Millimeter attenuation function

For fundamental physical reasons, the attenuation function for any process must vanish as to - °°. This expectation is borne out by far-infrared measurements of ct(u>) for a variety of molecular systems exhibiting a relaxation-type absorption in the microwave and millimeter-wave region (17-23). While H2O as a solute in nonhydrogenbonding solvents also shows this behavior (35), the millimeter-wave and far-infrared spectrum of 0(2.) is complicated by contributions to a (10) due to intermolecular vibrations involving a cluster of H2O molecules (libration and translation), in addition to the high-frequency tail of the relaxation absorption. A heuristic treatment of the general problem (30) makes the relaxation time,... 

Figure 3. Schematic of the total attenuation function arorf ). and its contributions, for H20(l) at millimeter and far-infrared wavelengths.

In spite of the complexity of the analysis of the millimeter-wave and far-infrared spectrum of l CKO, the principal contributor to the bulk attenuation function of typical biological systems in this frequency range, no compelling experimental evidence or theoretical constructs exist to disqualify the assumption that this system is a complex, but typical, collision-broadened system whose fundamental features (1) are well understood. 

As for all the systems relegated to Section 2 the attenuation function for structural H2O in the microwave and far-infrared region, as well as that for free H2O, can be understood in terms of collision-broadened, equilibrium systems. While the average values of the relaxation times, distribution parameters, and the features of the far-infrared spectra for these systems clearly differ, the physical mechanisms descriptive of these interactions are consonant. The distribution of free and structural H2O molecules over molecular environments is different, and differs for the latter case with specific systems, as are the rotational dynamics which govern the relaxation responses and the quasi-lattice vibrational dynamics which determine the far-infrared spectrum. Evidence for resonant features in the attenuation function for structural H2O, which have sometimes been invoked (24-26,59) to play a role in the microwave and millimeter-wave region, is tenuous and unconvincing. 

The Attenuation Function. In contrast to the existing exper-mental evidence which would suggest that the biological response fucntions 8- Dis(to) may have sharply frequency-selective features at millimeter wavelengths (145-14 7), M. cm- -, and that the Raman... 

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