## Abstract

Radio-frequency magnetic resonances have been studied between the states of the atomic level 5 $^{2}$P$_{\frac{3}{2}}$, excited in a coarse atomic beam of potassium by optical resonance radiation. The resonances were detected by observing changes in the intensity of the scattered resonance radiation. Hyperfine structure was observed in the Paschen-Back region, and in magnetic fields down to zero. Values of a and b, the magnetic dipole and electric quadrupole interaction constants, derived independently from measurements in the strong and zero field regions, are in agreement. The values a = 1$\cdot $97 $\pm $ 0$\cdot $1 Mc/s, b = 1$\cdot $7 $\pm $0$\cdot $3 Mc/s have been obtained for the interaction constants of $^{39}$K. Double quantum transitions between hyperfine structure levels were observed in zero field. The nuclear magnetic moment derived from a agrees with molecular beam magnetic resonance measurements; the nuclear electric quadrupole moment derived from b is 0$\cdot $11$_{3}\pm $0$\cdot $02$\times $10$^{-24}$ cm$^{2}$. Hyperfine structure was also observed in the level 5 $^{2}$P$_{\frac{1}{2}}$. A preliminary value for the a factor is 9$\cdot $3$\pm $0$\cdot $5 Mc/s.