silicide. Different annealings of the as-implanted samples at 900°C and 1150°C lead to the formation of the pure b- and a-FeSi2, respectively. Additionally to the measurements of the concentration profiles investigations of the lateral element distributions in the layers were performed with scanning Auger electron microscopy. It can be concluded that during the annealing at 1150°C precipitates of the a-FeSi2 grow in the Si matrix.

Iron implanted and subsequently annealed n-type Si(111) was studied by conversion electron Mössbauer spectroscopy and Auger electron spectroskopy. Phase analysis performed with Mössbauer spectroscopy indicates the formation of a mixture of ß- and a-FeSi2 due to the implantation (200 keV, 3 x 1017 cm-2, 350 °C) and a complete transition of the mixture to the ß-and the a-phase after annealing at 900 °C for 18 h and 1150 °C for 1 h, respectively. The iron concentration profiles were determined with Auger electron sputter depth profiling. The profile of the as-implanted sample is Gaussian shaped and boradened after annealing at 900 °C to a pleateau-like profile. For the sample annealed at 1150 °C only a slight broadening occurs and depth depending fluctuations of the iron concentration can be observed. Investigations with scanning Auger electron spectroscopy show in the case of the as-implanted sample only slight fluctuations of the iron distribution in the center of the profile. After annealing at 900 °C large separated ß-FeSi2 precipitates grow due to the process of Ostwald ripening. At 1150 °C Ostwald ripening and coalescence of the precipitates occur and a wide extended penetration a-FeSi2 network structure is formed. It is the first time that such large iron disilicide structures could be proved by scanning Auger electron spectroscopy.

Iron implanted and subsequently annealed n-type Si(111) was studied by conversion electron Mössbauer spectroscopy for phase analysis and Auger electron spectroscopy for sputter depth profiling and element mapping. During implantation (200 keV, 3 x 1017 cm-2, 350 °C) a mixture of ß- and a-FeSi2 and after the subsequent annealing (900 °C for 18 h and 1150 °C for 1 h) a complete transition to the ß- and a-phase can be detected. The as-implanted profile has Gaussian shape which becomes broad during annealing at 900 °C to a plateau-like profile and shows only a slight broadening and depth depending fluctuations of the iron concentration after the 1150 °C annealing. With scanning Auger electron spectroscopy the lateral iron and silicon distribution were investigated show for the sample annealed at 900 °C large separated ß-FeSi2 precipitates which grow due to the process of Ostwald ripening. At 1150 °C additionally coalescence of the precipitates occur and a wide extended penetration a-FeSi2 network structure is formed.