Measurement of free-free emission from low-energy-electron collisions with Ar

The production of free-free radiation in collisions of low-energy electrons with Ar atoms has been measured using the drift-tube technique. The excitation coefficients were obtained from measurements of the absolute intensity of continuum radiation at wavelengths of 500, 650, and 1300 nm. At 650 nm the electric-field—to—gas-density ratio $\frac{E}{N}$ was varied from 0.25×${10}^{-21\mathrm{}}$ to 10×${10}^{-21\mathrm{}}$ V ${\mathrm{m}}^2$ corresponding to mean electron energies from 1.2 to 5.4 eV. As expected, the emission was proportional to the argon density from 3×${10}^{24}$ to 15×${10}^{24}$ atoms/${\mathrm{m}}^3$. The experimental excitation coefficients are in good agreement with calculations using theoretical free-free emission cross sections and electron energy distributions and serve to demonstrate the usefulness of simple formulas relating the free-free emission cross section to measured momentum-transfer cross sections.