Soft X-rays from Iron, Cobalt, Nickel and Copper

Critical potentials of Fe, Co, Ni and Co, to 1500 v.—Using the photo-electric method previously ${\mathrm{described}}^1$, the study of iron has been extended, and critical potentials of Co and Ni have been determined throughout the range 0-1500 volts. The arrangement is more sensitive than that used by previous observers and readings were taken at intervals of 1/2 volt accelerating potential up to 100 volts and at corresponding small intervals for the higher ranges; hence the curves are more accurate and reproducible than those obtained by other investigators. Forty-six critical potentials were found for Ni, 48 for Co, and the number for Fe was increased to 46. More than 100 independent curves were obtained for each of these metals and the breaks reported were entirely reproducible. Copper was investigated only in the range 0-90 and 700-1200 volts; 25 critical potentials were found. The breaks are of two types with shapes which suggest that they correspond to ionizing and radiating potentials. There is a general correspondence, both in nature and value, between the critical potentials of iron, cobalt and nickel, with an upward shift of voltage with increasing atomic number. This correspondence is especially striking in the case of the nine breaks which have been designated as "ionizing" potentials. In the region of the L series, the critical potentials of copper also correspond accurately with those of the other metals, but in the region 0-90 volts there appears to be no similarity. The critical potentials which could be connected with x-ray spectroscopic terms with some certainty are: Fe, ${\mathrm{L}}_{\mathrm{I}}$ at 818.5 and ${\mathrm{L}}_{\mathrm{III}}$ at 704.3; Co, ${\mathrm{L}}_{\mathrm{I}}$ at 873.2 and ${\mathrm{L}}_{\mathrm{III}}$ at 764.8; Ni, ${\mathrm{L}}_{\mathrm{I}}$ at 948.0 and ${\mathrm{L}}_{\mathrm{III}}$ at 833.4; Cu, ${\mathrm{L}}_{\mathrm{I}}$ at 1017.0 and ${\mathrm{L}}_{\mathrm{III}}$ at 929.0 volts.