Cooling Infrared

By carrying out photolyses in liquid nitrogen- or liquid helium-cooled infrared cells using a special low-temperature apparatus (see Figure 4.2), one is often able to obtain direct spectroscopic evidence for intermediates of photochemical reactions. In this section we will briefly review how low-temperature techniques have been used to observe intermediates in type I cleavage reactions. 

Spitzer Space Telescope a cryogenically cooled infrared space telescope on a thermally stable Earth-trailing orbit operated by NASA. The telescope has very sensitive imaging capabilities with the IRAC (3 to 8 pm) and MIPS (24— 160 pm) cameras as well as spectroscopic capability with the IRS instrument (5-40 pm). 

Apart from innovative ideas in chemistry, progress in the field of matrix i.solation is closely coupled with the development of new instruments and devices. Fourier-transform infrared instruments have enormously increased the amount of information which is obtained from matrix-isolation experiments. With new helium-cooled infrared detectors and faster Fourier-transform processors, less time is required to obtain high-quality spectra over a large spectral range. Unusual species at low concentrations and isotopic molecules with a low natural abundance can therefore now be detected. Additionally, hidden data are easily accessible by spectra subtraction (e.g., of the spectra before and after photolysis). 

Characterization of the conformational probability of A-acetyl-phenylalanyl-NH2 by RHF, DFT, and MP2 computation and AIM analyses, confirmed by jet-cooled infrared data ... 

Figure 10. Mini pulse tube cryocooler used for cooling infrared sensors or superconducting devices in space. Courtesy TRW/NGST.

The energy transfer described by (8.16) can be monitored if M is excited by a short infrared laser pulse and the fluorescence of AB is detected by a fast cooled infrared detector (Vol. 1, Sect. 4.5) with sufficient time resolution. Such measurements have been performed in many laboratories [963]. For illustration, an experiment carried... 

A typical experimental arrangement for such investigations is depicted in Fig. 8.23. In a flow system, where the reactive collisions take place, the levels (u, J) of the reactant molecules are selectively excited by a pulsed infrared laser. The time-dependent population of excited levels in the reactants or the product molecules are monitored through their fluorescence, detected by fast, cooled infrared detectors (Vol. l,Sect. 4.5). 

In the IR with terrestrial backgrounds and cooled infrared sensitive devices, the ultimate limit in performance is set by the shot noise arising from fluctuations in the arrival rate of the background photons. For the wavelengths of interest the background photons follow Poisson statistics, and the standard deviation of the number of collected photoelectrons from a detector in a sampling time will be the square root of the number. For an individual detection cell of area A with quantum efficiency 7, the noise spectrum of the photocurrent will be Sf=e J tjA where is the background photon flux at the detection cell... 

DESIGN CONSIDERATIONS FOR CRYOGENIC LIQUID REFILL SYSTEMS FOR COOLING INFRARED DETECTION CELLS... 

There have been many methods used for cooling infrared cells. Among these are Joule-Thomson cryostats. They utilize a high pres sure gas, a Joule-Thomson nozzle, and a heat exchanger to provide regenerative counter flow which cools the input gas so that a certain percentage of the gas is liquefied at the end of the cryostat. This device is mounted inside of a small dewar in which the detection cell itself is mounted. 

Another method which has been utilized for cooling infrared cells is a cryostat operating on the expansion engine principle. Here the reliability of the cooling device is somewhat higher as it is not so prone to clogging by impurities. However, it can cause mechanical vibrations which may introduce noise into the infrared system, and hence, it has not always proved to be practical. 

A fourth method of cooling infrared detection cells is the thermoelectric device which utilizes the Peltier effect. This is a reverse thermocouple. When current is applied to it, one side of the dissimilar metal junction turns cold. These devices, while very reliable, are presently.capable of cooling to no lower than -100°F, whereas required cooling is to the order of -300°F, or lower. 

The energy transfer described by (13.16) can be monitored if M is excited by a short infrared laser pulse and the fluorescence of AB is detected by a fast cooled infrared detector (Sect. 4.5) with sufficient time resolution. Such measurements have been performed in many laboratories [13.6]. For illustration, an experiment carried out by Green and Hancock [13.77] is explained by Fig. 13.16a a pulsed HF laser excites hydrogen fluoride molecules into the vibrational level u = 1. Collisions with other molecules AB (AB = CO, N2) transfer the energy to excited vibrational levels of AB. The infrared fluorescence emitted by AB and HF has to be separated by spectral filters. If two detectors are used, the decrease of the density A(HF ) of vibrationally excited HF molecules and the build-up and decay of A(AB ) can be monitored simultaneously. 

Another advantage of nichrome is that it resists oxidation, which is important since this hot wire is exposed to the atmosphere. A common item that also contains an air-cooled infrared source is a toaster. The nichrome wire may in an FTlR s source be in thermal contact with a piece of ceramic, which allows it to operate at higher temperatures. Air-cooled sources can attain temperatures of 1200°K to 1400°K. In most FTIRs the source is backed by a mirror, as shown in Figure 2.24, or surrounded by a housing to collect as much infrared light as possible and send it toward the interferometer. 

Like the filament in any light source, air-cooled infrared sources will eventually fail. This makes the source one of the consumables in an FTIR. The author s experience is that most air-cooled infrared sources last for three or more years. For many FTIRs, replacing the source is something users can perform themselves. Air-cooled sources are not expensive, and it is a good idea to keep a spare one handy so when the day comes when your source dies you can replace it quickly. Alternatively, if you have a service contract on your instrument, the source is something the manufacturer should replace on a routine basis. 

Klipping, Lemke, Romisch, Telescope] Klipping, Gustav/Dietrich Lemke/Norbert Romisch Liquid Helium Cooled Infrared Telescope for Astronomical and Atmospherical Measurements from Spacelab, Advances in Cryogenic Engineering 23 (1978), p. 628-633. 

We constructed a liquid nitrogen cooled infrared camera (PICNIC), whidi uses a NICM0S3 256 X 256 format array detector and a new optical syst which consists of reflective optics (Fig. 1). Two additional componoits, a rotating half wave plate polarimeter and a prism, can be attadied in front of the camera, enabling us to obtain imaging polarimetry and multi-object spectroscopy. 

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