Spectroscopy of doubly resonant third-harmonic generation inI2

A technique of third-harmonic generation based on simultaneous resonances in one- and two-quantum transitions was applied to find a level of symmetry ${}_{}{}^1{}_{}{}^{}O_g^{+}{}_{}{}^{}$ of molecular iodine. The energy of this level is given, in ${\mathrm{cm}}^{-1\mathrm{}}$, by $E_T\mathrm{}(n,J)=(35\mathrm{}972.4\pm 0.5)+(132.9\pm 0.2) (n+\frac{1}{2})-(0.34\mathrm{}\pm 0.02){(n+\frac{1}{2})}^2+\left[\right(0.028\mathrm{}57\pm 0.00015)-(0.00015\pm 0.00002\left)n\right]\left(J\right)\mathrm{}(J+1\mathrm{})\mathrm{}$, where $n$ is an integer differing from the vibrational quantum number $v$ by an undetermined integer and $J$ is the angular momentum. This technique has relatively high efficiency in third-harmonic generation and may provide a basis for a new Doppler-free spectroscopy.