Applications of Cooled Atoms

Another application is the deflection of atoms by photon recoil. For sufficiently good beam collimation, the deflection due to single photons can be detected. The distribution of the transverse-velocity components contains information about the statistics of photon absorption [14.46]. Such experiments have successfully demonstrated the antibunching characteristics of photon absorption [14.47]. The photon statistic directly manifests itself in the momentum distribution of the deflected atoms [14.48]. 

A very interesting application of cold, trapped atoms is their aspect for an optical frequency standard [14.49]. They offer two major advantages  

For the realization of an atomic fountain, cold atoms are released in the vertical direction out of an atomic trap. They are decelerated by the gravitational field and return back after having passed the culmination point with V = 0. 

Assume the atoms start with Vq = 5 m/s. Their upwards flight time is then t = Vqz/S = 0-5 s, their path length is z = Vot-l/2gt2 = 1.25 m and their total flight time is 1 s. Their transit time through a laser beam with the diameter d = 1 cm close to the culmination point is T = 90 ms and the maximum transverse velocity is v 0.45 m/s. The transit-time broadening is then less than 10 Hz. 

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The sensitivity difference between direct flame and furnace atomisation has been bridged via the general method of atom trapping as proposed by Watling [1]. A silica tube is suspended in the air-acetylene flame. This increases the residence time of the atoms within the tube and therefore within the measurement system. Further devices such as water-cooled systems that trap the atom population on cool surfaces and then subsequently release them by temporarily halting the coolant flow are sometimes employed. The application of atomtrapping atomic absorption spectrometry to the determination of lead and cadmium has been discussed by Fiallam and Thompson [2]. 

West and co-workers11 achieved far greater sensitivity enhancement by trapping readily atomized elements, which had been nebulized conventionally for periods of up to a few minutes, onto the outer surface of a small bore, water-cooled quartz tube. If the water was then drained rapidly, the tube temperature quickly rose and the element was atomized off the surface. For cadmium in calcium chloride extracts of soil, for example, a detection limit of 4 ng (g soil) -1 was reported. A water-cooled double silica tube atom trap similarly has been very successfully employed for the determination of cadmium and lead in natural waters by flame AAS.12 Some further examples of applications of atom trapping are included in Chapter 7. 

Seventy grams (1.07 gram atoms) of zinc dust is added in small portions under a reflux condenser to a stirred solution of 40 g. (0.205 mole) of fluorenone oxime (p. 164) in 267 ml. of acetic acid and 13 ml. of water. The rate of addition of the zinc is such as to maintain an even, gentle reflux. The mixture is then refluxed for another hour by the application of heat, and this is followed by the addition of 400 ml. of water. The mixture is filtered and the residue extracted with 200 ml. of hot 5% acetic acid. The solution thus obtained is cooled and filtered, and the filtrate is mixed with an equal volume of concentrated hydrochloric acid and held at 0° for 12 hours. The precipitated amine hydrochloride is removed by filtration and treated with excess ammonium hydroxide solution, and the resulting free base is recrystallized from petroleum ether to give a 90% yield of 9-aminofluorene, m.p. 62-63 . 

In polystyrene, a benzene ring replaces one hydrogen atom of each ethylene monomer unit. Because such a ring is bulky, atactic polystyrene does not crystallize to any significant extent. The most familiar application of this polymer is in the polystyrene foam used in disposable containers for food and drinks and as insulation. A volatile liquid or a compound that dissociates to gaseous products on heating is added to the molten polystyrene. It forms bubbles that remain as the polymer is cooled and molded. The gas-filled pockets in the final product make it a good thermal insulator. 

It is impossible to overstate the importance of the role that CFCs have played in making refrigeration, air-conditioning, and, to some extent, aerosols such essential facets of modern existence. Life, as we know it, would be impossible in some parts of the world without the means for cooling food and houses. Of course, the very property that made the CFCs so useful, their stability, allows them to reach the stratosphere and there release chlorine atoms, which interact with the ozone layer. There will be a drastic decline in the use of CFCs in most of their high-volume applications of the heavier CFCs and the volumes will be inconsequential compared to the 2.5 billion pounds of volatile CFCs produced worldwide in 1986. 

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