  We investigate Rabi oscillation of an atom ensemble in Gaussian spatial
distribution. By using the ultrafast laser interaction with the cold atomic
rubidium vapor spatially confined in a magneto-optical trap, the oscillatory
behavior of the atom excitation is probed as a function of the laser pulse
power. Theoretical model calculation predicts that the oscillation peaks of the
ensemble-atom Rabi flopping fall on the simple Rabi oscillation curve of a
single atom and the experimental result shows good agreement with the
prediction. We also test the the three-pulse composite interaction
$R_x(\pi/2)R_y(\pi)R_x(\pi/2)$ to develop a robust method to achieve a higher
fidelity population inversion of the atom ensemble.
