  Optical pump-probe time- and magnetic-field-resolved Kerr rotation
measurements provide a window into the mechanisms that generate dynamic nuclear
polarization in bulk gallium arsenide. Previously, we have reported an
unexpected dependence of the direction of the nuclear polarization on the sweep
direction of the applied external magnetic field. In this paper, we present
numerical calculations based on a model for this nuclear polarization due to
the optical orientation and optical Stark effect produced by a train of
ultrafast optical pulses. We demonstrate the correspondence of the model to our
experimental measurements for different laser wavelengths and magnetic field
sweep rates. Finally, we show that the model reproduces the sweep direction
dependence and provides an explanation for this behavior.
