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Weak IR Modes as Structural Reporters via Dual-Frequency 2D IR Spectroscopy

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Abstract

We report a new approach in 2D-IR that implements interactions of weak IR modes with strong IR modes. It is illustrated on three-pulse photon-echo measurements on 2-cyanocoumarin C≡N, C=O, and C=C modes using a 4, 4, 6 µm pulse sequence. Prominent cross-peaks for the interaction of weak C≡N and strong C=O stretching modes were detected, indicating a positive correlation between C≡N and C=O frequency distributions. Additional cross-peaks were observed at (1608, 2240 cm−1), derived from the interaction of the C≡N and C=C modes, where C=C is a coumarin ring stretching mode. Investigating 2D IR spectra at larger delay times, long-lived vibrational states were detected in 2-cyanocoumarin: while the lifetime of the CN mode is only 1.4 ps, the strong cross-peaks could be seen even at 20 ps delay. Based on these observations, a new approach of measuring interactions of weak IR modes in proposed, which makes use of the relaxation process. A detailed analysis is presented which compares the direct coupling and relaxation induced contributions. These advances will allow enhancing substantially detectable distances via 2D IR, especially when weak IR modes are used.

© 2006 Optical Society of America

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