Level crossing and polarization rotation in doubly resonant third-harmonic generation inI2

The gyromagnetic ratio of several states in ${\mathrm{I}}_2$ is studied with the technique of polarization rotation and level crossing in doubly resonant third-harmonic generation. The level-crossing curve has a linewidth of approximately the Doppler linewidth of the corresponding non-coherent process divided by the angular momentum quantum number. In the polarization-rotation work, a rotated angle 80° is observed with little depolarization. The ratio of the angle rotated to the magnetic field of some 20 resonances can be divided into two groups: large ($\frac{\Delta \theta }{\Delta H}\sim 10$°/kG) and small ($\frac{\Delta \theta }{\Delta H}<2\mathrm{}$°/kG). A theory based on the interference between the different molecules is presented to explain the experimental data. The observed differences in $\frac{\Delta \theta }{\Delta H}$ is interpreted in terms of the difference in the transition branch in the one- and two-quantum transition. The gyromagnetic ratio of the ground state as determined by this technique is approximately -6 and that of the two-quantum excited state is approximately -20.