  Diffusion is the main limitation of storage time in spatially multimode
applications of warm atomic vapors. Precise knowledge of diffusional
decoherence in the system is desired for designing most of vapor memory setups.
Here we present a novel, efficient and direct method of measuring unbiased
diffusional decoherence, clearly distinguished from all other decoherence
sources. We found the normalized diffusion coefficients of rubidium atoms in
noble gases to be as follows: neon 0.20 cm$^{2}$/s, krypton 0.068 cm$^{2}$/s
and we are the first to give an experimental result for rubidium in xenon:
0.057 cm$^{2}$/s. Our method consists in creating, storing and retrieving
spatially-varying atomic coherence. Raman scattering provides a necessary
interface to the atoms that allows for probing many spatial periodicities of
atomic coherence concurrently. As opposed to previous experiments the method
can be used for any single sealed glass cell and it does not require any setup
alterations during the measurements and therefore it is robust and repeatable.
