Extended Appearance-Potential Fine-Structure Analysis: Oxygen on Al(100)

To measure O-Al separation at Al(100) surfaces disordered (low-energy electron-diffraction beams extinguished) by reaction with oxygen, the extended appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O 1s core. Calculation shows that the outgoing electron has angular momentum l=0, allowing simple Fourier inversion of the fine structure. The separation, 1.9+-0.05 A, suggests that oxygen lies under the top layer, a result undetectable in extended-x-ray-absorption fine-structure measurements on thicker films.https://doi.org/10.1103/PhysRevLett.44.49

Effect of the central atom potential on the extended fine structure above appearance potential thresholds

The formalism previously given for describing the extended fine structure above appearance-potential-spectroscopy (APS) thresholds is extended by incorporating the effects of the excited "central" atom potential in an exact manner. The excitation-matrix elements are expressed in terms of the exact wave functions of the central atom potential. This introduces a "phase renormalization" into the excitation-matrix elements and eliminates a previously noted "phase difference" between single- and multiple-scattering calculations employing a plane-wave basis set. A series of approximations is then made which leads to an expression for the APS extended fine structure in terms of sinusoidal functions and hence provides a rationale for a Fourier-transform analysis. Simple model calculations assuming a constant "bare" excitation-matrix element, a spherically symmetric electronic density of states, and only S-wave scattering from the atomic cores are performed for a cluster of atoms having the atomic geometry of bulk vanadium. These calculations display the major predictions of the formalism and indicate that for a given system there may be some optimal energy range for data analysis. The problem of electron characteristic losses is considered, and it is pointed out that in the small-momentum-transfer limit the simple dipole selection rules appropriate to a photon-excitation process again apply. This may obviate many of the problems introduced by multiple angular momentum final states in the APS process.https://doi.org/10.1103/PhysRevB.21.210

Uno studio di riferimento su 883 pazienti affetti da neoplasia della rinofaringe trattati in due Centri italiani dal 1977 al 2000. Parte I: evoluzione delle scelte tecniche e sopravvivenza

PurposE. The authors sought to define treatment results according to the different accrual periods and clinical- therapeutic features in a large series of nasopharyngeal cancer (NPC) patients treated in two Italian centres over more than two decades. Materials and methods. A total of 883 patients consecutively treated with radiotherapy between 1977 and 2000 at the Florence (FLO) and Brescia (IRA) Radiation Oncology centres were studied. Five-year overall (OS) and disease-specific (DSS) actuarial survival rates in the different pathological, clinical and therapeutic subgroups were calculated, along with the actuarial local-regional control (LRC) probabilitY. Results. At univariate analysis, survival and local control rates were significantly better in the more recent accrual periods and in the more favourable disease presentations; treatment-related parameters mainly affect LRC. At multivariate analysis, patient- and disease-related factors had a more evident prognostic effect than did therapeutic factors, although dose to the nasopharynx and treatment technique had a marginally significant impact on DSS and OS. Conclusions. Results of this benchmark study may be useful for understanding the development of new radio- therapy techniques for NPC, such as three-dimensional conformal radiotherapy (3D-CRT) and particularly intensity- modulated radiotherapy (IMRT). © Springer-Verlag 2011