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High-sensitivity measurements of the linearly-polarized solar limb spectrumproduced by scattering processes in quiet regions of the solar atmosphereshowed that the Q/I profile of the lithium doublet at 6708 A has an amplitude~10^{-4} and a curious three-peak structure, qualitatively similar to thatfound and confirmed by many observers in the Na I D_2 line. Given that aprecise measurement of the scattering polarization profile of the lithiumdoublet lies at the limit of the present observational possibilities, it isworthwhile to clarify the physical origin of the observed polarization, itsdiagnostic potential and what kind of Q/I shapes can be expected from theory.To this end, we have applied the quantum theory of atomic level polarizationtaking into account the hyperfine structure of the two stable isotopes oflithium, as well as the Hanle effect of a microturbulent magnetic field ofarbitrary strength. We find that quantum interferences between the sublevelspertaining to the upper levels of the D_2 and D_1 line transitions of lithiumdo not cause any observable effect on the emergent Q/I profile. Our theoreticalcalculations show that only two Q/I peaks can be expected, with the strongestone caused by the D_2 line of ^7Li I and the weakest one due to the D_2 line of^6Li I. Interestingly, we find that these two peaks in the theoretical Q/Iprofile stand out clearly only when the kinetic temperature of the thinatmospheric region that produces the emergent spectral line radiation is lowerthan 4000 K. The fact that such region is located around a height of 200 km instandard semi-empirical models, where the kinetic temperature is about 5000 K,leads us to suggest that the most likely Q/I profile produced by the sun in thelithium doublet should be slightly asymmetric and dominated by the ^7Li I peak.

The physical origin and the diagnostic potential of the scattering polarization in the lithium resonance doublet at 6708 A