Ultrasonic evidence of the existence of two hydrogen Snoek–Köster relaxations in high-purity single crystal copper

In this paper we present experimental data of ultrasonic velocity and attenuation of a hydrogenated high-purity crystalline copper sample. Two evident peaks are observed at temperatures higher than those corresponding to the Niblett–Wilks and Bordoni peaks, which are characteristic of an uncharged similar sample. The peaks we observed are interpreted in terms of the hydrogen Snoek–Köster relaxation (HSK) proposed by Seeger. This is the first ultrasonic observation of the two HSK relaxations predicted for face-centered cubic samples.