Narrow, dry, deep-etched GaAs/GaAlAs quantum wires with modulation doping are investigated by electronic Raman scattering (RS) and far-infrared magnetotransmission (FIR). Laterally-confined plasmon and magnetoplasmon dispersions and intensities lead to the determination of the confinement potential and the free and trapped electron densities. Free electrons are observed down to a critical widthwc,significantly smaller under strong illumination (RS,wc=50nm) than in dark conditions (FIR,wc=130nm). The induced changes of the external confining potential and lateral electron distribution are analyzed in terms of a semiclassical electrostatic approach. ©1998 American Institute of Physics.