Phys. Rev. A 110, 052813 (2024)
Cooling of Gold Cluster Anions, Au−𝑁 (𝑁=2–13,15), in a Cryogenic Ion-beam Storage Ring
Klavs Hansen1, Tian Weihao2, Emma K. Anderson3, Mikael Björkhage3, Henrik Cederquist3, MingChao Ji3, Stefan Rosén3, Alice Schmidt-May3, Mark H. Stockett3, Henning Zettergren3, Vitali Zhaunerchyk4 and Henning T. Schmidt3
1 Center for Joint Quantum Studies, Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China
2 School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China
3 Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
4 Department of Physics, University of Gothenburg, 41296 Gothenburg, Sweden
* klavshansen@tju.edu.cn
Abstract
We measured the spontaneous and photoinduced decays of anionic gold clusters, Au−𝑁, with sizes ranging from 𝑁=2 to 13 and 15. After production in a sputter ion source, the size-selected clusters were stored in the cryogenic electrostatic ion-beam storage ring DESIREE, and their neutralization decays were measured for storage times between 0.1 and 100 s. The dimer was observed to decay by electron emission in parallel to neutral atom emission at long times, implying a breakdown of the Born-Oppenheimer approximation, analogous to the behavior of copper and silver dimers. Radiative cooling is observed for all other cluster sizes. The decays of clusters 𝑁=3,6,8–13,15 show only a single radiative cooling time. For 𝑁=6–13 the cooling times have a strong odd-even oscillation with an amplitude that decrease with cluster size and with the even 𝑁 having the faster cooling. We compare our results with previous measurements of radiative cooling rates of the corresponding cationic gold clusters, Au+𝑁, which also show an odd-even effect with a similar oscillation amplitude but at orders of magnitude shorter timescales and out of phase with the anions. The tetramer and pentamer both show two cooling times, which we tentatively ascribe to different structural forms at different ranges of high angular momenta of the ions in the Au−4 and Au−5 beams. For Au−7, the shape of the decay curve suggests that the cluster cools by emission of low-energy photons. The calculated limit on photon energies strongly suggests that cooling is by vibrational transitions in this case. For Au−5, time-resolved studies of photoinduced decays were performed to track the evolution of the internal energy distribution. We conclude that the radiative cooling is dominated by sequences of vibrational transitions in the IR. The laser-enhanced neutralization rate of Au−5 was exponential, in contrast to its spontaneous decay rate, indicating that the cluster had already been cooled to a very narrow internal energy distribution at 120 ms as the total (integrated) laser-enhanced intensity was independent of the laser firing time at later times. The unimolecular rate constants decreased from 500 s−1 when laser excited at 0.12 s to 40 s−1 when laser excited at 0.62 s.
