A 2D-simulation of the dependence of the EMF of a steady resonance photoplasma two-chamber cell containing a NaAr mixture at the PAr = 0.1 - 100 and PNa = 0.02 torr pressures was carried out. The effect of collisional broadening of the D1 and D2 lines of Na on the radiation transfer is taken into account. The change in the sign of the photoEMF at the PAr = 5 - 10 torr was found caused by the transition from the ambipolar diffusion mode to the free diffusion one with increasing of the Ar pressure in the gas cell.