STP799: Simple Theory of Microwave Absorption in Alkali Halides

    Sparks, M
    Scientific Research Center, Santa Monica, California

    King, DF
    Scientific Research Center, Santa Monica, California

    Mills, DL
    Scientific Research Center, Santa Monica, California

    Pages: 16    Published: Jan 1983


    Abstract

    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 NaCℓ, KBr, and KI. As ω increases from ω<ωc/3 to ω=ωc293, the frequency and temperature dependence of β at 293 K (or 1000 K) change from the low-frequency limit β∼Tω2 to β∼Thω2+, where 2.0 ≤ ℓ ≤ 2.6 (or 2.2 ≤ ℓ ≤ 4.0) and 1.1 ≤ h ≤ 1.2 (or 0.57 ≤ h ≤ 1.6) for the three materials. Here ωC = 20 cm−1 for NaCl is the frequency difference between the transverse acoustical and transverse-optical phonon modes at the Brillouin zone edge and γ is the combined phonon inverse lifetime at 293 K. The temperature dependence of the phonon frequencies and other parameters must be included in order to explain the temperature dependence of the experimental results. The present lifetime-broadened results agree with previous energy-conservation results (apart from smoothing) and with experimental results in the region ω>ωc+γ. Previous energy-conserving results disagree with the T and ω dependence and magnitude of the experimental results in the region ω<ωc.

    Keywords:

    Alkali halides, microwave absorption, multiple-photon absorption


    Paper ID: STP37287S

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP37287S


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