Fast radiative cooling of anthracene was observed in a compact electrostatic storage ring by probing the evolution of the internal energy distribution of a stored (C(14)H(10))(+) molecular ensemble via laser excitation. We have measured the mean radiative decay rate to be about 120 to 250 s(-1) for internal energies in the range from 6.6 to 6.8 eV. Such a high decay rate is 2 orders of magnitude larger than the infrared emission cooling rate expected for vibrational transitions. It is attributed to fluorescence from thermally excited electrons. This fast cooling mechanism may have important implications in astrophysics concerning the lifetime and the critical size of polycyclic aromatic hydrocarbons in interstellar conditions.

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+ --> C(n-2)+ + C2, for sizes 42 < or = n < or = 90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values.

Fast radiative cooling of anthracene cations (C 14 H 10 ) + is studied with a compact electrostatic storage device, the Mini-Ring. The time evolution of the internal energy distribution of the stored ions is probed in a time range from 3 to 7 ms using laser-induced dissociation with 3.49-eV photons. The population decay rate due to radiative emission is measured to vary from 25 to 450 s -1 as a function of the excitation energy in the range from 6 to 7.4 eV. After corrections of the infrared emission effect via vibrational transitions, the fluorescence emission rate due to electronic transitions from thermally excited electronic states is estimated and compared with a statistical molecular approach. In the considered internal energy range, the radiative cooling process is found to be dominated by the electronic transition, in good agreement with our previous work [S. Martin et al.,