The Photo-Ionization of Potassium Vapor

Using a space-charge-neutralization method described in a recent publication the variation with wave-length of the ionization per unit light intensity $B_{\nu }$ in potassium vapor has been studied. $B_{\nu }$ exhibits maxima in the range 2100A to 3000A, one at the series limit, 2856A, and another at 2340A, the magnitude of the latter maximum being three times the former. A minimum value of $B_{\nu }$ occurs close to 2700A. Former results are shown to agree with the present much more accurate data though, because of the wide bands of wave-lengths used and the scantiness of observations, the earlier work failed to reveal the salient features here presented. The decrease of the ionization efficiency on both sides of the series limit is similar to the corresponding variation in caesium and rubidium and it seems probable, though not here proved, that the ionization by wave-lengths longer than the series limit is due to absorption of principal series lines. The increase of $B_{\nu }$ beyond the series limit has been attributed in the past to ionization of molecules. The present work shows that on this view the apparent molecular threshold occurs at 2700A and not at 2550A as estimated from earlier work. This fact in itself constitutes a serious objection to interpreting the observed variation of $B_{\nu }$ as a molecular phenomenon. It is pointed out that Williamson's observations of the emergent velocities of the photo-electrons show no evidence of molecularionization involving dissociation of the molecule and ionization of one of the atoms. Also Ditchburn's observations of the continuous optical absorption due to potassium atoms agree qualitatively with the present results. It thereby appears probable that the observed dependence on wave-length of $B_{\nu }$ is an atomic property of potassium.