Time-resolved nonlinear spectroscopy of a fermi doublet: The {${\nu }_1$,2${\nu }_2$} Fermi resonance in ${\mathrm{CO}}_2$ solid

It is shown that the intramolecular Fermi doublet {${\nu }_1$,2${\nu }_2$} in crystalline ${\mathrm{CO}}_2$ arising from resonant anharmonic coupling between a single-phonon state and a quasibound two-phonon state can be coherently driven and its dynamics determined by time-resolved nonlinear optical techniques. Coherence relaxation times for both components of the doublet are measured between 9 K and the fusion point (217 K). A mechanism for loss of coherence is proposed which involves modulation by low-lying intermolecular modes.