Helium expansion measurements

Determination of the volume of activated carbon Norit R1 Extra by helium expansion measurements at 298 K in a commercial gas pycnometer (Micromerites, Accu Pyc 1330). 

Several catalyst densities are used in the literature. True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. In a strict physical sense, this density can be calculated only through X-ray or neutron diffraction analysis of single crystal samples. The term apparent density has been used to refer to the mass divided by the volume including some portion of the pores and voids, and so values are always smaller than the true density. This term should not be used unless a clear description is given of what portion of the pores is included in the volume. So-called helium densities determined by helium expansion are apparent densities and not true densities since the measurement may exclude closed pores. 

V e volume of a (porous) sorbent measured by helium expansion experiments... 

The adsorbent (powder) is then brought into contact with the adsorbate and, when constant pressure, volume and temperature conditions show that the system has attained equilibrium, the amount of gas is again calculated. The difference between the amount of gas present initially and finally represents the adsorbate lost from the gas phase to the adsorbate phase. The accurate determination of the amount of gas nnadsorbed at equilibrium depends upon the accurate determination of the dead space or the space surrounding the adsorbent particles. The dead space is determirud by expansion measurements using helium, whose adsorption can be assumed to be negligible. 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Supersonic molecular beam (SMB) mass spectrometry (SMB-MS) measures the mass spectrum of vibra-tionally cold molecules (cold El). Supersonic molecular beams [43] are formed by the co-expansion of an atmospheric pressure helium or hydrogen carrier gas, seeded with heavier sample organic molecules, through a simple pinhole (ca. 100 p,m i.d.) into a 10 5-mbar vacuum with flow-rates of 200 ml. rn in. In SMB, molecular ionisation is obtained either through improved electron impact ionisation, or through hyperthermal surface ionisation... 

Also as in the case of helium, asymptotic expansion methods can be applied to the Rydberg states of lithium and compared with high precision measurements [73,74]. This case is more difficult because the Li+ core is a nonhydrogenic two-electron ion for which the multipole moments cannot be calculated analytically, and variational basis set methods must be used instead. However, the method is in principle capable of the same high accuracy as for helium. 

The flow of gas was controlled with mercury cutoffs. The pertinent volumes of the apparatus were determined by expansion of argon or helium from the calibrated volume of the McLeod gage and the pressure differences were observed with a cathetometer. The reproducibility of the McLeod gage measurements was 1% or better in the pressure range used in these measurements. 

Gas Thermometers. These are expansion thermometers that depend on the coefficient of thermal expansion. They use, for example, helium gas and have helped to establish the thermodynamic temperature scale, and also for measurements at very low temperatures. 

Figure 18. The top spectrum shows part of the (5,0,0) band for the 035CI0 iso-topomer, measured at high resolution (0.05 cm-1) under expansion conditions of 10% OCIO in 1 atm of helium. The bottom spectrum is from a 100 K Boltzmann rotational temperature calculation. The experimental rotational linewidth is consistent with a 0.28 cm-1 width calculated Lorentzian lineshape.

Figure 19. Jet-cooled emission spectrum of the X <- B transition of CN measured at 0.25 cm-1 resolution. The expansion conditions were 100 Torr of acetonitrile seeded in 1 atm of helium. Both the 0-0 and 1-1 transitions are shown. The arrows indicate the small perturbations due to A state rotational levels (see text).

The sampling loops were replaced by two stainless steel U-tubes of 1.5- and 20-cc. capacity. The expansion bomb is a 1.7-liter stainless steel cylinder. The trap between the helium supply and the Beckman valve is 1/4-inch stainless steel tubing. A null detector is used to measure pressures in the inlet system. Samples are obtained in 10-ml. stainless steel cylinders fitted with a Vg-inch stainless steel Hoke valve with a V-stem and Teflon packing. When the sample is liquid, it is entirely vaporized into the 1.7-liter expansion bomb, and a gaseous sample is taken for infrared, near infrared, and gas chromatographic analysis. 

The final electrical connections to the STM can be done with copper wires. A small amount of helium is used as an exchange gas to anchor the temperature of the whole assembly to the cryogenic fluid. The body of the STM can be made out of copper, which will respond quickly to temperature changes for variable temperature measurements and provide a uniform temperature environment for the tunnel junction. One has to estimate the differential thermal contraction of the component parts to make sure that a tunnel junction separation set at room temperature is sufficiently large to prevent tip crash on cooling. Other materials like Macor or Invar , which closely match the thermal expansion properties of the piezoelectric transducers, are used as well but take more time to thermally stabilize. Some references are given in [6.30-6.43]... 

It was still too hot for the electrons to join the hydrogen and helium ions to form neutral atoms. This occurred not until about 500 000 years later, wh tenq>erature had dropped to a few 1000 K. The disappearance of free electrons broke the thermal contact between radiation and matter, and radiation continued then to expand freely. An outside spectator would have observed this as a hugh flash and a rapidly expanding fireball. In the adiabatic expansion the radiation cooled further to the cosmic background radiation level of 2.7 K measured today. 

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