Trapping ions at high temperatures: thermal decay of C60+

An ensemble of trapped C60+ ions has been heated with a continuous CO2 laser to a stationary state where, in time average, the same energy is emitted as absorbed. With 10 W laser power, equilibria have been reached, which correspond to temperatures between 1800 and 2000 K. The ions are confined in a radio frequency quadrupole field created by a set of ring electrodes (split ring electrode trap). The number of stored ions can be determined in two ways, on one side by extracting and counting them with a Daly detector, on the other side via imaging their thermal emission onto an intensified CCD camera. Single photon sensitivity and a spatial resolution of a few μm provide precise information on the geometrical distribution and the total number of the trapped C60+ ions. The spectral distribution of the emitted photons or their total number provides information on the internal energy of the ions. Trapping times of many minutes make it possible to follow very slow thermal loss of C2 from hot C60+ resulting in fragmentation rates between 10−1 and 10−3 s−1. Correlating them to the internal temperature leads to a curved Arrhenius plot. The resulting parameters are smaller than the values derived from nonequilibrium ensembles.