This thesis discusses decelerating atomic beams using pulse trains, and studies the possibility of cooling atoms or molecules using frequency combs. Using frequency combs in laser cooling of multilevel systems may eliminate the need for multiple laser sources, which are currently required to pump the population between manifolds of energy states. It is shown that a small amount of momentum is transferred to two-level atoms during an interaction with a single weak laser pulse. However, there is a considerable change of the atomic momentum when the atoms interact with a pulse train due to the interference between successive pulses. |