Far infrared detection

An interesting potential application of Rydberg atoms is as a far infrared (or microwave) detector, a notion first suggested by Kleppner and Ducas.37 The basic 

While Rydberg atoms have been used to detect infrared radiation with wavelengths as short as 2.34 ftm, they are most effective as detectors of far infrared 

Absorption,1 field ionization Variable temperature black body 3.3 10 17 

Loudon, The Quantum Theory of Light (Oxford University Press, London, 1973). 

Gallagher, in Rydberg States of Atoms and Molecules, eds. R. F. Stebbings and F. B. Dunning (Cambridge University Press, Cambridge, 1983). 

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Table 5.3. Far infrared detection sensitivity in Rydberg atom experiments.

Aside from what these experiments reveal about the atoms, they provide in some cases an excellent means of studying the interaction of radiation with the atoms and may lead to some interesting applications such as far infrared detection. ... 

Matrix Raman spectroscopy allows detection of some additional vibrations which are inactive in IR spectra (e.g. symmetrical vibrations vi in AB3 molecules having 3 symmetry) or which tie in the far infrared region. In practice, matrix-isolated organic intermediates have not been studied by Raman spectroscopy the main objects of these investigations are inorganic molecules (AICI3, PbS, Gep2, SiO, etc.) which are evaporated from solids in effusion cells. 

Changes of fibre optical properties and thus changes of the analyte can be detected in the ultraviolet (UV), visible (VIS), near infrared (NIR) and middle/far infrared (IR) regions. There are only a few materials sufficiently transparent in the UV region, and among them, the pure silica is uniquely suitable for fibre drawing. From Figure 5 it can be seen that the UV... 

The measurement of the cosmic microwave background. Far infrared astronomers were the first to develop thermal detectors. Some of the resulting technologies, such as neutron transmutation doping (NTD) [3], have been transferred to particle detection sensors and have allowed many groups (e.g., ref. [4-11] to make rapid progress). 

R. W. McMillan, Osborne Milton, Jr., M. C. Hetzler, R. S. Hyde, W. R. Owens. Detection of Concealed Weapons Using Far-Infrared Bolometer Arrays , Proceedings of the 25th International Conference on Infrared and Millimeter Waves, Beijing, China, 12-15 September 2000. 

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

The C—C bond was found to be slightly more perturbed than in the monolithium species (i.e., LiC2H4) and the Li—C interactions somewhat more rigid. The normal coordinate analysis showed that such a model is capable of satisfactorily reproducing the measured isotopic shifts on the observed 12 fundamentals. However, the very important Li—Li vibration, which could prove the proposed geometry, was not detected in the expected far-infrared spectral region ... 

The Earth s atmosphere is composed primarily of non-polar molecules like N2 and O2, especially at greater altitudes where the H2O concentrations are small. One would therefore expect collision-induced contributions to the absorption of the Earth s atmosphere from N2-N2, N2-O2 and O2-O2 pairs. The induced rototranslational absorption of nitrogen has not been detected in the Earth s atmosphere, presumably because of strong interference by water absorption bands, but absorption in the various induced vibrational bands is well established (Tipping 1985). Titan (the large moon of Saturn) has a nitrogen atmosphere, somewhat like the Earth methane is also present. Collision-induced absorption by N2-N2 and N2-CH4 is important in the far infrared. 

Thermal emission spectroscopy can be used in middle- and far-infrared spectral regions to make stratospheric measurements, and it has been applied to a number of important molecules with balloon-borne and satellite-based detection systems. In this approach, the molecules of interest are promoted to excited states through collisions with other molecules. The return to the ground state is accompanied by the release of a photon with energy equal to the difference between the quantum states of the molecule. Therefore, the emission spectrum is characteristic of a given molecule. Calculation of the concentration can be complicated because the emission may have originated from a number of stratospheric altitudes, and this situation may necessitate the use of computer-based inversion techniques (24-27) to retrieve a concentration profile. 

Although the focus of this chapter is tropospheric HO measurements, it is worthwhile to mention techniques that have proven useful in the laboratory or in other regions of the atmosphere. As a small molecule in the gas phase, HO has a much-studied and well-understood discrete absorption spectrum in the near UV (29), shown in Figure 1, that lends itself to a variety of absorption and fluorescence techniques. The total atmospheric HO column density has been measured (30-32) from absorption of solar UV radiation, observed with a high-resolution scanning Fabry-Perot spectrometer. Long-path measurements of stratospheric HO from its thermal emission spectra in the far infrared have been reported (33-35). Long absorption paths in the atmospheric boundary layer have been used for HO detection from its UV absorption (36-42). 

The far-infrared emission of the idler frequency was also detected 81) with an InSb detector cooled with liquid He. The power of the pulses was estimated to be about 5 W. Their frequency was not directly measured but only inferred from energy conservation. Later measurements 83> gave a power from 0.25 W at 60 gm to 3 W at 200 jum and a linewidth of 0.1 to 0.5 cm"1 for the signal radiation. 

In this section we have described in considerable detail just one aspect of the spectroscopy of OH, namely, the measurement of zl-doubling frequencies and their nuclear hyperfine structure. This has led us to develop the theory of the fine and hyperfine levels in zero field as well as a brief discussion of the Stark effect. We should note at this point, however, that OH was the first transient gas phase free radical to be studied by pure microwave spectroscopy [121], We will describe these experiments in chapter 10. We note also that magnetic resonance investigations using microwave or far-infrared laser frequencies have also provided much of the most important and accurate information these studies are described in chapter 9, where we are also able to compare OH with the equally important radical, CH, a species which, until very recently, had not been detected and studied by either electric resonance techniques or pure microwave spectroscopy. 

As we mentioned in the introduction to this section, it was known forty years ago from optical spectroscopy that CH is a component of interstellar gas clouds and the search was on for a spectroscopic detection of the radical at higher resolution so that the /l-doubling in the lowest rotational level (J = 1/2) could be measured or predicted accurately. This would enable the detection of CH by radio-astronomers and so allow the distribution of CH in these remote sources to be mapped out. The race was won by Evenson, Radford and Moran [48] using the then new technique of far-infrared LMR in the Boulder laboratories of the NBS (now known as NIST). They realised that there was a good near-coincidence between the water discharge laser line at 118.6 qm (84.249 cm ) and the N = 3 <- 2, J = 7/2 5/2 transition of CH in the / ) spin... 

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