Gases, Liquids, and Solids

Figure: Rotational Interaction Strength shows the strength of interactions as a function of frequency and molecular mass [1]. These absorptions result from the interaction of the rotation of the molecule with the radiation and exist only in gases. This figure shows that the strengths of the interactions increase as $ν^2 - ν^3$. As a result, THz interactions are 10³ - 10⁶ times more intense than interactions in the conventional Microwave (MW) region. Beyond a mass dependent peak in the THz, the interaction strength falls exponentially towards the infrared. This very sharp peak in the THz is one of the most important features of the spectral region and is closely related to many of the current and potential applications.

also shows the atmospheric absorption as a function of altitude. This is a very important figure because it shows how rapidly atmospheric transmission changes as a function of altitude. For example, in windows around 500 GHz, at sea level the attenuation is ~100 db/km (virtually opaque), whereas at 16 km the attenuation is ~0.01 db/km (so small as to be difficult to detect).

All of the interactions discussed above are based on the interaction of molecular rotation with THz radiation. In liquids and solids there is no rotation and consequently no rotational spectra. However, for large molecules collective motions are possible which result in energy level spacings that correspond to THz frequencies. Solids and liquids are strongly interacting and as a result resonances are much broader, ordinarily leading to 'continua' spectra in the THz. However, evidence of specific features in biological substances has been reported. Although speculative, it would appear to us that such spectra are most likely at relatively high THz frequencies.