Properties of Cu-Ni solid solutions have long been studied in physical and materials sciences. Yet, their many-body properties have not been well understood. Here, we investigate ultrafast luminescence in near-infrared region for ${\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x$ alloys. The luminescence intensity was the highest in Cu and decreased dramatically by adding Ni, approaching close to the value for pure Ni at $x=0.45$. This composition dependence was well reproduced by calculations assuming two-body scattering of the energetic electrons. The luminescent decay rate was not straightforward, i.e., it decreased first by adding Ni up to $x=0.17$ and then started to increase approaching twice the initial value at $x=0.45$. This behavior was in good agreement with ab initio calculations of electron-phonon (e-ph) coupling strength. This work provides a new perspective on the electron relaxation dynamics in solid solution systems.

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• Electron relaxation
• Electron-phonon coupling
• Ultrafast phenomena
• Alloys
• Luminescence