SPECIAL SECTION: RAMAN SPECTROSCOPY CURRENT SCIENCE, VOL. 97, NO. 2, 25 JULY 2009 210 *For correspondence. (e-mail: umapathy@ipc.iisc.ernet.in) Ultrafast Raman loss spectroscopy: a new approach to vibrational structure determination A. Lakshmanna, B. Mallick and S. Umapathy* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India The rapid data acquisition, natural fluorescence rejec- tion and experimental ease are the advantages of the ultra-fast Raman loss scattering (URLS) which makes it a unique and valuable molecular structure-determin- ing technique. URLS is an analogue of stimulated Raman scattering (SRS) but far more sensitive than SRS. It involves the interaction of two laser sources, viz. a picosecond (ps) pulse and white light, with the sample leading to the generation of loss signal on the higher energy (blue) side with respect to the wave- length of the ps pulse, unlike the gain signal observed on the red side in SRS. These loss signals are at least 1.5 times more intense than the SRS signals. Also, the very prerequisite of the experimental protocol for sig- nal detection to be on the higher energy side by design eliminates the interference from fluorescence, which always appears on the red side. Unlike coherent anti- Stokes Raman scattering, URLS signals are not pre- cluded by non-resonant background under resonance condition and also being a self-phase matched process, it is experimentally easier. Keywords: Anti-Stokes Raman scattering, Raman scat- tering, ultrafast Raman loss spectroscopy, vibrational struc- ture. Introduction LIGHT on interaction with matter undergoes scattering. Most of the incident photons are elastically scattered (Rayleigh scattering), while one out of ten million is inelas- tically scattered. Inelastic scattering of photon is termed as Raman scattering. Raman scattering occurs due change in the polaraizability of a molecule, thereby leading to a change in its vibrational state. This results in the emission of a photon having energy lower (Stokes Raman) or higher (anti-Stokes Raman) than that of the incident photon depending upon the initial vibrational state of the mole- cule (Figure 1). The shift in Raman frequency provides chemical and structural information. However, Raman scattering is relatively weak, leading to low detection sensitivity. As a consequence, it is difficult to measure the vibration spectra of molecules of low concentration, and weak Raman scatterers. This can be overcome using resonance Raman technique, wherein the wavelength of the exciting photon lies within the electronic absorption of the molecular system. Under this condition, the Raman signals can be enhanced by a factor of 10 4 –10 6 . But, in the case of fluorescent molecules or in the presence of fluorescent impurities, the strong fluorescence signal masks the resonance Raman signals. Thus, the sensitivity of Raman spectroscopy is limited to the study of non- fluorescent and strong Raman scattering molecules. Advanced Raman spectroscopic techniques, such as coherent anti-Stokes Raman scattering (CARS) 1 , picosec- ond (ps) Kerr-gate 2 , stimulated Raman scattering (SRS) 3–11 , etc. have been reported in recent times to overcome the problem. All these processes are characterized by the third-order nonlinear susceptibility (χ [3] ) of the system. Both CARS and SRS involve a four-wave mixing process providing the signal. While in the case of ps Kerr-gate spectroscopy, a nonlinear phenomenon, the Kerr effect, is used as a gate for the detection of the instantaneous Raman scattering signals before being overwhelmed by the fluorescence signal. The Kerr effect occurs due to a nonlinear change in the refractive index of a material in the presence of a short laser pulse (gating pulse). These methods provide a Raman spectrum with a good signal- to-noise ratio and relatively efficient fluorescence rejec- tion compared to conventional Raman spectroscopy. Yet, these methods suffer from a number of difficulties. For example, in CARS 1,12 , (a) signal distortion occurs due to Figure 1. Raman scattering. a = Rayleigh scattering, b = Stokes Raman scattering, c = Anti-Stokes Raman scattering.