Electron impact emission cross sections of Ar were measured for the , and transitions from the threshold to 1000 eV by a polarization-free method. Two different methods were used in determining absolute emission cross sections for the first resonance lines. The first method is to measure the cascade cross sections to the 4s state by detecting visible to near-infrared radiation and the total cascade cross section was then combined with the level excitation cross section to the 4s state to give the emission cross section for the resonance line. The second method is to measure the vacuum ultraviolet sensitivity of the apparatus and an absolute emission cross section for the resonance line was obtained by comparing its spectral intensity with that of the Lyman- radiation produced from by electron impact. The two results showed good agreement within the experimental errors. The level excitation cross sections to the and states were obtained by subtracting the total cascade cross section to these levels from the corresponding emission cross sections.

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If the states of photons are bound, another type of spontaneous emission becomes possible for a relativistic electron due to the symmetry breaking in space. We obtain the radiation probability for a relativistic electron passing through a box-shaped cavity. The radiation spectrum is discrete in which line positions are determined by the boundary condition of the cavity.