  Fluorescence spectra of the $8P_{J} \rightarrow 6P_{J'}$ ($J$ and $J'$ = 3/2,
1/2) electric quadrupole transition of cesium atoms have been observed with a
heated cesium vapor cell. We determined the ratio of the transition
probabilities of $8P_{J}\rightarrow6P_{J'}$ to $8P_{J}\rightarrow5D_{3/2}$ by
comparing their respective photon emission rates. The results are in good
agreement with our theoretical calculations. These measurements provide crucial
parameters for tests of coherent amplification method and improve knowledge of
cesium properties which are essential to dark matter detection through atomic
transitions.
