Inhibited Absorption of Blackbody Radiation

A cutoff with wavelength is observed in the absorption of blackbody radiation by free atoms. The cutoff arises from a discontinuity in the density of modes between parallel conducting plates. Absorption at a wavelength of $\frac{2}{3}$ cm by atoms between planes $\frac{1}{3}$ cm apart is measured at a temperature of 180 K. The discontinuity in the absorption rate occurs when the absorption wavelength is varied across the cutoff of one of the parallel plate modes.