  Coherence relaxation in Na vapor was studied utilizing a pump-probe method
with femtosecond pulses. The coherence was created by the pump pulse, which
excited the Na atoms to the 4S state. The atoms were further excited to the 7P
state by the probe pulse. As a result a coherent UV radiation was emitted by
the atoms. This emission was used to study the relaxation of both the 4S-3S and
7P-3S coherent superpositions. The relaxation is mainly due to collisions with
the Ar atoms serving as a buffer gas. Coherence relaxation times and
collisional cross-sections for the Na-Ar collision pairs were extracted
experimentally.
