Cooling of gold cluster anions, Au$_N^-$, $N=2-13,15$, in a cryogenic ion-beam storage ring

TL;DR

This study investigates the decay and cooling behaviors of gold cluster anions in a cryogenic storage ring, revealing size-dependent radiative cooling rates and odd-even oscillations, and compares these with cationic clusters.

Contribution

It provides new measurements of decay rates and radiative cooling times for Au$_N^-$ clusters, highlighting size and parity effects, and contrasts these with cationic clusters to understand electronic and structural influences.

Findings

Decay rates vary with cluster size and show odd-even oscillations.

Radiative cooling occurs for all sizes except the dimer.

Cooling times are longer for odd N and decrease with increasing N.

Abstract

We have measured the spontaneous and photo-induced 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, analogously to the behavior of copper and silver dimers, implying a breakdown of the Born-Oppenheimer approximation. Radiative cooling is observed for all cluster sizes except for the dimer. The decay rates 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 fast 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 time scales, and out of phase with the anions.