Evidence of a green luminescence band related to surface flaws in high purity silica glass.

Using luminescence confocal microscopy under 325 nm laser excitation, we explore the populations of defects existing in or at the vicinity of macroscopic surface flaws in fused silica. We report our luminescence results on two types of surface flaws: laser damage and indentation on fused silica polished surfaces. Luminescence cartographies are made to show the spatial distribution of each kind of defect. Three bands, centered at 1.89 eV, 2.75 eV and 2.25 eV are evidenced on laser damage and indentations. The band centered at 2.25 eV was not previously reported in photo luminescence experiments on indentations and pristine silica, for excitation wavelengths of 325 nm or larger. The luminescent objects, expected to be trapped in sub-surface micro-cracks, are possibly involved in the first step of the laser damage mechanism when fused silica is enlightened at 351 nm laser in nanosecond regime.

Raman spectroscopy and ab initio investigations of transient complex formation in CO2-benzene mixtures.

The polarized and depolarized Raman spectra of CO(2) have been measured as a function of CO(2) concentration (0.02-0.7 molar fractions) in the dense phase of the binary mixtures obtained by introducing under pressure (from 0.2 up to 6.0 MPa) supercritical carbon dioxide (at 313 K) in liquid benzene. Four main experimental features are observed. A new weak polarized band centered at approximately 660 cm(-1) has been detected in the region of the Raman inactive nu(2) bending mode of carbon dioxide. The analysis of the polarized band shapes of the Fermi dyad shows that CO(2) molecules probe two environments. In one of them carbon dioxide interacts "specifically" with benzene molecules, whereas in the other it interacts "nonspecifically" with its neighbors. The analysis of the depolarized Fermi dyad profiles shows that the rotational dynamics of CO(2) specifically interacting with benzene is strongly hindered. Finally, a new weak polarized band has been detected between the two components of the dyad. These observations rationalized at the light of ab initio calculations show that CO(2)-benzene transient complexes are formed. It is argued that ab initio predictions, limited here to a pair of molecules, are still valid in dense phase because the elementary act of formation of the transient complex can be probed on the observation time and spatial range of vibrational Raman spectroscopy.

Enhancement of the Raman scattering signal due to a nanolens effect.

The Raman scattering signal of a substrate is investigated using a polystyrene nanolens of a few hundred nanometers inserted within the light path of a confocal microspectrometer. As observed in solid immersion microscopy, the nanolens is at the origin of the improvement of the spatial resolution. Furthermore, enhancement of the Raman scattering signal of the substrate is observed when measuring through the polystyrene bead. The enhancement factors have been measured for silicon, highly ordered pyrolytic graphite, and gallium arsenide substrates. This setup provides a new way of enhancing the Raman signal by means of a nanolens.

Dynamics and folding of single two-stranded coiled-coil peptides studied by fluorescent energy transfer confocal microscopy.

We report single-molecule measurements on the folding and unfolding conformational equilibrium distributions and dynamics of a disulfide crosslinked version of the two-stranded coiled coil from GCN4. The peptide has a fluorescent donor and acceptor at the N termini of its two chains and a Cys disulfide near its C terminus. Thus, folding brings the two N termini of the two chains close together, resulting in an enhancement of fluorescent resonant energy transfer. End-to-end distance distributions have thus been characterized under conditions where the peptide is nearly fully folded (0 M urea), unfolded (7.4 M urea), and in dynamic exchange between folded and unfolded states (3.0 M urea). The distributions have been compared for the peptide freely diffusing in solution and deposited onto aminopropyl silanized glass. As the urea concentration is increased, the mean end-to-end distance shifts to longer distances both in free solution and on the modified surface. The widths of these distributions indicate that the molecules are undergoing millisecond conformational fluctuations. Under all three conditions, these fluctuations gave nonexponential correlations on 1- to 100-ms time scale. A component of the correlation decay that was sensitive to the concentration of urea corresponded to that measured by bulk relaxation kinetics. The trajectories provided effective intramolecular diffusion coefficients as a function of the end-to-end distances for the folded and unfolded states. Single-molecule folding studies provide information concerning the distributions of conformational states in the folded, unfolded, and dynamically interconverting states.