A Critical Potential of Methane and Its Absorption in the Ultra-Violet.

In a previous paper' it was stated that the maximum in the current potential curve found in a three-electrode tube with methane must be due either to a resonance potential of methane or to a large transparency for slow electrons. It was further pointed out thatthe interpretationof maxima in current-potential curves obtained in four-electrode tubes must also take into account the possibility of abnormal transparency at certain velocities. In the meantime Brode2 has as a matter of fact found a variable mean free path for electrons in methane using the method of Ramsauer,3 which presumably accounts for the maximum, since it occurs at about the expected velocity. Nevertheless, the whole problem is of such importance that it seemed wise to make a further investigation to see if, in addition to the effect due to transparency, there might not also be a resonance potential occurring in the same neighborhood, accounting for part of the effect. The possibility must also be considered of a critical potential (as distinguished from a resonance potential) at which the molecule assumes a metastable state, without immediate re-emission as in the case of a resonance potential. (The hypothesis of a metastable state has been made before for instance in the case of helium by Franck and Knipping;4 and, since nothing is known concerning the energy-levels of methane, this possibility must also be considered.) In this paper experimental work is described which shows that no part of the maximum obtained in three or four-electrode tubes filled with methane can be due to a resonance potential. The work does not eliminate, however, the possibility of a non-radiating critical potential occurring at the voltage in question. The experiments consist in the measurement of the absorption of methane for ultraviolet light and in the study of Lenard photo-electric curves in methane. Before describing them the current-potential curves obtained by the resonance method will be given.