Saturated Helium

Radiostearic acid was adsorbed from n-hex-adecane onto mica and thin vapor-deposited films of iron, gold, and copper that had been exposed to dry and water-saturated helium or air. Adsorption was measured directly and continuously by a recently developed technique. The mica substrate showed essentially zero adsorption. None of the metals adsorbed more than one stable compact monolayer. Iron and gold showed a large difference in adsorption in dry helium or air, but adsorbed about the same amoxmt, 0.2 to 0.5 monolayer, when exposed to either wet helium or air. Copper adsorbed 0.3 to 0.7 monolayer in all atmospheres except wet air, in which it showed a weak adsorption of nine monolayers rinsing with hexaiie removed all but one monolayer. 

A. films of iron, gold, and copper that had been exposed to purified dry helium or air [20]. Because the presence of moisture is important in many surface phenomena, corresponding experiments have been conducted in which the metal films were exposed to water-saturated helium or air. 

Saturated hydrocarbons such as neopentane, notbomane, and cyclooctane have been converted to the corresponding perfluoro derivatives in 10-20% yield by gas-phase reaction with fluorine gas diluted with helium at —78°C. Simple ethers can be completely fluorinated under similar conditions. Crown polyethers can be fluorinated by passing an Fa/He stream over a solid mixture of sodium fluoride and the crown ether. Liquid-phase fluorination of hydrocarbons has also been observed, but the reaction is believed to be ionic, rather than radical, in character. A variety of milder fluorination agents have been developed for synthetic purposes and will be discussed in Chapter 6 of Part B. 

An experiment is performed to determine the vapor pressure of formic acid. A 30.0-L volume of helium gas at 20.0°C is passed through 10.00 g of liquid formic acid (HCOOH) at 20.0°C. After the experiment, 7.50 g of liquid formic acid remains. Assume that the helium gas becomes saturated with formic acid vapor and the total gas volume and temperature remain constant. What is the vapor pressure of formic acid at 20.0°C ... 

The gas feed and mixing system consists mainly of glass flowmeters or electronic mass flowmeters connected to gas bottles. For reactants that are in liquid state at room conditions (e. g. methanol) a saturator is normally used through which helium is sparged and then mixed with the other reactants. In this case all lines connected to the reactor are heated (e.g. at 150°C) to avoid condensation in the lines. In certain cases the gases from the bottles should be pretreated in order to avoid contamination of the catalyst. For example, a... 

A fiber-diffraction pattern is recorded on a flat-film camera in which the fiber-to-photographic film distance is typically in the range of 3 to 4 cm. During exposure to X-rays, the specimen chamber is continuously flushed with a slow and steady stream of helium gas that has been bubbled through a saturated salt solution so that (a) the fiber is maintained at a constant desired r.h. and (b) fogging of the photographic film from air scattering is reduced. 

Accessibility to Cu sites was determined by temperature programmed desorption of NO (NO TPD), using an experimental setup similar to that used for TPR, except the detector was a quadrupole mass spectrometer (Balzers QMS421) calibrated on standard mixtures. The samples were first activated in air at 673 K, cooled to room temperature in air, and saturated with NO (NO/He 1/99, vol/vol). They were then flushed with He until no NO could be detected in the effluent, and TPD was started up to 873 K at a heating rate of 10 K/min with an helium flow of 50 cm min. The amount of NO held on the surface was determined from the peak area of the TPD curves. 

After the catalyst was saturated with carbon dioxide, a temperature programmed desorption (TPD) was carried out by heating the sample in helium (40 cm3min 1) from room temperature to 873 K (10 Kmin 1). The mass spectrometer was used to follow water (mass 18), carbon monoxide (mass 28), carbon dioxide (mass 44) and oxygen (mass 32). 

Fig. 13.10 Sonoluminescence spectrum of potassium-atom emission from helium-saturated KC1 aqueous solution at 148 kHz. The spectrum shows slightly asymmetric broadening toward blue side, which is in contrast with the potassium line in argon-saturated solution...

Fig. 14.6 (a) Concentration of hydrogen peroxide formed after 30 min sonication as a function of dissolved gas concentration for helium and argon saturated NaN03 and NaC104 solutions (data adapted from reference [41])... 

It can be noticed that the extent of SL quenching (and the extent of temperature lowering, as shown later in Fig. 14.15) is significantly lower in helium saturated... 

Fig. 14.14 The effect of ethanol concentration on the relative SL intensity in argon and helium saturated water normalised with respect to pure water) at 363 kHz [60]...

Fig. 14.15 Cavitation bubble temperature at different alcohol concentrations at 363 kHz in (a) helium saturated and (b) argon saturated solutions, (c) Temperature at different alcohol concentrations in argon saturated solutions at 1,056 kHz [60]...

Assuming a cooler temperature for helium, this means the amount of heat energy available for pyrolysis is less, which would produce less hydrocarbon products. This ultimately would lead to a relatively smaller SL quenching compared to that observed in argon saturated solutions. The relative change in bubble temperature would also be less in helium saturated solutions due to the lower amount of hydrocarbon products generated. Solubility differences would re-enforce this. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

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