By the time-domain method this article deals with the interaction between a three-level quantum system (vibrational energy level of the molecule of a dilute gas) and the infrared electric field of CO2 laser. The semiclassical Bloch-Maxwell's equations have been changed into the equivalent differential equations of real function using the rotating wave approximation. This article studies their solutions of the rate-equations approach under the condition of long pulse and short pulse. The area theorem of coherent short pulses which is propagating through a three-level system of equal spacing is obtained. The article contains the derivations of the formula of separate coefficient, the conditions of the selective optical excitation and the excited rate for isotope separation as an actual application.