Millimeter Wavelength Dispersion of Water Vapor

Dispersion due to water vapor in the wavelength region ∞ to 0.6 mm is computed by means of quantum-mechanical formulas. The dispersion is attributed to: (1) The contributions of the 11 rotational lines in the region considered; these are obtained with a dispersion equation characteristic of collision theory specialized to sharp lines. (2) The combined contribution of all rotational lines of shorter resonance wavelength. This is obtained with the aid of the well-known shape factor (νn2—ν2)−1, which is shown to be valid for sharp lines independent of any special model. The strongest line, 31−4−3, occurs at 0.794 mm, and it gives rise to a maximum change in susceptibility ε′−1 of 8.2×10−6 for 1 atmos of air containing 1 percent water molecules. The increase in ε′−1 from ∞ to 0.6 mm is about twice this value. If the very small effect of the 1.35-cm line is neglected, a simple equation for ε′−1 holds from ∞ to 2 mm.