▎ 摘　　要

In this paper we clarify the nature of pi and pi + sigma electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that pi and pi + sigma plasmons do exist in graphene, though occurring only for a particular range of wave vectors and with finite damping rate. Particular attention is paid to comparing the theoretical results with available EELS measurements in optical (Q approximate to 0) and other (Q not equal 0) limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach.