Abstract
Determination of the water vapor continuum absorption from 0.35 to 1 THz is reported. The THz pulses propagate though a 137 m long humidity-controlled chamber and are measured by THz time-domain spectroscopy (THz-TDS). The average relative humidity along the entire THz path is precisely obtained by measuring the difference between transit times of the sample and reference THz pulses to an accuracy of 0.1 ps. Using the measured total absorption and the calculated resonance line absorption with the Molecular Response Theory lineshape, based on physical principles and measurements, an accurate continuum absorption is obtained within four THz absorption windows, that agrees well with the empirical theory. The absorption is significantly smaller than that obtained using the van Vleck-Weisskopf lineshape with a 750 GHz cut-off.
Highlights
Water vapor plays the dominant role in the propagation of mm and THz electromagnetic waves in the atmosphere, since it has several thousand rotational absorption lines from the mm wave range through the terahertz region up to 5 THz
The atmospheric mm and THz windows, which are located between the strong resonant lines from 10 GHz up to approximately 2 THz allow for variable range transmission of THz pulses depending on the window
An additional continuum absorption exists within the transparent THz windows, which cannot be explained by the far-wing absorption of the resonant lines
Summary
Water vapor plays the dominant role in the propagation of mm and THz electromagnetic waves in the atmosphere, since it has several thousand rotational absorption lines from the mm wave range through the terahertz region up to 5 THz. The poorly understood continuum absorption is determined empirically and is defined as the difference between the measured total absorption and the absorption of the resonant lines [4] The latter is usually calculated as the sum of all of the resonant lines based on a lineshape function with the corresponding line intensity and linewidth of individual lines, taken from a spectroscopic database, such as the Millimeter wave Propagation Model (MPM) [5], HITRAN [6] and JPL [7]. Many recent works have obtained the relative continuum absorption by subtracting the resonance absorption based on the quadratic pre-factor vV-W lineshape function, but with a cutoff frequency of 750 GHz from the line center. Under these same conditions, the approximate vV-Wc absorption is calculated to be only 33 dB/km, thereby giving the increased continuum absorption of 23 dB/km
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