Phys. Rev. Research 3, 033225 (2021)

Unravelling the electronic nature of the radiative cooling of cobalt clusters

Kristien Peeters¹, Ewald Janssens¹, Klavs Hansen^1,2, Peter Lievens¹, and Piero Ferrari^1,*

1 Quantum Solid-State Physics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200d box 2414, 3001 Leuven, Belgium

2 Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China

* piero.ferrari@kuleuven.be

ABSTRACT

In this paper, we report on the experimental determination of photon emission rates of laser-excited cobalt clusters, Co_n⁺ (n = 5–23), deduced from fragmentation mass spectrometry and metastable decay fractions. The rates are so high that they can only be ascribed to recurrent fluorescence (RF), a process where emitting states are populated by inverse internal conversion, followed by photon emission. Cooling via electronic states is confirmed by quantitative agreement with calculated rates using the low-lying electronic transitions predicted by time-dependent density functional theory calculations for n = 5–10, which are performed considering all electrons and including relativistic effects implicitly. The outstanding agreement between experiment and theory provides clear evidence that the clusters radiate via electronic states, being a consistent theoretical and experimental study invoking RF.