Radiative cooling of Al?4 and Al?5 in a cryogenic environment

We have investigated the radiative cooling of initially hot Al ^- ₄ and Al ^- 5 cluster anions during storage in a cryogenic ion beam trap at an ambient temperature of <15 K. By applying a statistical rate model to the ions' measured delayed detachment rate following excitation with a laser, we have obtained their vibrational temperature as a function of cooling time. The temperature curves for Al ^- ₄ suggest that the vibrational cooling slows down considerably once the ion reaches approximately room temperature. This suggests the participation of transitions from recently found low-lying electronic states of the anion in the cooling process, prior to reaching approximately room temperature. The experimental results for Al ^- ₅ suggest slightly slower radiative cooling than for Al?4