  We investigate the effect of buffer gases on the coherent population trapping
resonance induced by a $\sigma$-polarized optical field in $^{87}$Rb atoms. Our
experimental results show that inert gases, which depolarize the excited state
of the alkali-metal atoms, provide higher contrast than nitrogen that
effectively quenches their fluorescence. We also demonstrate that elimination
of the spontaneous radiation does not significantly decrease the width at
moderate temperatures of an atomic medium. Therefore, a mixture of inert gases
can be preferable over a mixture with nitrogen for atomic clocks.
