Comparisons between 2D doubly vibrationally enhanced four wave mixing and site selective spectroscopy

We have constructed a nonlinear spectroscopic system for performing multire sonant four-wave mixing with infrared lasers. The system consists of three coherent sources, two of which an tunable in the infrared region of the spe ctrum. The sources are tuned to different vibrational resonances and the fo ur-wave mixing output is monitored as a function of the two infrared freque ncies. When the frequencies match direct infrared absorption or Raman trans itions, the four-wave mixing output is enhanced. A two-dimensional display of the data shows the output intensity as a function of the two infrared fr equencies. We observe that cross-peaks appear in the 2D spectra a hen multi ple resonances are excited. We have named the method "doubly vibrationally enhanced four-wave mixing (DOVE-FWM)". This method represents the long soug ht optical analogue to 2D nmr. It should provide a method that is complemen tary to nmr because of the difference in the time scales of the dephasing p rocesses. Spin-lattice interactions fix the dephasing times for NMR measure ments at millisecond time scales so nmr senses the ensemble average of a ma terial's structure. Vibrational dephasing times occur on the picosecond tim e scale so the DOVE-FWM measurement represents a more instantaneous measure ment of material structure. (C) 2000 Elsevier Science B.V. All rights reser ved.