LOW-ENERGY-ELECTRON ATTACHMENT TO FULLERENES C-60 AND C-70 IN THE GAS-PHASE

We have investigated the low-energy (0-10 eV) electron attachment on C -60 and C-70, using an improved energy resolution close to zero incide nt energy (0.030-0.060 eV). We have found evidence of a zero-energy at tachment process (within 0.030 eV), for both C-60 and C-70. indicating that a s-wave capture process is present at low energy for these two molecules. For C-60, although in contrast with symmetry arguments, and with previous free-electron attachment experiments, this result is in full agreement with the conclusions presented by several groups, usin g the Rydberg atom energy transfer technique to investigate the low-en ergy attachment in these systems. Energy positions of several resonant states, appearing between 0.5 and 5 eV in the ion yield versus electr on energy spectrum, are clearly established in C-60 and C-70, and assi gnments are discussed. Lifetime measurements are reported for the full erene anions. In the incident energy range 7-11 eV the lifetime tau of C-60(-) was found to vary from 500 to 45 mu s, respectively, in agree ment with previous measurements. The thermionic model for C-60(-) deta chment does not seem to describe the values obtained, and their observ ed variation in this energy range. The lifetime of C-70(-) is longer a nd could not be measured precisely with our time-of-flight conditions. An estimation of tau approximate to 700 mu s has been made at E-i = 1 1 eV. Below E-i = 7 eV the lifetime of C-60(-) was larger than 700 mu s and could not be measured precisely.