| Summary: | The color of a thin copper oxide layer formed on a copper plate was transformed from reddish-brown into dark blue-purple by irradiation with 5 keV Ar<sup>+</sup> ions to a fluence as low as 1 × 10<sup>15</sup> Ar<sup>+</sup> cm<sup>−2</sup>. In the unirradiated copper oxide layer, the copper valence state of Cu<sup>2+</sup> and Cu<sup>+</sup> and/or Cu<sup>0</sup> was included as indicated by the presence of a shake-up satellite line in a photoemission spectrum. While for the irradiated one, the satellite line decreased in intensity, indicating that irradiation resulted in the reduction from Cu<sup>2+</sup> to Cu<sup>+</sup> and/or Cu<sup>0</sup>. Furthermore, nuclear reaction analysis using a <sup>16</sup>O(d, p)<sup>17</sup>O reaction with 0.85 MeV deuterons revealed a significant loss of oxygen (5 × 10<sup>15</sup> O atoms cm<sup>−2</sup>) in the irradiated layer. Thus, the chromatic change observed in the present work originated in the irradiation-induced reduction of a copper oxide.
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