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.

The laser, detection system, and methods that enable femtosecond broadband stimulated Raman spectroscopy (FSRS) are presented in detail. FSRS is a unique tool for obtaining high time resolution (<100 fs) vibrational spectra with an instrument response limited frequency resolution of <10 cm -1 . A titanium:Sapphire-based laser system produces the three different pulses needed for FSRS: (1) A femtosecond visible actinic pump that initiates the photochemistry, (2) a narrow bandwidth picosecond Raman pump that provides the energy reservoir for amplification of the probe, and (3) a femtosecond continuum probe that is amplified at Raman resonances shifted from the Raman pump. The dependence of the stimulated Raman signal on experimental parameters is explored, demonstrating the expected exponential increase in Raman intensity with concentration, pathlength, and Raman pump power. Raman spectra collected under different electronic resonance conditions using highly fluorescent samples highlight the fluorescence rejection capabilities of FSRS. Data are also presented illustrating our ability: (i) To obtain spectra when there is a large transient absorption change by using a shifted excitation difference technique and (ii) to obtain high time resolution vibrational spectra of transient electronic states.

The assembly of noble metal nanoparticles is an appealing means to control the plasmonic properties of nanostructures. Dimers are particularly interesting because they are a model system that can provide fundamental insights into the interactions between nanoparticles in close proximity. Here, we report a highly efficient and facile assembly method for dimers and other forms of assemblies. Gold nanoparticles (AuNPs) adsorbed on aminosilanized glass surfaces protect the silanes underneath the nanoparticles from hydrolysis. This masked desilanization allows us to prepare AuNP homodimers on glass slides with remarkably high yield (∼90%). The interparticle distance and, accordingly, the surface plasmon coupling are readily tuned at the molecular level using self-assembled monolayers of alkanedithiols. As the interparticle distance is reduced, the resonance surface plasmon coupling progressively redshifts, following the classical electromagnetic model. When the interparticle distance enters the subnanometer regime, however, the resonance band begins to blueshift and significantly broadens. The comparison of our observations with theoretical studies reveals that quantum tunneling effects play a significant role in the plasmonic response of AuNP dimers in the subnanometer gap region. The assembly method based on the masked desilanization is extendable to the formation of various other forms of nanoassemblies and, thus, will further our understanding of plasmonic interactions in nanoassemblies.

The assembly of noble metal nanoparticles offers an appealing means to control and enhance the plasmonic properties of nanostructures. However, making nanoassemblies with easily modifiable gap distances with high efficiency has been challenging. Here, we report a novel strategy to assemble gold nanoparticles (AuNPs) into Janus-type asymmetric core-satellite nanostructures. Markedly different desorption efficiency between large and small AuNPs in ethanol allows us to prepare the asymmetric core-satellite nanoassemblies in a dispersed colloidal state with near 100% purity. The resulting nanoassemblies have well-defined structures in which a core AuNP (51 nm) is covered by an average of 13 ± 3 satellite AuNPs (13 nm) with part of the core surfaces left unoccupied. Strong surface plasmon coupling is observed from these nanoassemblies as a result of the close proximity between the core and the satellites, which appears significantly red-shifted from the surface plasmon resonance frequencies of the constituting nanoparticles. The dependence of the surface plasmon coupling on a gap distance of less than 3 nm is systematically investigated by varying the length of the alkanedithiol linkers. The asymmetric core-satellite nanoassemblies also serve as an excellent surface-enhanced Raman scattering substrate with an enhancement factor of ~10(6). Finally, we demonstrate that the presented assembly method is extendible to the preparation of compositionally heterogeneous core-satellite nanoassemblies.

Hot-electron chemistry at gold nanoparticle (AuNP) surfaces has received much attention recently because its understanding provides a basis for plasmonic photocatalysis and photovoltaics. Nonradiative decay of excited surface plasmons produces energetic hot charge carriers that transfer to adsorbate molecules and induce chemical reactions. Such plasmon-driven reactions, however, have been limited to a few systems, notably the dimerization of 4-aminobenzenethiol to 4,4′-dimercaptoazobenzene. In this work, we explore a new class of plasmon-driven reactions associated with a unimolecular bond cleavage process. We unveil the mechanism of the decarboxylation reaction of 4-mercaptobenzoic acid and extend the mechanism to account for the β-cleavage reaction of 4-mercaptobenzyl alcohol. Combining the construction of well-controlled nanogap systems and sensitive Raman spectroscopy with methodical changes of experimental conditions (laser wavelengths, interface materials, pH, ambient gases, etc.), we track the hot charge carriers from the formation to the transfer to reactants, which provides insights into how plasmon excitation eventually leads to the C–C bond cleavage of the molecules in the nanogap.

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Experimental and theoretical studies explore the reactivity of the symmetric and the antisymmetric stretching vibrations of monodeuterated methane (CH3D). Direct infrared absorption near 3000 cm−1 prepares CH3D molecules in three different vibrationally excited eigenstates that contain different amounts of symmetric C–H stretch (ν1), antisymmetric C–H stretch (ν4), and bending overtone (2ν5) excitation. The reaction of vibrationally excited CH3D with photolytic chlorine atoms (Cl, 2P3/2) yields CH2D products mostly in their vibrational ground state. Comparison of the vibrational action spectra with the simulated absorption spectra and further analysis using the calculated composition of the eigenstates show that the symmetric C–H stretching vibration (ν1) promotes the reaction seven times more efficiently than the antisymmetric C–H stretching vibration (ν4). Ab initio calculations of the vibrational energies and eigenvectors along the reaction coordinate demonstrate that this difference arises from changes in the initially excited stretching vibrations as the reactive Cl atom approaches. The ν1 vibration of CH3D becomes localized vibrational excitation of the C–H bond pointing toward the Cl atom, promoting the abstraction reaction, but the energy initially in the ν4 vibration flows into the C–H bonds pointing away from the approaching Cl atom and remains unperturbed during the reaction. A simple model using vibrational symmetries and vibrational adiabaticity predicts a general propensity for the greater efficiency of the symmetric stretch for accelerating the reaction in the vibrationally adiabatic limit.

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...