The Si 3s(2) 3p(1)md(1) (1)D(2)degrees Rydberg series is investigated. Experimentally, the series has a large constant quantum defect (QD) up to 3s(2)3p(1)6d(1), which increases abruptly there after. Experimentalists consider that the increase in QD is brought about by a perturber 3s(1)3p(3). We show that 3s(1)3p(3) works as a strong perturber, giving a large and constant QD for the lower members of the series (m <= 6), but does not contribute to the abrupt observed increase in QD. It is found that 3s(2)3p(1)md(1 3)F(2)degrees acts as a perturber for the higher member of the (1)D(2)degrees series through spin-orbit interaction, giving a sharp increase of QD. (c) 2005 Elsevier B.V. All rights reserved.