Photoelectron spectroscopy of Co^-_n and product anions of Co^-_n with O₂ and N₂

The photoelectron spectra of Co^-_n (3 ≤ n ≤ 70), Co_mN^- and Co_mO^- (3 ≤ m ≤ 6) were measured by a XeCl excimer laser (4.025 eV). Also the reactivities of Co^-_n with O₂ and CO were measured. A clear distinction between the cluster size of n ≤ 6 and that of n ≥ 7 was observed both in the photoelectron spectra and in the reactivities. (i) In the size range above n = 7, the electron affinities of the cobalt clusters determined by the photoelectron spectra are linearly dependent on the reciprocal of the cluster radius, but below this size they deviate from this relation. (ii) The reactivity of Co^-_n with O₂ in the size range below n = 6 is higher than that above n = 7. However, the reactivity with CO in the size range below n = 6 is extremely lower than that above n = 7. These findings suggest that a transition of electronic structure in size dependence, which is correlated with the reactivity, occurs at n ≃ 7.