*Article* **Chromatic Change in Copper Oxide Layers Irradiated with Low Energy Ions**

**Takuya Kobayashi <sup>1</sup> , Fumitaka Nishiyama <sup>2</sup> and Katsumi Takahiro 1,\***


**Abstract:** 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 <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>15</sup> Ar<sup>+</sup> cm−<sup>2</sup> . In the unirradiated copper oxide layer, the copper valence state of Cu2+ 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 Cu2+ to Cu<sup>+</sup> and/or Cu<sup>0</sup> . Furthermore, nuclear reaction analysis using a <sup>16</sup>O(d, p)17O reaction with 0.85 MeV deuterons revealed a significant loss of oxygen (5 <sup>×</sup> <sup>10</sup><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.

**Keywords:** ion beam; copper oxide; chromatic change; photoemission spectrum; beam viewer

**Citation:** Kobayashi, T.; Nishiyama, F.; Takahiro, K. Chromatic Change in Copper Oxide Layers Irradiated with Low Energy Ions. *Quantum Beam Sci.* **2021**, *5*, 7. https://doi.org/10.3390/ qubs5010007

Academic Editor: Akihiro Iwase

Received: 2 February 2021 Accepted: 9 March 2021 Published: 10 March 2021

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