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

Abstract

We have constructed a nonlinear spectroscopic system for performing multiresonant four-wave mixing with infrared lasers. The system consists of three coherent sources, two of which are tunable in the infrared region of the spectrum. The sources are tuned to different vibrational resonances and the four-wave mixing output is monitored as a function of the two infrared frequencies. When the frequencies match direct infrared absorption or Raman transitions, 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 frequencies. We observe that cross-peaks appear in the 2D spectra when multiple resonances are excited. We have named the method “doubly vibrationally enhanced four-wave mixing (DOVE-FWM)”. This method represents the long sought optical analogue to 2D nmr. It should provide a method that is complementary to nmr because of the difference in the time scales of the dephasing processes. Spin-lattice interactions fix the dephasing times for NMR measurements at millisecond time scales so nmr senses the ensemble average of a material's structure. Vibrational dephasing times occur on the picosecond time scale so the DOVE–FWM measurement represents a more instantaneous measurement of material structure.