Emergence of circularity at linear polarized excitation of molecules

General conditions causing partial transformation of optical alignment of excited state angular momenta J into orientation at weak linear polarized broad line excitation of molecules are examined. As is shown, the phenomenon takes place under the effect of an external perturbing factor leading to such magnetic sublevel M splitting ω_MM′ when ω _MM±1≠ω_-M∓1-M and ω_MM′ is of the order of molecular state relaxation rate. An analytical expression is obtained describing the appearing circularity of fluorescence for arbitrary J values. A numeric example is given for ¹Σ state molecules with J= 10 assuming a quadratic Stark effect in a homogeneous external electric field.