Femtosecond Stimulated
Raman Spectroscopy A new approach for obtaining vibrational Raman spectra and for studying chemical reaction dynamics.
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 advanPhilipp Kukura tage. Clinical applications of FT Raman range Sangwoon Yoon from identification of cancerous tissues to microscopic imaging (4). However, it can suffer from Richard A. Mathies weak resonance enhancement and intense University of California, Berkeley 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 (
