A description is given of an elementary method for the determination of the effective cross-section for the ionization of gas atoms by metastable atoms, together with some preliminary results.
The process of ionization of gas molecules by collisions with metastable atoms (m.a.) is called the P e n n i n g-effect. This ionization is possible when the ionization energy of the gas molecule is lower than the excitation energy of the metastable level. The efficiency of the process has been determined by Penning1), by Kruithof and Druyvesteyn2), and recently by Hoffmann3). P e n n i n g measured the breakdown potential in Ne-Ar mixtures, calculated from this the T o w n s e n d ionization coefficient ~. and concluded that almost every gas-kinetic collision of a Ne m.a. with an Ar atom results in the ionization of the latter. K r u i t 11 o f and D r u y v e s t e y n started from measurements of this ~ in T o w ns e n d discharges and found an efficiency near 1/2-(effective crosssection 50 cm2/cm 3. mm pressure). H o f f m a n n determined lifetimes of Ne m.a. from his measurements on the conductivity of Ne-Ar mixtures immediately after the passing of a glow discharge, and with these lifetimes he calculated that only about one in 4000 collisions results in an ionization. As these contradictory results were both obtained in rather indirect ways, it seems reasonable to publish some preliminary results of much more direct measurements of the Pe n n i n g-effect, made in the laboratory at Utrecht.
The apparatus (except for a few small alterations) has been described at length in a paper by D o r r e s t e i n 4). Fig. 1 shows the