Magnetic resonance frequency shift of na atoms during collisions in a mixture of potassium and sodium atoms

Collisions between alkali sodium and potassium atoms at an optical orientation of potassium atoms are considered. As a result of collisions, spin exchange process occurs between the colliding atoms and leads to a shift of the magnetic resonance frequency of the atoms. The calculations showed, that the shift of the magnetic resonance frequency of Na atoms in the upper hyperfine state ($\mathbf{F}$ = 2) changes sign from negative to positive when the temperature in the absorption chamber increase from $T$ = 300 to 500 K. This shift passes through zero in the region near the temperature $T\sim$ 450 K. At the same time the frequency shift of the lower hyperfine state ($\mathbf{F}$ = 1) remains negative over the entire range of the studied temperatures. A phenomenon of this kind was discovered for the first time in the case when the main contribution to the magnitude of the shift of the magnetic resonance frequency is made by spin exchange.