Excitation of C_(60)~+ by electrons in a Fourier transform ion cyclotron resonance ion trap: an experimental determination of the radiative cooling rate through its competition with C_2 evaporation

摘要

C_(60)~+ ions are produced in an ion trap by crossing a collimated effusive beam of C_(60) and an electron beam of energy lower than the C_(58)~+ fragment appearance energy. Being trapped, the C_(60)~+ ions can be further excited by another electron impact, inducing the well-known C_2 evaporation. By varying the electron pulse length while counterbalancing it with the intensity in order to maintain a constant total number of electrons in a pulse, the mean time between two successive electron impacts can be modified. The influence of this time on the amount of fragmentation is direct experimental evidence of the competition between radiative cooling and dissociation, invoked to explain the abnormally high internal energies needed to observe fragmentation even at long times. For ions with internal energies in the range 15.4-22.4 eV, a mean radiative rate constant of 3.3 * 10~2 s~(-1) has been determined.