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. (paper)