Multiple resonance-enhanced four-wave-mixing processes inI2

Resonance-enhanced four-wave-mixing processes were studied in molecular iodine by detection of a wave generated at frequency ${\mathrm{\omega }}_3$=2${\mathrm{\omega }}_1$-${\mathrm{\omega }}_2$ for a wide range of Raman frequencies (${\mathrm{\omega }}_1$-${\mathrm{\omega }}_2$). On the basis of known spectroscopic constants of the two electronic states involved, the electronic ground state ${\mathit{X}}^1$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ and the electronic excited state ${\mathit{B}}^3$ ${\mathrm{\Pi }}_{\mathit{u}0}^{+}$, an interpretation of the spectral features in terms of three essentially different four-wave-mixing processes is given. Resonance enhancement through bound states with discrete energy levels and the enhancement effect by dissociative continuum states are considered. In addition to resonance-enhanced coherent anti-Stokes Raman-scattering (CARS) processes, in which the Raman resonances are in the electronic ground state, two resonant schemes are identified in ${\mathrm{I}}_2$, in which Raman resonances in an excited state are probed: excited-state parametric and nonparametric CARS. The latter process is an example of electronic population transfer in a four-wave-mixing cycle.