Two-Quantum Transitions in the Microwave Zeeman Spectrum of Atomic Oxygen

Two-quantum transitions have been observed in the microwave Zeeman spectrum of atomic oxygen in its ground ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ state by the method of magnetic resonance absorption spectroscopy. The three lines observed originally by Rawson and Beringer are identified as arising from two-quantum transitions between Zeeman levels with the selection rule $\Delta M=\pm 2$. Each of the three lines is observed at the mean frequency of the two corresponding $\Delta M=\pm 1$ transitions to within the experimental accuracy of a few parts per million. The line width of the two-quantum transitions is approximately one-half that of the normal transitions, and the line intensity varies more rapidly with rf power, both in agreement with the theory. The principal features of these transitions are explained by second-order time-dependent perturbation theory. The two-quantum transitions reported here are essentially similar to the double-quantum transition reported by Hughes and Grabner in the electric quadrupole spectrum of ${\mathrm{Rb}}^{85}$F and to the multiple-quantum transitions seen by Kusch in the Zeeman spectra of K and ${\mathrm{O}}_2$.