Terahertz Spectroscopy
The Terahertz Spectroscopy , also Submillimeterwellen- , far infrared or far-IR called spectroscopy, investigated the interaction of matter with electromagnetic waves in the terahertz region ( terahertz radiation ).
Frequency range
As a continuation of microwave spectroscopy , terahertz spectroscopy begins at around 100 GHz. Beyond approx. 10 THz, it changes into infrared spectroscopy . The table provides information about the wavelength and the radiation energy. For blackbody radiation, Wien's law of displacement defines the radiation temperature as a function of the radiation maximum and the wavelength.
frequency | wavelength | energy | Blackbody radiation temperature |
---|---|---|---|
0.1 THz | 3 mm | 0.4 meV | 1 K |
1 THz | 0.3 mm | 4 meV | 10 K |
10 THz | 0.03 mm | 40 meV | 100 K |
interaction
The low energy of the THz radiation stimulates bonds with heavy binding partners, for example low-frequency lattice vibrations in a crystal lattice (phonons). Light binding partners react with small binding energies. Hydrogen bonds in water and dipole-dipole in general show a high level of interaction. For example, it is possible to distinguish water of hydration from free water molecules.
The water vapor in the atmosphere absorbs the far infrared radiation from space around 100 dB / km almost completely. Above the troposphere at approx. 15 km height the attenuation is only 0.01 dB / km.
Metals reflect electromagnetic radiation. Non-polar materials such as paper, many plastics, textiles or ceramics appear transparent to THz radiation. A full-body scanner uses this property . The radiation from the moist skin passes through the surrounding clothing unhindered.
Examples
Gases
Similar to infrared spectroscopy , molecules show a characteristic absorption and emission spectrum.
The graphic on the right shows a THz emission spectrum of the higher earth atmosphere. As the altitude and pressure decrease, the interaction between the molecules decreases. The natural frequencies become sharper, the line width decreases. The red line corresponds to a pressure of 100 hPa, the green line to 30 hPa and the blue line to 10 hPa. The emissions from different altitudes are superimposed. Nevertheless, the line widths are developed and the concentration of the gas components is measured as a function of altitude with an error of the order of magnitude of the scale heights (approx. 10 km).
Solid
In solids, many atoms interact with each other, the absorption lines are smeared accordingly. Nevertheless, under favorable circumstances, it is possible to identify connections non-destructively and contact-free. It is hoped that explosives such as hexogen can also be detected in closed vessels. Some plastics also have characteristic absorptions in the terahertz frequency range. This allows a selective identification of different polymers. For example, polyethylene (PE) has an absorption peak at 2.2 THz, which allows a clear differentiation from polypropylene (PP). Furthermore show plastics with polar molecular chains, such as. B. polyamides (PA), characteristic absorption bands.
Superconductor
The energy gaps of many superconductors are in the meV range and can therefore be characterized using terahertz spectroscopy.
Examples: spectrometers for astronomy
MLS
The MLS ( Microwave Limb Sounder ) is a payload on the Aura satellite . It receives radiation in the bands 118, 190, 240, 640 and 2250 GHz, and derives the concentration of gases such as HO 2 , NO, NO 2 , O 2 , H 2 O, OH or O 3 from it. The spectrometer is aligned tangentially to the earth in the direction of flight of the satellite. It captures the edge of the atmosphere (hence the English term limb ) and sees the radiation against the cold background of space. With a nadir alignment , the thermal radiation would be superimposed on the earth's surface.
SMILES
SMILES ( Superconducting Submillimeter Wave Limb Emission Sounder ) is a superconducting receiver for submillimeter waves, designed for tangential sounding of the earth's atmosphere. It was installed on the ISS in September 2009 and was in operation there until the LO failure in April 2010.
SOFIA
The SOFIA stratospheric telescope observes space in the wavelength range from 0.3 to 1600 µm.
Herschel space telescope
The satellite carries the two sub-millimeter wave detectors HIFI (Heterodyne Instrument for Far Infrared ) for the range 157–625 µm, and SPIRE ( Spectral and Photometric Image Receiver ) for the range 200–670 µm.
literature
- D. Stich, S. Kremling, F. Minolts, M. Bastian, T. Hochrein: Useful addition: Assessing plastic products using terahertz measurement technology . In: QZ , 58, 2013, p. 7
- N. Krumbholz, T. Hochrein, DM Mittleman, J. Grunenberg, U. Schade, M. Koch: Temperature sensitive characteristics of polyamides . In: Infrared, Millimeter and Terahertz Waves (2009), IRMMW-THz, 34th International Conference
Web links
Individual evidence
- ↑ harvard.edu: Scientific Prospects for a Terahertz Telescope , example: fingerprint of various components in the atmosphere
- ↑ Example: Measuring the ozone concentration as a function of the altitude. (From: The first observation data from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) ( Memento from December 28, 2009 in the Internet Archive ))
- ↑ MLS
- ↑ MLS on Aura
- ↑ SMILES
- ↑ Failure of SMILE