Probing electron correlations in molecules by two-dimensional coherent optical spectroscopy

The-nonlinear optical signal generated in phenol by three femtosecond pulses with wavevectors k(1), k(2), and k(3) in the phase-matching direction k(1) + k(2) - k(3) is simulated. This two-dimensional coherent spectroscopy (2DCS) signal has a rich pattern containing information on double-excitation states. The signal vanishes for uncorrelated electrons due to interference among quantum pathways and, thus, provides direct signatures of correlated many-electron wavefunctions. This is illustrated by the very different 2DCS signals predicted by two levels of electronic structure calculations: state-averaged complete active space self-consistent field (SA-CASSCF) and multistate multiconfigurational second-order perturbation theory (MS-CASPT2).