Unravelling the electronic nature of the radiative cooling of cobalt clusters
Kristien Peeters, Ewald Janssens, Klavs Hansen, Peter Lievens, Piero Ferrari
Abstract
In this paper, we report on the experimental determination of photon emission rates of laser-excited cobalt clusters, ${{\mathrm{Co}}_{n}}^{+}$ ($n=5--23$), 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.