# Remote Spectral Identification in the THz Band with Reflection Spectroscopy in an Open Atmosphere
## 2 Materials and Methods
### Samples and Reference Data  
In THz spectroscopy, pure materials have very high attenuation. This is good for the reflection measurement, where characteristic frequencies are selectively absorbed by the material. Unfortunately, at the same time, high attenuation renders transmission spectroscopy challenging. For this reason, two sets of samples were prepared. Pure samples, for reflection measurements, containing 100% of the investigated substance were ground in a mortar into fine powder. Then, the powder was loaded into a form and pressed with 2 tons of pressure for 3 min. As a result, disc-shaped samples with a diameter of 30 mm and a thickness of 3-4 mm were produced. In this case, the amount of the substance  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Detector Type** & **Sensitivity** & **Speed** & **Spectral Range** \\ \hline Golay Cells & Good & Slow & High \\ Pyroelectric Detectors & Average & Slow & High \\ Photoconductive Detectors & High & Good & High \\ Photomixers & Good & Good & Average \\ Schottky Diodes & High & High & Limited \\ Semiconductor Bolometers & High & High & Limited \\ Hot-electron Bolometers & High & High & High \\ Hot-electron Bolometers Mixers & High & High & Limited \\ SIS Mixers & High & High & Limited \\ Electro-optic Sampling & High & High & Limited \\ \hline \hline \end{tabular}
\end{table}
Table 2: Overview of the currently available THz radiation detectors [11].  
was not important, because in reflection measurements, only the signal reflected from the front surface is analyzed.  
For transmission measurements, the pellet preparation methods were adopted from FTIR spectrometry [38], which are well established. The samples were made of 10% of a substance and 90% polyethylene to obtain a weight of 400 mg per pellet. For the transmission reference measurements, the samples were prepared with a 13 mm diameter to fit a standard holder for the commercially available Time Domain Spectrometer (TDS). A TDS spectrometer, TeraView3000, was used to collect the reference measurements for both the 30 mm pure material for reflection and the 13 mm 10% material, for transmission configuration. A gold mirror and a 100% polyethylene pellet were used as a reference for reflection and transmission reference data measurements, respectively. The TDS measurements were carried out in an atmosphere with a humidity below 1% and with a THz optical path of about 30 cm. The reference data of the water vapor were also measured with the TDS spectrometer in an atmosphere with a humidity of 41%.  
The availability of the data on the spectral absorption features was satisfactory; nonetheless, samples were prepared, measured using a TDS spectrometer for the reference data, and compared with the data from the literature, before being used in the actual experiment.  
To ensure clarity, all measurements conducted in the TDS spectrometer are referred to as reference data and serve only that exact purpose, i.e., reference data. When the term reference measurement is used hereafter, it means a background measurement needed to calculate the transmission or reflectance spectrum in an experimental setup.