X-ray absorption near-edge structure ͑XANES͒ measurements were used to probe the H-charging-induced electronic structure changes of a 2400 Å Nb film capped with Pd. These results are discussed in terms of ab initio linear augmented plane-wave ͑LAPW͒ band-structure calculations for this material. The Pd-L 3 -edge XANES clearly manifested the spectral ͑Pd-d state related͒ changes expected for Pd-hydride formation, a white line feature degradation, and the appearance of a Pd-H antibonding feature at 6 eV above the threshold. The Nb-L 2,3 edge changes with H charging show a distinct enhancement of the white line strength; a feature 6 eV above the edges, associated with Nb-H antibonding states in analogy with the Pd results; the suppression of a threshold-onset feature of Nb metal; and a shift of the centrum of the white line feature towards the threshold. Comparison of the Nb sphere projection of the d 3/2 component of the LAPW density of states ͑DOS͒ to the Nb-L 2 -edge spectra yields good basic agreement with the observed spectral changes. In particular, the substantial theoretical reduction in the DOS at, and just above, the Fermi energy (E f ) is directly related to the near threshold Nb-L 2,3 spectral changes. The more modest white line enhancement in the theoretical DOS is noted and discussed. Nb-K-edge XANES are also discussed in terms of the Nb-site p-state projected LAPW DOS. This last comparison indicates a p-state reduction near E f upon H charging of the Nb.

The effect of large hydrogen concentrations ͑Ϸ30% atomic͒, on the superconducting transition temperature T c of Nb films is discussed. The hydrogen is incorporated in solutionlike phases which occur at these concentrations because the ͑␣ to ␤͒ hydride transition in these films is suppressed. X-ray-diffraction data show an asymmetric expansion of the ͓110͔ Nb interplanar spacing. This can be as high as a 6-8 % expansion perpendicular to the plane of the film for the addition of ϳ70% hydrogen, with only a 1-2 % change in the plane of the film. T c is depressed to a value near 50% of that of the undoped film with about 30% atomic hydrogen, and returns to its initial value when the hydrogen is removed. A discussion is given of how both disorder and changes in the electronic structure can affect T c .

Hydrogen absorption by 200-2000-Å-thick Pd-capped Nb films, between 5 and 110°C, was studied by simultaneous four-probe resistivity and volumetric measurements. The resistivity as a function of hydrogen concentration was measured while charging the films with hydrogen, and was used to compute the change in hydrogen concentration in the film, during the reaction with oxygen. For the thinnest films ͑200 Å thick͒, the hydrogen charging and discharging curves indicate that a first-order gas-liquid-like phase transition with a T c of 70-75°C takes place. The H-Nb phase diagram for the 200-Å film looks like the H/bulk Nb ␣-␣Ј phase diagram which has a higher T c ͑173°C͒. We attribute the substantial modifications of the film's phase diagram to the clamping of the Nb film at its interfaces with glass and Pd and to the nanostructure of the films.

Photoabsorption or photoionization spectra of CH3I are discussed as a function of perturber pressure for 11 different binary gas mixtures consisting of CH3I and each one of 11 different gaseous perturbers. Five of the perturbers were rare gases and six were nondipolar molecules. The energy shifts of CH3I Rydberg states become independent of n, the principal quantum number, for n≥10. The energy shifts for n≥10 vary in a linear fashion with perturber number density. The electron scattering lengths for the perturbers are extracted from the shifts using Fermi theory in which the polarization term is that of Alekseev and Sobel’man. These scattering lengths are compared with those from swarm and time-of-flight experiments. It is found that the uncorrected shift scattering lengths correspond to the zero energy or near-zero energy scattering lengths obtained from extrapolated swarm and time-of-flight data. It is found that plots of scattering length vs polarizability α (ᾱ for molecules) define two linearities, one for the rare gases and one for molecules, CO2 being an exception to the latter linearity (presumably because of its large quadrupole moment). For a given polarizability, it is also found that molecules exhibit a larger scattering length than the rare gases. These results are discussed and consequences for scattering cross sections are elaborated.

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