  We study optical pumping of resident electron spins under resonant excitation
of trions in n-type CdTe/(Cd,Mg)Te quantum wells subject to a transverse
magnetic field. In contrast to the comprehensively used time-resolved
pump-probe techniques with polarimetric detection, we exploit here a single
beam configuration in which the time-integrated intensity of the excitation
laser light transmitted through the quantum wells is detected. The transmitted
intensity reflects the bleaching of light absorption due to optical pumping of
the resident electron spins and can be used to evaluate the Larmor precession
frequency of the optically oriented carriers and their spin relaxation time.
Application of the magnetic field leads to depolarization of the electron spin
ensemble so that the Hanle effect is observed. Excitation with a periodic
sequence of laser pulses leads to optical pumping in the rotating frame if the
Larmor precession frequency is synchronized with the pulse repetition rate.
This is manifested by the appearance of Hanle curves every 3.36 or 44.2 mT for
pulse repetition rates of 75.8 or 999 MHz, respectively. From the experimental
data we evaluate the g factor of |g|=1.61 and the spin relaxation time of 14 ns
for the optically pumped resident electrons, in agreement with previous
time-resolved pump-probe studies.
