Phenomenon of the Metastable State Coherently Excited by Electron Impact

Detailed information is given on an experiment which we have reported previously, studying intensity beats in the resonance absorption due to nondegenerate magnetic sublevels of the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ mercury metastable state coherently excited by electron impact. Rather straightforward theory is used to explain the experimental result. It is found that regardless of the exciting energy, as long as there is sufficient selective excitation of certain magnetic sublevels, the absorption beat frequency is modulated at twice the Larmor frequency when the incident electron beam is perpendicular to the externally applied field $H_0$ and the linearly polarized monitoring light is perpendicular to the electron beam.