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...

Resonance Raman intensity analysis was used to investigate the initial excited-state nuclear dynamics of cis- and trans-azobenzene following S1 (npi*) excitation, and fluorescence quantum yield measurements were used to estimate the excited-state lifetimes. trans-Azobenzene exhibits the strongest Raman intensities in its skeletal stretching and bending modes, while torsional motions dominate the nuclear relaxation of cis-azobenzene as indicated by intense Raman lines at 275, 542, 594, and 778 cm(-1). The very weak fluorescence quantum yield for cis-azobenzene is consistent with its approximately 100 fs electronic lifetime while trans-azobenzene, with a fluorescence quantum yield of 1.1 x 10(-5), has an estimated S1 lifetime of approximately 3 ps. The absorption and Raman cross-sections of both isomers were modeled to produce a harmonic displaced excited-state potential energy surface model revealing the initial nuclear motions on the reactive surface, as well as values for the homogeneous and inhomogeneous linewidths. For cis-azobenzene, this modeling predicts slopes on the S1 potential energy surface that when extrapolated to the position of the harmonic minimum give excited-state changes of approximately 6-20 degrees in the CNNC torsion angle and a < or =3 degrees change in the CNN bending angle. The relatively large excited-state displacements along these torsional degrees of freedom provide the driving force for ultrafast isomerization. In contrast, the excited-state geometry changes of trans-azobenzene are primarily focused on the CNN bend and CN and NN stretches. These results support the idea that cis-azobenzene isomerizes rapidly via rotation about the NN bond, while isomerization proceeds via inversion for trans-azobenzene.

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Resonance Raman spectra of electrons solvated in liquid methanol, ethanol, and n-propanol are presented. At least five distinct solvent modes exhibit resonantly enhanced scattering, including the OH torsion, CO/CC stretches, the OH in-plane bend, methyl deformations, and the OH stretch. The 200-350 cm-1 frequency downshift of the OH stretch indicates a strong H-bond interaction between the electron and the hydroxyl group. The multiple modes including alkyl vibrations that are coupled to the electronic transition of the solvated electron reveal the extension of the electron's wavefunction into the alkyl solvent environment.

The phosphorescence and zero field optically detected magnetic resonance (ODMR) of the tryptophan (Trp) residues of alkaline phosphatase from Escherechia coli are examined. Each Trp is resolved optically and identified with the aid of the W220Y mutant and the terbium complex of the apoenzyme. Trp(109), known from earlier work to be the source of room-temperature phosphorescence (RTP), emits a highly resolved low-temperature phosphorescence (LTP) spectrum and has the narrowest ODMR bands observed thus far from any protein site, revealing a uniquely homogeneous local environment. The decay kinetics of Trp(109) at 1.2 K reveals that the major triplet population (70%) undergoes inefficient crystallike spin-lattice relaxation by direct interaction with lattice phonons, the remainder being relaxed efficiently by local disorder modes. The latter population is smaller than is typical for protein sites, suggesting an unusual degree of local rigidity and order consistent with the long-lived RTP. Trp(220) emits a broader LTP spectrum originating to the blue of Trp(109). It has typically broad ODMR bands consistent with local heterogeneity. The LTP of Trp(268) has an ill-defined origin blue shifted relative to Trp(220) and ODMR frequencies consistent with a greater degree of solvent exposure. Trp(268) has noticeable dispersion of its decay kinetics, consistent with quenching at the triplet level by a nearby disulfide residue.