The terahertz frequency regime is often used as the ‘chemical fingerprint’ region of the electromagnetic spectrum since many molecules exhibit a dense selection of rotational and vibrational transitions. Water is a major component of the atmosphere and since it has a large dipole moment the propagation of terahertz radiation will be dominated by atmospheric effects. This study will present the results of high-‐resolution broadband measurements of the terahertz atmospheric absorption and detail the technique for directly measuring the pressure broadening coefficients, absolute absorption coefficients, line positions, and continuum effects. Differences between these measured parameters and those tabulated in HITRAN will be discussed. Once the water vapor absorption was characterized, the same technique was used to measure the line parameters for methanol, a trace gas of interest within Earth’s atmosphere. Methanol has a dense absorption spectrum in the terahertz frequency region and is an important molecule in fields such as environmental monitoring, security, and astrophysics. The data obtained in the present study will be of immediate use for the remote sensing community, as it is uncommon to measure this many independent parameters as well as to measure the absolute absorption of the transitions. Current models rely on tabulated databases of calculated values for the line parameters measured in this study. Differences between the measured data and those in the databases will be highlighted and discussed.
The terahertz frequency regime is often used as the ‘chemical fingerprint’ region of the electromagnetic spectrum due to
the large number of rotational and vibrational transitions of many molecules of interest. This region of the spectrum has
particular utility for applications such as pollution monitoring and the detection of energetic chemicals using remote
sensing over long path lengths through the atmosphere. Although there has been much attention to atmospheric effects
over narrow frequency windows, accurate measurements across a wide spectrum are lacking. The water vapor continuum
absorption is an excess absorption that is unaccounted for in resonant line spectrum simulations. Currently a semiempirical
model is employed to account for this absorption, however more measurements are necessary to properly
describe the continuum absorption in this region. Fourier Transform Spectroscopy measurements from previous work are
enhanced with high-resolution broadband measurements in the atmospheric transmission window at 1.5THz. The
transmission of broadband terahertz radiation through pure water vapor as well as air with varying relative humidity
levels was recorded for multiple path lengths. The pure water vapor measurements provide accurate determination of the
line broadening parameters and experimental measurements of the transition strengths of the lines in the frequency
region. Also these measurements coupled with the atmospheric air measurements allow the water vapor continuum
absorption to be independently identified at 1.5THz. Simulations from an atmospheric absorption model using
parameters from the HITRAN database are compared with the current and previous experimental results.
Remote sensing over long path lengths has become of greater interest in the terahertz frequency region. Applications
such as pollution monitoring and detection of energetic chemicals are of particular interest. Although there has been
much attention to atmospheric effects over narrow frequency windows, accurate measurements across a wide spectrum
is lacking. The water vapor continuum absorption spectrum was investigated using Fourier Transform Spectroscopy.
The continuum effect gives rise to an excess absorption that is unaccounted for in just a resonant line spectrum
simulation. The transmission of broadband terahertz radiation from 0.300THz - 1.5THz through air with varying relative
humidity levels was recorded for multiple path lengths. From these data, the absorption coefficient as a function of
frequency was determined and compared with model calculations. The intensity and location of the strong absorption
lines were in good agreement with spectral databases such as the 2008 HITRAN database and the JPL database.
However, a noticeable continuum effect was observed particularly in the atmospheric transmission windows. A small
discrepancy still remained even after accounting for continuum absorption using the best available data from the
literature. This discrepancy, when projected over a one kilometer path length, typical of distances used in remote
sensing, can cause a 30dB difference between calculated and observed attenuation. From the experimental and resonant
line simulation spectra the air-broadening continuum parameter was calculated and compared with values available in