Liquefied rare gases

The liquefied rare gases fall into two classes with respect to their electron mobility, indicating that two different transport mechanisms are operative. In liquid helium and neon, near boiling, the electron mobility is much smaller than 1 cm V- s L The charge carrier is depicted as a bubble with a 10- to 20-A radius occupied by the electron. In the other liquids the electron mobility is much higher (see Table 1). 

1992 Schmidt, W.E, IEEE Trans. Electr. 

In liquid neon the mobility is thermally activated up to the critical temperature. In both liquids, the drift mobility is independent of the electric field strength up to about 60 kV/cm, the highest value employed (Sakai et al., 1982 1983 1992). I 

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Excited states of hydrocarbon molecules often undergo nondissociative transformation, although dissociative transformation is not unknown. In the liquid phase, these excited states are either formed directly or, more often, indirectly by electron-ion or ion-ion recombination. In the latter case, the ultimate fate (e.g., light emission) will be delayed, which offers an experimental window for discrimination. A similar situation exists in liquid argon (and probably other liquefied rare gases), where it has been estimated that -20% of the excitons obtained under high-energy irradiation are formed directly and the rest by recombination (Kubota et al., 1976). 

In liquefied rare gases (LRG) the ejected electron has a long thermalization distance, because the subexcitation electrons can only be thermalized by elastic collisions, a very inefficient process predicated by the small mass ratio of the electron to that of the rare gas atom. Thus, even at a minimum of LET (for a -1-MeV electron), the thermalization distance exceeds the interionization distance on the track, determined by the LET and the W value, by an order of magnitude or more (Mozumder, 1995). Therefore, isolated spurs are never seen in LRG, and even at the minimum LET the track model is better described with a cylindrical symmetry. This matter is of great consequence to the theoretical understanding of free-ion yields in LRG (see Sect. 9.6). 

Ionization in the condensed phase presents a challenge due to the lack of a precise operational definition. Only in very few cases, such as the liquefied rare gases (LRG), where saturation ionization current can be obtained at relatively low fields, can a gas-phase definition be applied and a W value obtained (Takahashi et al., 1974 Thomas and Imel, 1987 Aprile et al., 1993). 

Liquefied rare gases(LRGs) are very important both from the fundamental point of view and in application to ionization chambers. In these media, epithermal electrons are characterized by a very large mean free path for momentum transfer -10-15 nm, whereas the mean free path for energy loss by elastic collision is only -0.5 nm. This is caused by coherence in momentum transfer scattering exhibited by a small value of the structure factor at low momentum transfers... 

Table 10.2 lists the critical field Ec in various nonpolar liquids along with the approximate nature of field dependence of mobility when E > Eq. It is remarkable that the higher the zero-field mobility is, the smaller is the value of Ec, indicating the role of field-induced heating. Also note that in the sublinear case, Ec is larger in the case of molecular liquids than for liquefied rare gases,... 

Table 6.4 Properties of liquefied rare gases close to boiling points...

In this book we are mainly concerned with nonpolar dielectric liquids which are comprised of atoms or molecules with high ionization potentials. The liquids are insulators from a physical point of view. A filled valence band and an empty conduction band are formed. In the liquefied rare gases both bands are probably several... 

Values for A and B determined experimentally for liquefied rare gases and methane are compiled in Table 8. In the diffusion experiments an excess pressure of several bars is usually applied in order to prevent turbulences in the liquid due to boiling. 

This book deals with electronic processes occurring mostly in hydrocarbons, liquefied rare gases, cryogenic molecular liquids, and some other nonpolar liquids. In addition, solutions are considered. Since many readers may not be familiar with their chemical properties and the chemical nomenclature, we shall give a short summary. 

Cryogenic liquids are the liquefied rare gases ( He, Ne, Ar, Kr, Xe) and the simple molecular liquids liquid hydrogen (H2), liquid deuterium (D2), liquid nitrogen (N2), liquid oxygen (O2), liquid carbon monoxide (CO), and others. 

Organic liquids with low vapor pressure and liquefied rare gases are amenable to electron injection from the vapor phase (Sato et al., 1956 Sommer, 1964 Watson and Clancy, 1965). Usually, a hot filament is used as the electron source. The electrons are drawn to the liquid surface by an electric field. The principle of the method is depicted in Figure 4a. Depending on the energy barrier for electron transfer at the... 

Table 1 Electron Mobility and Saturation Velocity in Liquefied Rare Gases...

In the heavier liquefied rare gases, the mobility depends on temperature in acomplicated fashion (see Figure 2). The electron drift velocity increases subpropor-tionally with electric field strength (see Figure 3) or, in other words, the electron mobility decreases with the electric field strength. 

Sowada, U., Schmidt, W. E, and Bakale, G., The influence of nonelectronegative molecules on the mobility of excess electrons in liquefied rare gases and tetramethylsilane. Can. J. Chem., 55,1885,1977. 

Figure 6 Fast electron-induced free ion yields in liquefied rare gases. (Redrawn from the data of Doke, T., Portgal Phys., 12, 9,1981.)...

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