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.
Highlights
The emission of photons by isolated and thermally equilibrated clusters can occur via two different internal transitions: vibrational cooling (VC) and recurrent fluorescence (RF)
RF processes occur because excited electronic states can be populated thermally via inverse internal conversion (IIC), in which vibrational energy is converted into electronic energy, followed by photon emission from such states if the transitions are optically allowed [4,5]
To extract rates of radiative cooling, a small and variable time delay is added between the creation of the hot cationic clusters by the UV laser excitation and the time the clusters are extracted into the mass
Summary
The emission of photons by isolated and thermally equilibrated clusters can occur via two different internal transitions: vibrational cooling (VC) and recurrent fluorescence (RF). RF, in contrast, occurs at time scales that can be as short as microseconds. Such a fast stabilization of a hot particle has direct consequences in diverse fields, including the size-selected production of nanoparticles, catalysis, and astrophysics [3]. RF processes occur because excited electronic states can be populated thermally via inverse internal conversion (IIC), in which vibrational energy is converted into electronic energy, followed by photon emission from such states if the transitions are optically allowed [4,5]. Despite all the experimental evidence in favor of RF, theoretical calculations assuming this process did not support
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