Simple Theory of Microwave Absorption in Alkali Halides

The intrinsic microwave absorption coefficient β of alkali halides is well explained by lifetime-broadened two-phonon difference processes, in contrast to energy-conserving two- and three-phonon processes as previously thought. A simple closed-form expression for β with no adjustable parameters gives excellent agreement with the magnitude, frequency dependence, and temperature dependence of β in NaC, KBr, and KI. As w increases from ω ω C +γ. Previous energy-conserving results disagree with the T and ω dependence and magnitude of the experimental results in the region ω<ω C .