The Behaviour of Methane Molecules and Argon Atoms in Collisions with very slow Electrons

Abstract

IN a paper by E. Brüche on “Wirkungsquerschnitt und Molekülbau”¹ it is stated on p. 1105 that the curve which shows the variation of the effective cross-section of methane molecules with respect to slow electrons for different velocities of the electrons (we shall call these curves Q-V-curves, since the velocity is given in terms of volts) is of the argon type, namely, that as the velocity of the electrons decreases from 4 volts to 1 volt (that is, from 2V to 1V) the Q-V-curve passes through a maximum and then rapidly sinks to a much lower value. This result was also obtained by R. B. Brode.² In a later paper by C. Ramsauer and R. Kollath,³ Brüche's Q-V-curve is extended to still lower electronic velocities (to about 0.39V) and is found to pass through a minimum. We have now determined the Q-V-curve for methane by Townsend's diffusion method and have also found that the absolute value of the effective cross-section Q of methane molecules is very low, like that of argon, and that the Q-V-curves of argon and methane closely resemble each other over the range of velocities investigated, which in our case was from 1.74V down, to 0.3V. The minimum Q obtained for methane by Ramsauer and Kollath was 4.7 cm.²/cm.³ at a velocity of 0.62V; our value is 4.3 cm.²/cm.³ at a velocity of 0.52V. The minimum in our curve is a little less sharp than in the curve of Ramsauer and Kollath.