Optical pumping and detection methods were used to determine the resonant frequency of the ${\mathrm{K}}^{39}$ $m=0\mathrm{}\to m=0\mathrm{}$ hyperfine line in various environments. Frequency shifts were observed in noble gases and in hydrogen; no shift was observed from a paraffin wall coating. The shifts, expressed as fractional changes in the hyperfine frequency, are similar to those in rubidium though generally somewhat smaller. The hyperfine constant itself was determined for both ${\mathrm{K}}^{39}$ and ${\mathrm{K}}^{41}$.

• Received 13 May 1960

DOI:https://doi.org/10.1103/PhysRev.119.1946