A novel technique in time resolved luminescence spectroscopy called population mixing using a subpicosecond cw mode‐locked dye laser has been developed and applied top‐type GaAs at low temperatures. Using this technique the relaxation lifetime for electron recombination was measured to be 39±7 ps forp‐type GaAs with Zn at 6×1018cm−3hole concentration. This is comparable to the relaxation time measured by a streak camera.