Femtosecond stimulated Raman spectroscopy in 1D and 2D – direct observation of intramolecular motions and intermolecular interactions

M. Kloz1, R. Grondelle1, J.T.M Kennis1

1. Department of Physics and Astronomy, Faculty of Sciences, VU University, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands

It is exactly half century now since the discovery of stimulated Raman scattering (SRS) (1). Despite numerous proof-of-the-principle experiments it is only about a decade since the mechanism got a general analytical use through the special phenomena called femtosecond stimulated Raman scattering (FSRS). When two strongly different light pulses are time spatially overlapped in a sample, one spectrally ultra narrow and one ultra short in time, the entire Raman spectra of the sample are imprinted on the broad spectral envelope of the ultra short pulse with a high signal gain. While the spectral resolution is determined by the narrow pulse the time gating precision is set by the ultrafast pulse so the time-energy resolution is no longer bound by the time-bandwidth uncertainty principle. This mechanism was recently successfully harnessed in mapping the fastest know bio reactions (2, 3) yet robust FSRS experiment is still under development (4). The supreme time resolution of FSRS opened the door for a fully coherent time domain 2D-Raman experiments (5) where the vibrational motion is mapped beyond the period of single oscillation. After initial optimism 2D Raman signals were discovered to be strongly overwhelmed by parasitic cascading signals (6) and at the moment it is unclear if the problem is fully solvable. Frequency domain 2D Raman experiments were proposed as well and their applicability is currently being evaluated. Never the less 1D Raman techniques are already obtaining strong recognition as an irreplaceable toll for studding of vibrations with low IR cross section.

References [1] Eckhardt G, et al. (1962) Stimulated Raman Scattering From Organic Liquids. Physical Review Letters 9(11):455-457. [2] Fang C, Frontiera RR, Tran R, & Mathies RA (2009) Mapping GFP structure evolution during proton transfer with femtosecond Raman spectroscopy. Nature 462(7270):200-204. [3] Kukura P, McCamant DW, Yoon S, Wandschneider DB, & Mathies RA (2005) Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman. Science 310(5750):1006-1009. [4] Kloz M, Grondelle Rv, & Kennis JTM (2011) Wavelength-modulated femtosecond stimulated raman spectroscopy-approach towards automatic data processing. Physical Chemistry Chemical Physics 13(40):18123-18133. [5] Kukura P, Frontiera R, & Mathies RA (2006) Direct Observation of Anharmonic Coupling in the Time Domain with Femtosecond Stimulated Raman Scattering. Physical Review Letters 96(23):238303. [6] Mehlenbacher RD, Lyons B, Wilson KC, Du Y, & McCamant DW (2009) Theoretical analysis of anharmonic coupling and cascading Raman signals observed with femtosecond stimulated Raman spectroscopy. The Journal of Chemical Physics 131(24):244512.

Figure 2 Mechanism of femtosecond stimulated Raman scattering.

Figure 1 Evolution of vibration manifold of -carotene molecule after excitation of its backbone consisting of conjugated double bonds monitored by femtosecond stimulated Raman spectroscopy.

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