Surface-enhanced resonance Raman spectroscopy (SERRS) [1] is a powerful technique for obtaining vibrational spectra of fluorescent molecules on metal surfaces. Raman scattering is strongly enhanced by two mechanisms:[2] 1) molecular resonance occurs when the probe laser lies within the molecular electronic absorption, 2) electromagnetic or chemical enhancement occurs due to interaction with the metal surface. The combination of these effects results in a 10 14 -10 15 -fold enhancement in scattering, enabling single-molecule SERRS spectroscopy.[3] However, due to the lack of resonance Raman (RR) spectra of fluorophores used in SERRS, the enhancement contributions from resonance and surface effects are difficult to separate and quantify. Although Raman techniques such as picosecond RR spectroscopy using Kerr gating [4] and coherent anti-Stokes Raman scattering (CARS) [5] are capable of rejecting fluorescence, these techniques are not ideal. Kerr-gated RR yields poor collection efficiency, while CARS has more complex lineshapes resulting in spectra that are more difficult to analyze. In contrast, the recently developed femtosecond stimulated Raman spectroscopy (FSRS) enables us to acquire and quantify RR cross-sections even in the presence of strong fluorescence.[6] Here we exploit this fluorescence rejection capability of FSRS to obtain a RR spectrum of the highly fluorescent dye rhodamine 6G (R6G) and quantify its resonance Raman scattering cross-sections. This result allows an estimate of the magnitude of surface and resonance enhancements in SERRS.Recent progress has advanced SERRS technology to the single-molecule detection limit.[3] SERRS experiments performed at wavelengths close to the absorption maximum of R6G resulted in a Raman cross-section of 10 À14 cm 2 molecule À1 .[3] Comparable or higher enhancements were reported for molecules adsorbed on colloidal silver or gold clusters in SERS experiments performed at near-infrared excitation: however, the anomalous enhancements may be due to the presence of colloidal clusters where the concentration of adsorbed molecules is higher.[7] Therefore, SERRS experiments with excitation wavelengths close to the absorption maximum are the most direct way of achieving optimal sensitivity for single molecule detection. [3,8] However, it is difficult to determine how much of the enhancement is due to field effects, because conventional resonance Raman intensities are difficult to measure directly on resonance.FSRS is a powerful new structural probe of chemical and biological systems in both steady-state and time-resolved studies.[9] Following a femtosecond actinic pulse, the simultaneous interaction of a 800 nm narrow-bandwidth picosecond Raman pump and a broadband femtosecond continuum Raman probe leads to the production of sharp vibrational gain features on top of the dispersed probe envelope. These gain features constitute the broadband stimulated Raman spectrum (SRS). FSRS has been successfully used to study a variety of chemical reaction dynamics-such as the first s...

Femtosecond stimulated Raman spectroscopy is extended to probe ground state anti-Stokes vibrational features. Off resonance, negative anti-Stokes features are seen that are the mirror image of the positive Stokes side spectra. On resonance, the observed dispersive lineshapes are dramatically dependent on the frequencies of the picosecond pump and femtosecond probe pulses used to generate the stimulated Raman spectra. These observations are explained by the contributions of the inverse Raman and hot luminescence four-wave mixing processes discussed by Sun et al. [J. Chem. Phys. 128, 144114 (2008)], which contribute to the overall femtosecond stimulated Raman signal.

Light-activated proton translocation in halobacteria is driven by photoisomerization of the retinal chromophore within the membrane-bound protein bacteriorhodopsin. The molecular mechanism of this process has been widely debated due to the absence of structural information on the time scale of the reactive dynamics (the initial 0.1-1 ps). Here we use tunable femtosecond stimulated Raman spectroscopy to obtain time-resolved resonance Raman vibrational spectra of bacteriorhodopsin's key J and K photoisomerization intermediates. The appearance of the J state is delayed by approximately 150 fs relative to the zero of time and rises after this dwell with a 450 fs time constant. The J state is characterized by a 16 cm(-1) red-shifted C=C stretch, which blue shifts by 5 cm(-1) coincident with the rise of the K state. The delayed 3 ps rise of the C(15)-H HOOP mode with enhanced intensity in K reveals the appearance of strain near the Schiff's base once the 13-cis configuration is fully formed. The delay in the initial appearance of J is assigned to nuclear dynamics on the excited state that precede the formation of the proper geometry for reactive internal conversion.

Surface-enhanced Raman scattering (SERS) of graphene on a SiO(2)(300 nm)/Si substrate was investigated by depositing Au nanoparticles using thermal evaporation. This provided a maximum enhancement of 120 times for single-layer graphene at 633 nm excitation. SERS spectra and scan images of single-layer and few-layer graphene were acquired. Single-layer graphene provides much larger SERS enhancement compared to few-layer graphene, while in single-layer graphene the enhancement of the G band was larger than that of the 2D band. Furthermore, the D bands were identified in the SERS spectra; these bands were not observed in a normal Raman spectrum without Au deposition. Appearance of the D band is ascribed to the considerable SERS enhancement and not to an Au deposition-induced defect. Lastly, SERS enhancement of graphene on a transparent glass substrate was compared with that on the SiO(2)(300 nm)/Si substrate to exclude enhancement by multiple reflections between the Si substrate and deposited Au nanoparticles. The contribution of multiple reflections to total enhancement on the SiO(2)(300 nm)/Si substrate was 1.6 times out of average SERS enhancement factor, 71 times.

An alternating stack (SG/GN) consisting of SnO₂-functionalized graphene oxide (SG) and amine-functionalized GO (GN) is prepared in solution. The thermally reduced SG/GN (r(SG/GN)) with decreased micro-mesopores and completely eliminated macropores, results in a high reversible capacity and excellent capacity retention (872 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹) when compared to a composite without GN.

We have developed a tunable femtosecond stimulated Raman spectroscopy (FSRS) apparatus and used it to perform time-resolved resonance Raman experiments with <100 fs temporal and <35 cm(-1) spectral resolution. The key technical change that facilitates this advance is the use of a tunable narrow-bandwidth optical parametric amplifier (NB-OPA) presented recently by Shim et al. (Shim, S.; Mathies, R. A. Appl. Phys. Lett. 2006, 89, 121124). The practicality of tunable FSRS is demonstrated by examining the photophysical dynamics of beta-carotene. Using 560 nm Raman excitation, the resonant S1 state modes are enhanced by a factor of approximately 200 compared with 800 nm FSRS experiments. The improved signal-to-noise ratios facilitate the measurement of definitive time constants for beta-carotene dynamics including the 180 fs appearance of the S1 vibrational features due to direct internal conversion from S2 and their characteristic 9 ps decay to S0. By tuning the FSRS system to 590 nm Raman excitation, we are able to selectively enhance vibrational features of the hot ground state S hot 0 and monitor its approximately 5 ps cooling dynamics. This tunable FSRS system is valuable because it facilitates the direct observation of structural changes of selected resonantly enhanced states and intermediates during photochemical and photobiological reactions.

Photochromic ring opening reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene in solution has been studied by femtosecond time-resolved fluorescence and transient absorption measurements. Time profiles of the transient absorption at several different probe wavelengths are identical, showing two time constants, 4 and 22 ps. The spontaneous fluorescence reveals time profiles identical to that in the transient absorption. A simple one step ring opening reaction mechanism is proposed, where the closed form in the excited state reaches the open form in the ground state through nonadiabatic curve crossing.The ring opening reaction rate is determined to be in the range (1.7-4) × 10 10 s -1 . A single 66 cm -1 wave packet motion in the excited state is observed, whose role on the ring opening reaction is speculated upon.

Photochromic ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluoro cyclopentene and its derivatives in solution has been studied by femtosecond time-resolved fluorescence. Time-resolved spontaneous fluorescence of the open isomer reveals a fast component of around 1 ps and a slow component on the order of 100 ps. Fluorescence time profiles, reaction quantum yields, and relative populations of the parallel (C(s) symmetry) and antiparallel (C(2) symmetry) conformations indicate that both time components are attributable mostly to the C(2) conformer that undergoes the ring closure reaction. The fast component is assigned to the direct ring closure reaction, and the slow component is assigned to the reaction through conformation change. Time constants of the slow component for the derivatives are inversely proportional to the reaction quantum yields, suggesting that the rate of the conformational dynamics is comparable to the rate of other population relaxation processes. The relative amplitude and exact time constant of the fast component depend on the detection wavelength displaying a higher relative amplitude with shorter time constant at longer wavelengths. The results allow us to propose a conformational inhomogeneity model, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.