Daniel M. Besemann

Nonlinear Two-Dimensional Vibrational Spectroscopy

There is currently great interest in developing the vibrational analog to two-dimensional NMR spectroscopy. One approach to implementing two-dimensional vibrational spectroscopy is to use doubly vibrationally enhanced (DOVE) four-wave mixing (FWM). Nonlinear signals occur because of correlations and mode coupling that are induced by interactions involving the driven modes. Since cross peaks do not occur between modes if interactions are absent, spectral congestion is removed and only the coupled modes remain. We describe the development of a two-dimensional doubly vibrationally enhanced four wave mixing method that extends the doubly resonant nonlinear spectroscopies to vibrational nonlinearities. We demonstrate the selective enhancements of coupled modes that are possible with the double resonances where the intensity of the enhancements reflects the strength of the interactions that are responsible for the vibrational mode coupling. We also demonstrate the capabilities for selectively enhancing specific sample components in an isotopic mixture. Since biological applications of DOVE require aqueous environments, we have examined the ability of DOVE methods to discriminate against the strong water absorption and have found that water has a small vibrational nonlinearity that allows DOVE of the solutes. Our results demonstrate the feasibility and features required to make DOVE methods practical for a wide range of scientific applications where identification of intra- and intermolecular interactions is important.

Interference, dephasing, and vibrational coupling effects between coherence pathways in doubly vibrationally enhanced nonlinear spectroscopies

Abstract Doubly vibrationally enhanced (DOVE) four-wave mixing is capable of producing coherent two-dimensional (2D) vibrational spectra with cross-peaks that reflect intramolecular and intermolecular mode coupling. These methods are based on double vibrational coherences that are analogous to the double spin coherences in 2D nuclear magnetic resonance methods. In particular, there are two pathways for DOVE infrared spectroscopy that interfere to change the line shapes of doubly resonant cross-peaks. The interference depends on the relative dephasing rates of the coherences involved in the different pathways. We derive relationships between the dephasing rates of the different coherences in DOVE pathways and show how these relationships affect the line shapes of DOVE resonances. Comparisons are made with experimental 2D DOVE spectra.

Isotope and mode selectivity in two-dimensional vibrational four wave mixing spectroscopy

Abstract Two-dimensional doubly vibrationally enhanced four wave mixing (2D-DOVE-FWM) spectra have been recorded for a mixture of CH3CN, CD3CN, and C6D6. Cross-peaks in these spectra indicate coupling between molecular vibrations that involve common vibrational modes. Modes are coupled by through-bond interactions and therefore the strongest cross-peaks are observed for vibrations on the same molecule. This coupling produces the vibrational anharmonicities that are required for multi-resonant four wave mixing. The absence of cross-peaks for different molecules demonstrates the isotope selectivity possible with doubly vibrationally enhanced (DOVE) spectroscopy. Model spectra were generated that account for the resonant spectral features observed.

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 materials structure. Vibrational dephasing times occur on the picosecond time scale so the DOVE–FWM measurement represents a more instantaneous measurement of material structure.