Plasmon response in K, Na and Li clusters: systematics using the separablerandom-phase-approximation with pseudo-Hamiltonians

The systematics of the plasmon response in spherical K, Na and Li clusters in a wide size region (8 less than or equal to N less than or equal to 440) is studied. Two simplifying approximations whose validity has been establi shed previously are considered: (a) a separable approach to the random-phas e-approximation, involving an expansion of the residual interaction into a sum of separable terms, (b) the electron-ion interaction is modeled within the pseudo-Hamiltonian jellium model (PHJM) including nonlocal effects by m eans of realistic atomic pseudoHamiltonians. In cases where nonlocal effect s turn out to be negligible, the Structure Averaged Jellium Model (SAJM) ha s been used. The leading role of Landau damping in forming the plasmon widt h in medium and large clusters is demonstrated. Good agreement with availab le experimental data is achieved for K, Na (using the SAJM) and small Li cl usters (invoking the PHJM). The trends for peak position and width are gene rally well reproduced, even up to details of the Landau fragmentation in se veral clusters. Less good agreement, however, is found for large Li cluster s. The possible reasons of the discrepancy are discussed.