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

Abstract

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\ifmmode^\circ\else\textdegree\fi{} 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 ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}\ensuremath{\sim}10$\ifmmode^\circ\else\textdegree\fi{}/kG) and small ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}<2$\ifmmode^\circ\else\textdegree\fi{}/kG). A theory based on the interference between the different molecules is presented to explain the experimental data. The observed differences in $\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}$ 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.