Formation and characterization of long-lived negative molecular ions of C60H18

Long-lived molecular negative ions are generated from C60H18 following the application of several ionization methods such as laser desorption/ionizati on, negative ion chemical ionization, and resonant free electron capture. T he mass spectrometric observation of stable negative fullerene ions showing such a high degree of hydrogen attainment is unprecedented. The identity o f the negative molecular ion is confirmed by high-resolution accurate mass measurements. The attachment of monochromatic electrons to C60H18 is Studie d in the energy range from 0 to 20 eV, and the resulting negative ion effec tive yield curve is discussed together with the mean lifetime curve of the ion. Negative molecular ions that result from capture of thermal electrons at a molecular temperature of 600 K possess a maximum mean lifetime in the range of 1 ms. From the experimental data the electron affinity of C60H18 i s estimated to lie between 1.4 and 1.6 eV, which is in excellent agreement with the results obtained here from semiempirical calculations.