Femtosecond transient absorption spectroscopy of single-walled carbon nanotubes in aqueous surfactant suspensions: determination of the lifetime of the lowest excited state.

Femtosecond one- and two-colour pump-probe spectroscopy of single-walled carbon nanotubes (SWNTs) individual in aqueous surfactant suspensions has been used to assess the "intrinsic" lifetime of the lowest excited states. We demonstrate that such measurements can be perturbed by several competing photophysical processes thus making lifetime deconvolution difficult. Furthermore we show how these effects, arising primarily from sample heterogeneity, can be reduced. Measurements of induced transients in the near IR yield lifetimes of (35 +/- 10) ps and (56 +/- 10) ps, for nanotubes having mean diameters of 0.95 and 1.2 nm, respectively. Furthermore, a fast decay component in the ps to sub-ps regime is also observed. We tentatively attribute this to relaxation in SWNT bundle components.

Ultrafast dynamics of excess electrons in molten salts: part II. Femtosecond investigations of Na-NaBr and Na-NaI melts.

Ultrafast dynamics of excess electrons in Na-NaBr and Na-NaI molten solutions at elevated temperatures (T = 953-1128 K) were investigated over an extended wavelength range. Modelling the time profiles resulted in two time constants tau 1 = (200 +/- 40) fs and tau 2 = (2.8 +/- 0.4) ps for both systems at 1073 K. All transients can be understood in terms of dynamical equilibria between polaron and Drude-type electrons as well as polaron and Drude-type electron forming bipolarons. In agreement with our earlier results for K-KCl melts the fast component is assigned to the relaxation of Drude-type electrons into polarons while the longer component, tau 2, represents the time during which Drude-type electrons recombine with polarons leading to bipolarons. In addition, the temperature dependence was studied in Na-Nal: Decreasing the temperature to 953 K resulted in an increase of the time constants to tau 1 = (360 +/- 50) fs and tau 2 = (4.3 +/- 0.7) ps, respectively. At temperatures, where the ionic diffusion in Na-NaI melts becomes comparable to Na-NaBr melts, the time constants for the relaxation processes also coincide. The temperature-dependent investigations resulted in an Arrhenius activation energy of (25 +/- 5) kJ mol(-1) for Na-NaI melts in good agreement with literature data.

Reactions of methyl radicals with propene at temperatures between 750 and 1000 K.

The pyrolysis of propene, initiated by methyl radicals, has been studied in the temperature range 750-1000 K and at a pressure of 0.13 bar in a quasi-wall-free reactor using laser heating by fast vibrational-translational (V-T) energy transfer. This is a convenient method to study homogeneous high-temperature kinetics since the reactor walls remain cold. The radial temperature distribution in the reactor has been investigated by four different methods: a stationary heat balance, optical absorption, pressure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl radicals were produced via the fast thermal dissociation of di-tert-butyl-peroxide and the products were analysed using GC-MS. The main products of the overall reaction of the model system propene and methyl (C3H6 + CH3) were isopentane (iso-C5H12) and but-1-ene (1-C4H8), whereas allene (C3H4), trans-but-2-ene (trans-2-C4H8) and cis-but-2-ene (cis-2-C4H8) were minor components, all showing a strong dependence on temperature. The product distribution and the temperature dependence were analysed by a kinetic model of 61 species and 166 reactions developed for the high-temperature oxidation of butane and the low-temperature oxidation of n-pentane and isopentane. It was necessary to include a few missing reactions and to adjust some rate constants to make the modeling agree with the experimental investigations. This extended mechanism has to be evaluated further in forthcoming experiments.

Autofluorescence spectroscopy using a XeCl excimer laser system for simultaneous plaque ablation and fluorescence excitation.

Laser-induced fluorescence may be used to guide laser ablation of atherosclerotic lesions. This study was performed to evaluate arterial autofluorescence spectroscopy in vitro using a single XeCl excimer laser (308 nm) for simultaneous tissue ablation and fluorescence excitation. The laser beam was coupled to a 600-microns silica fiber transmitting 40-50 mJ/mm2 per pulse. The fluorescence radiation emanating retrogradely from the fiber was collected by a concave mirror spectroscopic analysis over a range of 321-657 nm. The arterial media (n = 26), lipid plaques (n = 26), and calcified lesions (n = 27) of aortic specimens from ten human cadavers were investigated in air, saline, and blood. Whereas the spectrum of calcified lesions changed with the surrounding optical medium, the other spectra remained constant. In air and blood, the spectra of arterial media, lipid plaques, and calcified lesions could be differentiated qualitatively and quantitatively (P < 0.0001). In saline, there was no clearcut spectroscopic difference between lipid plaques and calcified lesions. However, normal arterial media and atherosclerotic lesions (lipid plaques plus calcified lesions) could still be discriminated. Thus spectroscopy and plaque ablation can be combined using a single XeCl excimer laser. These encouraging results should stimulate further studies to determine the potential use of this approach to guide laser angioplasty in humans.