Direct observation of internal energy distributions of C5(-).

Photon induced decay of C5(-) has been measured in an electrostatic storage ring. The time dependence of the photo-enhanced decay is close to a 1∕t decay which indicates a thermal process. The deviation from the expected power of -1 is quantitatively explained by the small heat capacity of the anion. Measurements of the photo-enhanced decay at different storage times and photon energies allow a determination of the radiative cooling rate and the energy distribution of the ions. The average energy content between 15 and 70 ms is found to vary as time to the power -0.72, and at 50 ms the ions contain an average excitation energy of 0.5 eV. The time dependent energy distribution is consistent with cooling by infrared photon emission if published oscillator strengths are reduced by a factor 2.5, in contrast to cooling of larger molecular carbon-based ions where electronic transitions cause a much stronger cooling.