Using a generalized formulation of Kirchhoff's law it is possible to relate the equilibrium electromagnetic radiation spectrum of a body to its absorptivity spectrum. The microwave absorptivity of a uniform anisotropic plasma slab to a normally incident electromagnetic wave is obtained by matching fields at the boundaries and is solved completely for the model chosen.The absorptivity spectrum of the plasma slab is computed for waves propagating parallel to and normal to an applied static magnetic field for various electron densities, electron collision frequencies, and slab thicknesses. Peaks in the absorptivity spectrum occur around the edges of the "stop-bands". In general, the absorptivity increases with slab thickness and collision frequency. The effect of internal reflections is included and gives rise to undulations in the absorptivity spectrum.
A tunable high-Q Fabry-Perot resonator and related instrumentation which has been used to measure the shapes of nitric oxide rotational lines is described. The cavity was used around 150 GHz, but is suitable for any frequency above 120 GHz. The transmitter klystron is phase-locked to a tunable 15 MHz crystal standard. A sawtooth sweep is applied to this standard resulting in a linear-frequency sweep of the 150 GHz signal across the cavity resonance. The shape of the cavity response is a measure of the gas absorption. A beamsplitter is used to couple energy into the resonator. It is demonstrated that such resonators are well suited for certain microwave spectroscopy experiments.
The linewidths and intensities of the 10 lines around 150 GHz (5 cm−1) corresponding to the J = 12→32 transition in nitric oxide have been measured with a Fabry–Perot resonator. Collisions are the dominant broadening mechanism at the pressures of interest in this study (0.2–2 torr), although Doppler and saturation broadening are seen at lower pressures. The measured self-broadening linewidth parameter, averaged over the various lines, is 2.82 MHz/torr. This gives a nitric oxide collision diameter of 5.26 Å, which compares with 3.53 Å from kinetic theory. Line broadening due to a number of atomic and molecular species has also been measured. Collision diameters again exceed kinetic theory ones. This is attributed to dipole–dipole, dipole–quadrupole, quadrupole–quadrupole, and dipole–induced-dipole interactions.
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