Photon echoes following two pulses of C${\mathrm{O}}_2$ laser radiation incident on S${\mathrm{F}}_6$ gas have been observed. Homogeneous relaxation times for S${\mathrm{F}}_6$ colliding with S${\mathrm{F}}_6$, He, Ne, and ${\mathrm{H}}_2$ are shown to be easily measurable from the decay of photon-echo amplitude.

• Received 18 March 1968

DOI:https://doi.org/10.1103/PhysRevLett.20.1087