Two-Quantum Transitions in the Microwave Magnetic Resonance Spectrum of Atomic Chlorine

Two-quantum microwave magnetic resonance transitions corresponding to $\Delta M_J=\pm 2$ have been observed for both stable isotopes of atomic chlorine, ${\mathrm{Cl}}^{35}$ and ${\mathrm{Cl}}^{37}$. The experimentally observed and theoretically predicted positions of the two-quantum transitions agree within 13 parts per million, which was the precision of the experiment. The power dependence of the two-quantum transitions has been measured and checked with a "variation of constants" treatment of the microwave-induced probability amplitudes, and agreement has been found. Variations in intensity between corresponding $\Delta M_J=\pm 1$ and $\Delta M_J=\pm 2$ resonances were observed and analyzed on the basis of time-dependent perturbation theory and the Schwinger-Karplus expression for the line shape of a collision-broadened resonance with saturation. Satisfactory agreement with theory is found for these intensity variations. The linewidths for the $\Delta M_J=\pm 2$ lines are approximately half of that for the $\Delta M_J=\pm 1$ lines; this is in agreement with the results of time-dependent perturbation theory.