  We relate the photochromic response of rare-earth oxy-hydride thin films
(YHO, NdHO, GdHO and DyHO) synthesized by reactive magnetron sputtering to
chemical composition. Depth profiles of the sample composition are extracted by
a multi-method ion beam analysis approach. The total areal density of the thin
films is deduced from Rutherford Backscattering Spectrometry while coincidence
Time-of-Flight/Energy Elastic Recoil Detection Analysis provides depth-profiles
of the film constituents. High-resolution depth profiles of the concentration
of light species, i.e. hydrogen and oxygen, are additionally extracted from
Nuclear Reaction Analysis and Elastic Backscattering Spectrometry,
respectively. The photochromic response of the films is measured by optical
transmission spectroscopy before and after illumination. We report photochromic
properties for YHO, NdHO, GdHO and DyHO for chemical compositions described by
the formula $REH_{2-\delta}O_{\delta}$ in the range of $0.45 {<} {\delta} {<}
1.5$.
