Femtosecond Stimulated Raman Spectroscopy

Abstract

R aman spectroscopy is a powerful analytical technique for revealing vibrational structure. It is widely used in biology, chemistry, and studies of molecular reaction dynamics (1-3). One of the technique's greatest advantages is that a single spectrum contains a wealth of vibrational structural information about the sample. The large number of well-resolved vibrational bands provides unique information analogous to a human fingerprint, and the technique is very sensitive to molecular structure. Changes in bond lengths of as little as 0.01 Å can produce clear differences in the vibrational spectrum. This specificity and sensitivity make Raman an excellent tool for trace analysis and for studies of chemically induced structural changes.Although a wide variety of Raman techniques have been developed over the years and applied to analytical problems, significant room for improvement still exists. FT Raman methods are often used in industrial applications to avoid background fluorescence-via intense laser radiation in the NIR-and to exploit the FT multiplex advantage. Clinical applications of FT Raman range from identification of cancerous tissues to microscopic imaging (4). However, it can suffer from weak resonance enhancement and intense Rayleigh scattering caused by the noise distribution of the FT (5). In surface-enhanced Raman spectroscopy, spectra of molecules adsorbed to roughened metallic surfaces or nanostructures can be taken with extreme sensitivity; however, the technique relies on the not-easily-controlled interaction of the metallic substrates with the analyte (6). Raman imaging has recently been advanced through the use of nonlinear Raman techniques, in particular coherent anti-stokes Raman scattering (CARS), which has been used, for example, to monitor biological processes in real time (7,8). Despite these advances, limitations exist that prevent these methods from exploiting the full potential of Raman scattering. We asked: Can an analytical resonance Raman technique be developed that can produce high-S/N spectra that are immune to background fluorescence, with short data-acquisition times?Time-resolved vibrational spectroscopies are also extensively used in structural studies of dynamics of chemical and biological reactions. The earliest (<50-fs) events in photochemical and photophysical processes are best studied by resonance Raman intensity analysis, an extremely powerful technique that can provide detailed excited-state structural information (9, 10). Spontaneous time-resolved resonance A new approach for obtaining vibrational Raman spectra and for studying chemical reaction dynamics. 9 4 A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 6Raman spectroscopy is limited to the picosecond time domain because of the transform limit; this prevents the technique from being applied to chemical reactions that occur on the order of 10 f s-1 ps. This limitation has recently been overcome by the availability of femtosecond pulses in the IR. As a consequence, pump-probe approaches for measu...

Ultrafast excited-state isomerization in phytochrome revealed by femtosecond stimulated Raman spectroscopy

“…Spectra from 200 to 400 fs consist of dispersive Raman features at 1,668, 1,330, 1,245, and 816 cm Ϫ1 , each of which correlates with a prominent P r ground-state feature. The characteristic dispersive line shape is caused by a nonlinear effect known as Raman induced by nonlinear emission (RINE) (19), a coherent hot luminescence signal that occurs when a stimulated emission band is resonant with both the Raman pump and probe wavelengths. Although direct information about the chromophore structure is masked, the decay time of RINE features indicate how fast the stimulated emission region depopulates.…”

“…FSRS has the advantage of providing vibrational structural information with high temporal (Ϸ50 fs) and spectral (Ϸ10 cm Ϫ1 ) resolution. This enhanced capability has been particularly revealing for structurally timing a wide variety of photochemical reactions in biomolecules (19). Here, we use FSRS to investigate the primary process of Cph1 P r -to-P fr photoconversion in the 100-fs to 100-ps time scale.…”

Signatures and control of strong-field dynamics in a complex system

“…energy shifts and phase shifts in our model can, in reality, be caused by a number of coupling mechanisms that exist in molecules, e.g., Stark effect, strong coupling of near-resonant electronic states, or Raman coupling of vibrational states (38)(39)(40)(41)(42)(43)(44)(45). Typically, the higher the pulse energy, the stronger are the energy shifts and, hence, the larger are the phase shifts of the dipole responses (see Eq.…”

Two-dimensional stimulated resonance Raman spectroscopy of molecules with broadband x-ray pulses

“…This pulse could prepare the molecule in some nonstationary state which may then be probed by an SXRS experiment, in much the same way as is currently done in vibrational spectroscopy. 80 We do not pursue this course here, rather we look at another advantage inherent in the time-domain experiment. We can use a two color scheme, where the two pulses are tuned to be resonant with different core transitions, providing an additional experimental knob to turn.…”