Abstract
In experiments preliminary to the design of an X-ray-excited optical luminescence (XEOL)-based chemical mapping tool we have used X-ray micro (4.5 × 5.2 μm) and macro (1 × 6 mm) beams with similar total fluxes to assess the effects of a high flux density beam of X-rays at energies close to an absorption edge on inorganic surfaces in air. The near surface composition of corroded cupreous alloys was analyzed using parallel X-ray and optical photoemission channels to collect X-ray absorption near-edge structure (XANES) data at the Cu K edge. The X-ray fluorescence channel is characteristic of the composition averages over several micrometers into the surface, whereas the optical channel is surface specific to about 200 nm. While the X-ray fluorescence data were mostly insensitive to the X-ray dose, the XEOL-XANES data from the microbeam showed significant dose-dependent changes to the superficial region, including surface cleaning, changes in the oxidation state of the copper, and destruction of surface compounds responsible for pre-edge fluorescence or phosphorescence in the visible. In one case, there was evidence that the lead phase in a bronze had melted. Conversely, data from the macrobeam were stable over several hours. Apart from localized heating effects, the microbeam damage is probably associated with the O3 loading of the surface and increased reaction rate with atmospheric water vapor.
Highlights
I maging techniques which can chemically map surfaces in air or a controlled environment are few in number
This channel can carry similar X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) information to the X-ray fluorescence channel leading to a technique sometimes known as optically detected X-ray absorption spectroscopy (ODXAS)
We explain a small fraction of the pre-edge signal in the X-ray-excited optical luminescence (XEOL)-XANES from I18 by optical fluorescence from whereas the Cu edge rises around 3 orders of magnitude in the surface contamination, a comparison between the edges in the XANES data, the XEOL-XANES shows a high level of pre-edge XANES and XEOL-XANES data shows that most of the prefluorescence in the UV−NIR region sampled
Summary
The geometrical similarity of the edge and postedge regions in both types of spectrum (especially the similar relative positions of the midedge feature) shows that, as for copper, the pre-edge background in the XEOL-XANES comes from emission channels which turn off at the edge. For example, the 5 XANES spectra overlaid and were characteristic of copper (thin cuprite layer), whereas the XEOL XANES showed high levels of pre-edge fluorescence (rather than phosphorescence), which decreased by a total of 10% in the 50 min, and a noisy edge, and a postedge signal characteristic of mixed copper and cuprite spectra (copper seen through thin or patchy cuprite). Lead, or a lead compound, contributes increasingly to the copper preedge signal in the XEOL-XANES, and a small increase in X-ray backscattering from the lead enhances the cuprite yield
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