J M Esteva

An approximation of the Voigt I profile for the fitting of experimental X-ray absorption data

Abstract An analytical expression of the Voigt integral, derived by numerical methods, is proposed. It spans a wide range in X-ray spectral region in which the Lorentzian broadening of atomic origin and the Gaussian (instrumental) broadenings are of comparable magnitude. The indefinite integral of this expression is easy to evaluate and provide a good approximation for the X-ray threshold profiles. The proposed expressions are compared with previously proposed Voigt profiles and an example of experimental data analysis by fitting the lines and threshold profiles is given.

Probing matrix isolated SiO molecular clusters by X-ray absorption spectroscopy

TheK absorption edge of Si in matrix isolated SiO molecular clusters was studied for various dilutionsR(Ar/SiO). The spectra from the clusters at different dilutions show a dramatic evolution in their relative intensity. Two types of spectral features from Si atoms present in the clusters could be detected in the spectra. The first is due to the Si atoms which are tetrahedrally coordinated to Si and O atoms responsible for the presence of different Si oxidation states. The Si+, Si3+ and Si4+ oxidation states in SiO clusters and in bulk were readily identified. The Si2+ oxidation state whose abundance in SiO clusters is predicted by structural considerations is not clearly observed. The second type of feature, whose intensity decreases with the dilution R, can be attributed to the presence of Si atoms possessing unsaturated bonds. In order to explain this the clustering of 4–6 SiO molecules is simulated to yield different tetrahedral bonding microstructures in which the well coordinated as well as under coordinated Si atoms are present. The behaviour of the spectral features resulting from these microstructures is discussed.

On the profiles and linewidths of the 3d→4f transitions in the lanthanides

Profiles and linewidths of the 3d to f transitions in many lanthanides have been determined using synchrotron radiation emitted by the ACO storage ring and a new double-crystal monochromator. The results are compared with existing theory and a more detailed interpretation of the structure is given than has been possible to date.

Unresolvable Rydberg lines in X-ray absorption spectra of free atoms

The authors investigate the contribution of the unseparated Rydberg states which are usually neglected, in atomic K-edge X-ray spectra. They suppose that the core hole broadening is large, so that the global contribution of all unresolved Rydberg levels introduces an apparent threshold shift with a value nearly equal to the separation of the first unresolvable Rydberg line from the series limit. The apparent threshold has the same dependence on the photon energy as the true one. The region of unresolvable Rydberg lines can be described by means of a quasi-particle approach, by introducing a pseudo-wave vector formalism. The authors perform an analysis of Ne and Ar K-edge absorption spectra. Using the present theory, they derive a very simple procedure for the evaluation of the instrumental and the core hole broadening in X-ray spectra.

Photoabsorption and electron yields of La and Gd in the vicinity of the 3d thresholds

Abstract A comparative study of the 3d photoabsorption and electron-yield spectra of La and Gd using synchrotron radiation emitted by the Orsay storage ring (ACO) is presented. The effects of sample thickness on each kind of spectrum are discussed. The mean sampling depths for La and Gd are determined from the variations in intensity of the main excitation lines as a function of film thickness in the yield spectra. The advantages and potential of the yield technique for the study of lanthanide spectra are pointed out.

K-shell excitation spectrum of Si in SiO vapour

The K absorption spectrum of Si in SiO vapour has been obtained at a temperature of 1200 degrees C. A modified King furnace is used for vapour containment. The absorption structures in the spectrum of the SiO vapour are understood in the framework of the equivalent core model by comparing their energies with those of corresponding terms of the PO molecule.

Sodium 1s photoabsorption spectra of Na and NaF clusters deposited in rare gas matrices

Abstract Na and NaF clusters were investigated by Na 1s edge excitation using synchrotron radiation provided by the Super Aco storage ring in Orsay. The Na cluster spectra show a close resemblance to the solid Na spectrum which consists essentially in an absorption jump. Only for very small clusters discrete structures appearing below the absorption spectrum are visible with small oscillator strength. This contrasts with the spectrum of Na vapour which show intensive lines attributed to Rydberg states, indicating that even in small Na clusters the valence electrons are delocalised over the whole aggregates. The NaF cluster spectra present a strong decrease in the excitonic structures with decreasing size, approaching a metallic threshold for the smallest ones. This, together with mass spectroscopy observations, allow us to infer the presence of non-stoichiometric clusters for the smallest aggregates which undergo a ‘metallisation’. The discrete excitations in NaF clusters are described in terms of a model which considers perturbation of the ion-like Rydberg states by the environment of the absorbing cation and this allows simulations in satisfactory agreement with the experimental data.

The 3d X-ray absorption spectra of matrix-isolated Tm atoms and clusters

The 3d-4f spectra of matrix-isolated clusters of Tm are obtained for the first time. These spectra are compared with the newly obtained 3d-4f spectrum of Tm vapour. The observed multiplet structure, yielding signatures of the ground-state configurations of Tm, exhibits a cluster-size dependence which enables a quantitative and straightforward interpretation of coordination-dependent surface valence in metallic Tm and Sm.

K edge photoabsorption spectra in gas phase alkali halides

Abstract High resolution X-ray absorption spectra of 12 alkali halide vapours at the sodium, potassium and chlorine K edges are obtained for the first time by using synchrotron radiation. Among these NaBr and KBr are given as examples in this communication. In addition to the usual broad molecular lines, the Na halides show some new extremely fine lines which are thought to be due to a negative charge state of the Na ion. The importance of the spectral contribution from the different types of ions present in small clusters in the halide vapours is emphasized.

INNER-SHELL ABSORPTION SPECTRA OF POTASSIUM FLUORIDE CLUSTERS

Abstract Potassium K-shell absorption data of KF clusters obtained with a new cluster source are presented. The clusters are trapped in rare gas matrices and analyzed with soft X-ray radiation provided by the DCI storage ring in Orsay. The inner-shell exciton lines of KF show distinct changes with the aggregate size, which can be used to identify their origin. Aggregates of smaller size are formed predominantly with nonstoichiometric compositions with potassium in excess.