Atmosphere windows

Figure 9. Quantum efficiency of a Rockwell 2Kx2K HAWAII array. The atmospheric windows of I, H, and K are shown. Note the relatively constant QE across the 1-2.5 p,m wavelength region, with peak QE of 84% in the K-band (centered at 2.2 /um). Eigure courtesy of J. Garnett, Rockwell Scientific.

There are four atmospheric windows each revealing, broadly speaking, properties of atoms or molecules (Figure 3.3) ... 

Much of the electromagnetic spectrum has been used to investigate the structure of matter in the laboratory but the atmospheric windows restrict astronomical observations from Earth. Irritating as this is for astronomers on the ground, the chemical structure of the atmosphere and the radiation that it traps is important to the origins of life on Earth. The light that does get through the atmosphere, however, when analysed with all of the tools of spectroscopy, tells the molecular story of chemistry in distant places around the Universe. 

Astronomical measurements have some limitations not present in laboratory investigations but a detailed knowledge of the spectra of atoms and molecules can be used to overcome the restrictions of resolution and atmospheric windows. The detailed knowledge is the key to this success, however, and confidence in the conclusions of astrochemical observations must come from the understanding in the laboratory. 

Rotational spectroscopy and microwave astronomy are the most accurate way to identify a molecule in space but there are two atmospheric windows for infrared astronomy in the region 1-5 im between the H2O and CO2 absorptions in the atmosphere and in the region 8-20 xrn. Identification of small molecules is possible by IR but this places some requirements on the resolution of the telescope and the spacing of rotational and vibrational levels within the molecule. The best IR telescopes, such as the UK Infrared Telescope on Mauna Kea in Hawaii (Figure 3.13), are dedicated to the 1-30 xm region of the spectrum and have a spatial resolution very close to the diffraction limit at these wavelengths. 

Telescopes Instruments for collecting the light from the stars either outside the Earth s atmosphere in orbit or on the Earth collecting the light through the atmospheric windows. Each telescope has a finite spatial resolution... 

Which of the named sequences in the spectrum of the H atom can be observed in atmospheric windows ... 

Atmospheric windows Regions of the electromagnetic spectrum that allow radiation to pass through the atmosphere to reach the ground. 

The most common application of MMW systems to security needs is the MMW radiometric imager. Systems of this type are very useful because of their ability to detect a variety of concealed weapons hidden by clothing. Millivision Technologies of South Deerfield, MA has been a leader in this field by developing a family of imagers that operate in the 94 GHz atmospheric window and that use a superheterodyne receiver... 

The major absorbers of infrared radiation in the atmosphere are HjO, CO2, and O3. The infrared wavelength ranges of 2-2.5 pm, 3-5 pm, and 8-12 pm are known as atmospheric windows. The radiative energies transmitted from the Sun to the Earth s surface within these atmospheric windows are shown in Table 12.3. 

Infrared emissions are dependent on the nature of the emissive body and its temperature. For example, a jetfighter emits strongly in the infrared around the wavelength ranges of 2 pm and 3-5 pm from the exhaust nozzle, 4—5 pm from the exhaust gases, and 3-5 pm and 8-12 pm from the flight body. Infrared sensors used in the aforementioned atmospheric windows are semiconductors composed of the chemical compounds PbS for 2.5 pm, InSb for 3-5 pm, and HgCdTe for 3-5 pm and 10 pm. 

For gases that satisfy these conditions, the effects can be proportionately quite large. For example, addition of one molecule of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 is equivalent to the addition of 104 additional molecules of C02 due to the stronger absorption cross sections of the CFCs that occur in the atmospheric window and to the dependence of absorption on concentration for the CFCs but on the logarithm of concentration for C02 (Ramanathan et al., 1987). 

FIGURE 14-10 Intrinsic infrared absorption band strengths of some potential greenhouse gases in the atmospheric window (from Ramanathan, 1988a, 1988b). 

Therefore, ground-based measurements are restricted to atmospheric windows where the water vapour absorptions are relatively small. For limb space-based atmospheric sounding, this is less of a problem but retrieval of trace gases in the upper troposphere are restricted to a limited number of candidates. For many gases such measurements are optimal for sounding the stratosphere and mesosphere. For the lower stratosphere and upper troposphere, some interesting possibilities exist, but the range of application is more restricted. 

Fig. 13. Attenuation of electromagnetic radiation in the atmosphere. The solid curve indicates the altitude where the intensity of the external radiation is reduced to 1/2 of its original value at a given wavelength (after Giacconi et al., 1968). Physical phenomena and observational techniques are indicated for the different wavelength regions. All observed interstellar radio frequency lines lie within the atmospheric windows...

The wide variety of interstellar molecules detected so far in our Galaxy (see Table 6) are composed of the most abundant chemically reactive elements, i.e. H, C, N, 0, Si and S. The selection of detected molecules is influenced by molecular and observational considerations i) the molecules must be polar ii) they must have sufficient vapor pressure for their laboratory spectra to be known, iii) of the known spectra, only the most intense transitions can be expected to be observable in interstellar space, and iv) the frequencies of these transitions have to be located within the Earth s atmospheric windows Only molecules which satisfy these conditions are amenable to radio techniques. 

Contrary to all the laser lines reported in Table 4, lasing at 3.9 /tm with ZBLAN Ho3+ fiber is achieved at liquid-nitrogen temperature. CW output power of 11 mW is obtained with 900 mW launched pump power at 885 nm. It must be noted that such lasers are of prime importance for military and space applications because they lie within an atmospheric window transparent from 3 /tm to 5 /tm. Very few lasers exist in this spectral region. 

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