Many of the experiments on muonium interacting with photo-excited carriers are conducted at pulsed muon beam facilities and typically concentrate on studying the muon polarization in a longitudinal magnetic field. In this paper, we discuss some of the early progress we have recently made in carrying out optical excitation experiments on semiconductors at a continuous beam facility, i.e. TRIUMF. Here, additional information is potentially available because the muon precession signatures associated with the various paramagnetic and diamagnetic muonium signals can be monitored directly. Some results in high-resistivity Si are presented. (c) 2005 Elsevier B.V. All rights reserved