the substrate surface (27) with DOS of polymers could occur, or there could be charge transfer (28) based on the interaction between the polymers and the I-Au(111). These electronic interactions between the polymers and the surface might reduce the HOMO-LUMO gaps compared with those in solution. Chem. Soc. 124, 11862 (2002). 11. H. Sakaguchi, H. Matsumura, H. Gong, Nat. Mater. 3, 551 (2004 Science 271, 1705Science 271, (1996. Adv. Mater. 4, 282 (1992 The primary event that initiates vision is the light-induced 11-cis to all-trans isomerization of retinal in the visual pigment rhodopsin. Despite decades of study with the traditional tools of chemical reaction dynamics, both the timing and nature of the atomic motions that lead to photoproduct production remain unknown. We used femtosecond-stimulated Raman spectroscopy to obtain time-resolved vibrational spectra of the molecular structures formed along the reaction coordinate. The spectral evolution of the vibrational features from 200 femtoseconds to 1 picosecond after photon absorption reveals the temporal sequencing of the geometric changes in the retinal backbone that activate this receptor. Understanding the mechanism of a chemical reaction requires measuring the structure of the reactant as it evolves into product. Many of the most intriguing and efficient photochemical and photobiological reactions take place on ultrafast time scales and their kinetics have been well characterized by femtosecond absorption and fluorescence spectroscopies (1-5). Although x-ray diffraction is being developed for timeresolved structural studies of reactions, this approach is challenging to apply in the condensed phase and currently limited to processes slower than È100 ps (6). Ultrafast vibrational spectroscopy is advantageous in this quest because it offers both excellent temporal and structural information (7). The traditional picosecond timeresolution limitation (8) is being transcended through the use of femtosecond pulses in the infrared (IR) in multidimensional as well as direct time-resolved experiments of ultrafast chemical and biological processes (9-11). The complementary Raman vibrational techniques have also advanced with the recent development of stimulated Raman in the femtosecond time domain (12, 13), which is valuable because of its ability to interrogate biological processes in aqueous media. Here, we demonstrate the capabilities of femtosecond-stimulated Raman spectroscopy (FSRS) in studies of reaction dynamics by elucidating the molecular mechanism of the primary photochemical events in vision. In FSRS, two laser pulses drive the Raman transition: a picosecond BRaman pulse[ and a femtosecond broadband continuum Bprobe pulse[ that stimulates the scattering of any vibrational modes with frequencies between 600 and 2000 cm j1 . The use of the additional probe pulse to induce the Raman scattering offers a number of notable improvements over traditional timeresolved spontaneous Raman spectroscopy (14), such as greatly enhanced cross sections and an order-of-magnitude improvement in time resolution (G100 fs) while maintaining excellent energy resolution (G15 cm j1 ) (15, 16). The impulsive creation of vibrational coherence by the Raman and probe pulses reveals highly time-resolved vibrational structural information that is not accessible by incoherent processes such as spontaneous Raman. The primary step in vision is the photochemical cis-trans isomerization of the 11-cis retinal chromophore in rhodopsin We address these questions by acquiring femtosecond time-resolved vibrational spectra of retinal in rhodopsin throughout the reaction. Modeling of the vibrational structural features after rapid internal conversion to the ground state reveals the highly distorted structure of photorhodopsin. Surprisingly, a large fraction of the atomic rearrangement leading to the formation of fully isomerized bathorhodopsin is shown to occur in the ground electronic state. Vivid details of thi
