Stabilization of benzene radical anion in ammonia clusters

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH(3))(N)·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH(3))(n−1)NH(2)(−) ions are produced via dissociative electron attachment (DEA) to NH(3) molecules. (NH(3))(n)(−) ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH(3))(n)(−) ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH(3))(n)·Bz(−) cluster ions exhibit essentially the same energy dependence. The (NH(3))(n)·Bz(−) ions are metastable and evaporate NH(3) molecule(s), while pure (NH(3))(n)(−) ions are stable. The lifetime for NH(3) molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz(−) ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH(3) molecule(s).