Multiply enhanced odd-order wave-mixing spectroscopy.

Extending current coherent multidimensional spectroscopy (CMDS) methods to higher order multiwave mixing requires excitation intensities where dynamic Stark effects become important. This paper examines the dynamic Stark effects that occur in mixed frequency/time domain CMDS methods at high excitation intensities in a model system with an isolated vibrational state. The phase-matching restrictions in CMDS define the excitation beams that interact by nonlinear mixing while the dynamic Stark effects create vibrational ladders of increasingly more energetic overtone and combination band states. The excited quantum states form coherences that reemit the output beams. This paper uses the phase-matching conditions k(out) = k(1) - k(2) + k(2') and k(out) =- k(1) + k(2) + k(2'), where the subscripts denote the excitation frequencies of each excitation pulse and the output pulse. The phase-matching condition constrains each pulse to have an odd number of interactions so the overall mixing process that creates the output coherence must also involve an odd number of interactions. Tuning the excitation frequencies and spectrally resolving the output intensity creates three-dimensional spectra that resolve the individual overtone states. Changing the excitation pulse time delays measures the dynamics of the coherences and populations created by the multiple excitations. The multidimensional spectra probe the highly excited states of a molecular potential energy surface. This paper uses tungsten hexacarbonyl (W(CO)(6)) as a model for observing how dynamic Stark effects change the multidimensional spectra of a simple system. The simplicity of the W(CO)(6) system provides the experimental data required to develop the nonperturbative theoretical methods that will be necessary to model this new approach to CMDS.