Ramsauer effect

Here the left-hand side is the ratio of power loss at time t, when the mean electron energy is (E), to that at thermalization, and C and n are determinable constants. This idealized equation is not expected to be valid in presence of the Ramsauer effect, but Warman and deHaas apply it anyway to N2, Ar, and He at atmospheric pressure. The method relates the gradual decrease of collision frequency to an increase in conductivity, which finally rides to a plateau interpreted to be the thermal conductivity. The time needed to reach 90% of the thermal conductivity is called the thermalization time (see Table 8.1). 

Sowada and Warman (1982) have described a dc conductivity method for Ar gas at 295 K and 45 atm. Following a 20-ns pulse of irradiation, the conductivity rises to a peak at -50 ns, due to the Ramsauer effect, before settling to a plateau, which is ascribed to thermal conductivity since the collecting field is very low. Since there is little electron loss, the conductivity profile is proportional to the mobility profile this in turn can be considered a kind of image of collision frequency as a function of electron energy. The time to reach the conductivity plateau, -150 ns, is the measure of thermalization time in the present case. At a density of -9 X 1021 cm-3, the conductivity maximum vanishes, indicating the disappearance of the Ramsauer minimum according to Sowada and Warman. 

Figure 4.38 Total cross sections for the scattering of electrons on rare gases. This cross section is given by

Hund attended the universities at Gottingen and Marburg and took courses in mathematics, physics, geology, and geography. In Gottingen, he took a course on quantum theory given by Debye and studied partial differential equations with Richard Courant. When Hund decided to become a theoretical physicist, he started to work with Born on the physics of crystals (Gittertheorie), as many of Bom s students did, but he finally wrote his dissertation on the Ramsauer effect. 

Kauppila, W.E., Stein, T.S. and Jesion, G. (1976). Direct observation of a Ramsauer-Townsend effect in positron-argon collisions. Phys. Rev. Lett. 36 580-584. 

This equation becomes important if, for example, one considers scattering of 1 eV electrons on xenon in a deflection analyser with 0 = 45°. From Fig. 4.38 one would conclude that scattering effects are small because the scattering cross section is small (Ramsauer minimum), but the reduction of kinetic energies towards the turning point increases [Pg.147]

An important and unique study using PR-TRMC was reported by Shimamori et al. in 992 In this work they made use of the extremely low thermalization efficiency of xenon which is caused by the Ramsauer minimum effect. This allowed them to increase the average equilibrium electron energy by applying even relatively low microwave power levels. In this way, detailed information was obtained on the energy dependence of the attachment coefficient for a variety of halogen-containing compounds. 

The ab initio PSCF approach provides quantitative descriptions of the Ramsauer-Townsend minima in methane and silane where very low energy experimental data are available. In the calculations performed to date it appears that this effect is ubiquitous at least in non-polar hydrocarbons, but that conclusion awaits experimental studies at low energies on more hydrocarbons. 

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