Radiative cooling of isolated Al−₅ cluster ions observed on ultralong timescales as a test case for the validity of the harmonic cascade model

While discussing the vibrational radiative cooling of isolated cluster molecules, it is common to apply statistical models based on the density of vibrational states and calculated transition strengths. In the current work, we put this assumption to the test by studying the cooling rates of isolated Al⁻₅ ions across very long timescales ranging up to 2000 s. To this end, we stored Al⁻₅ ions in the Cryogenic Storage Ring (CSR) and monitored their internal excitation by laser-induced delayed electron emission. We find that once the internal energy drops below the first electronically excited state, the cooling rates scale linearly with energy. Our results are consistent with the calculated vibrational relaxation rates through the emission of infrared photons, based on the harmonic cascade model, thus validating quantitatively the assumption of a statistical cooling process for the relaxation of small cluster molecules with infrared-active vibrational modes.