Two types of individualized single-walled nanotubes (SWNTs) in aqueous surfactant suspensions have been studied by femtosecond two-color absorption spectroscopy. A careful selection of pump and probe wavelengths allows for the determination of the “intrinsic” lifetimes of the lowest excited states which depend on the diameter of the SWNTs. Furthermore, a fast decay component in the ps to sub-ps regime was also observed and tentatively attributed to bundle relaxation. Likewise, this experimental approach can also assess spectral regions with decays faster than 1 ps, which is of great interest for all-optical switching devices near the optical telecommunication wavelength.

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.

The kinetics of the C3H3+C3H3 reaction was investigated behind incident shock waves at temperatures ranging from 995 to 1440K and at pressures between 600 and 1000mbar with argon as bath gas. The C3H3 radicals were generated by thermal decomposition of propargyl iodide with initial concentrations between 3×1015 and 6×1015cm−3 corresponding to initial mole fractions between 750 and 800ppm. Measurement of the UV absorption at 332.5nm was used to monitor the C3H3 concentration, and the rate coefficients for the C3H3+C3H3 reaction were determined by fitting a second-order rate law to the absorption–time profiles. Values between 2×10−11cm3s−1 near 1000K and 7.5×10−12cm3s−1 near 1400K were obtained with no significant pressure dependence. The negative temperature dependence can be expressed in the form k1=5.8×10−13exp(3534K/T) cm3s−1 with an estimated maximum error of ±30% in k1. The reaction channel leading to C6H5+H is estimated to contribute with less than 10% to the overall reaction at temperatures between 1000 and 1300K and, pressures ranging from 80 to 500mbar.

Internal conversion processes of 1,3,5-cycloheptatriene (CHT) and its perdeuterated counterpart (CHT-d8) in liquid cyclohexane have been studied by performing pump-probe experiments. The excitation and probe wavelength were 263 nm and 395 nm, respectively; the time resolution was 50fs. Temporal absorption and transient anisotropy profiles were monitored. The experimental findings are consistent with the following mechanism: After excitation, the system undergoes a fast internal conversion associated with a molecular reorientation. The characteristic times are (110 +/- 10) fs for CHT and ( 150 +/- 20) fs for CHT-d8. The transient anisotropy as well as the isotope effect are consistent with a [1,7] hydrogen shift as reorientation mechanism. Moreover, initial anisotropies indicate that coherence effects between two excited states at early times may occur.

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.

The ultrafast photophysics of D2O/sodium dodecylbenzene sulfonate surfactant dispersions of single-walled carbon nanotubes enriched in individual tubes (versus tube bundles) were studied by femtosecond pump-probe spectroscopy in the near-IR (NIR) spectral range. Measurements at 920 nm excitation and variable probe wavelengths showed evidence of superimposed transient bleaching as well as induced absorption behaviour. Our results indicate that such nanotube samples manifest ultrafast pump-induced switching of probe transmission with switching times of less than 1 ps under appropriate conditions. Given their high photochemical and photophysical stability these materials may be suitable candidates for the development of ultrafast NIR optical switches and logic gates.

We investigated the ultrafast dynamics in a Na-NaBr melt at 1073 K by fs pump probe absorption spectroscopy. A simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons. The relaxation times for polarons and bipolarons are 210 fs and 3 ps, respectively, The existence of an isosbestic point at similar to 1.35 eV indicates an inter-conversion between bipolarons and Drude-type electrons.

Experimentally determined incubation times in the thermal decomposition of methyl radicals were used to obtain collisional energy transfer probability information by adopting a discrete vibrational energy level master-equation scheme with specific rate constants from the statistical adiabatic channel model. The agreement with information from classical molecular dynamic, MD, simulations of CH3–Ar collisions was shown to be remarkably good. Results from MD simulations also support the assumption of thermally equilibrated rotations used here and in earlier work. The sensitivity of the pressure fall-off behaviour of the decomposition channels to remaining uncertainties in the energy transfer profiles is shown to be significant, in this case, as a consequence of the large number of collisions needed to reach activation. Nevertheless, we find classical molecular dynamics simulation to be useful and a good starting point in obtaining the collisional energy transfer kernel to be used in master-equation calculations treating the most obvious quantum effects through the use of discrete energy levels at low energies.

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.

The potential energy surface of the C2H5O system was studied by high level ab initio methods. Unimolecular rate constants have been computed using a simple transition state theory approach. The good agreement between predicted and experimental high pressure limiting rate constants supported the reliability of the proposed procedure. The direct bimolecular H-atom abstraction from ethylene by OH is unimportant and the reaction proceeds via the intermediate adduct. We predict high pressure bimolecular rate constants above 600 K for the following reactions under the condition that the pre-equilibrium is established:(OH)-O-. + C2H4 double left right arrow (CH2CH2OH)-C-. --> C2H3OH + H-.;k(R3) = 4 x 10(12) exp(-20.4 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(OH)-O-. + C2H4 double left right arrow (CH2CH2OH)-C-. --> CH3CH2O. -->CH2O + CH3.;k(R4) = 1.4 x 10(12) exp(-13.6 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(OH)-O-. + C2H4 double left right arrow (CH2CH2OH)-C-. --> (CH3CHOH)-C-. --> CH3CHO + H-.;k(R5) = 4 x 10(12) exp(-45.4 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)We also predict that the addition of H-atoms to acetaldehyde proceeds without an appreciable barrier and that redissociation is efficient above 400 K and a thermal equilibrium will be established. We found the barrier for addition of CH3 to formaldehyde to be 12 kJ mol(-1) lower than the currently accepted barrier for the competing hydrogen abstraction reaction leading to CH4+CHO.

The rate of H-atom formation in the thermal decomposition of o-, m-, and p-xylene was studied behind reflected shock waves between 1300 and 1800 K and at pressures of 1.6 to 4 bar by using atomic resonance absorption spectroscopy at 121.6 nm. For the thermal decomposition of p-xylene, m-xylene, and o-xylenep-CH(3)C(6)H(4)CH(3) --> p-CH(3)C(6)H(4)CH(2) + H (R1)m-CH(3)C(6)H(4)CH(3) --> m-CH(3)C(6)H(4)CH(2) + H (R2)o-CH(3)C(6)H(4)CH(3) --> o-CH(3)C(6)H(4)CH(2) + H (R3)the following rate constants were determined: k(1) = 1 x 10(16) exp(-380 kJ mol(-1) /RT) s(-1), k(2) = 1 x 10(16) exp(-382 kJ mol(-1)/RT) s(-1). and k(3) = 1 x 10(16) exp(-378 kJ mol(-1)/RT) s(-1). The rate constants are all close to the high-pressure limit, and the accuracy was estimated to be 30%. The H-atom-concentration time profiles were modeled using a simple mechanism in which the parallel C-C bond split, and secondary reactions have been taken into account. We found that within our experimental conditions, the C-C bond scission contributes about 15% to the overall decomposition rate constant of the xylenes.

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The temperature and pressure dependence of the rate constant for the unimolecular decomposition of i-propoxy radicals has been determined using the laser photolysis/laser induced fluorescence technique. Important features of the potential energy surface have been calculated by nb initio methods. Experiments have been performed at total pressures between 0.01 and 60 bar of helium and in the temperature range 330-408 K. The low and the high pressure limiting rate constants have been extracted from a complete falloff analysis: k(0) = [He] x 1.0 x 10(-8) exp(-43.8 kJ mol(-1)/RT) cm(3) s(-1) and k(infinity) = 1.2 x 10(14) exp(- 63.7 kJ mol(-1)/RT) s(-1). We estimate an uncertainty for these rate constants of +/-30%. Both rate constants have been discussed in terms of statistical unimolecular rate theory. Very good agreement between the calculated and the experimental rate constants has been found.

The reaction of toluene with molecular oxygen was studied behind reflected shock waves. Mixtures of 0.5-1 mol % toluene and 5-10 mol % oxygen in argon were investigated in the temperature range between 1050 and 1400 K at total pressures between 2 and 4 bar. We followed the rate of formation of the benzyl radicals by time-resolved UV absorption at 257 nm. The measured concentration-time profiles of the benzyl radicals were numerically reproduced using a simple reaction mechanism. For the initial reactionC6H5CH3 + O-2 --> C6H5CH2 + HO2 (R1)a rate coefficient k(1) ofk(1) = 3 x 10(14) exp[-180 kJ/mol/RT] cm(3)/mol s (1050 K < T < 1400 K)was determined with an accuracy of 30%. The rate constant k(2) of the subsequent reactionC6H5CH3 + HO2 --> C6H5CH2 + H2O2 (R2)was determined to bek(2) = 3 x 10(14) exp[-92 kJ/mol/RT] cm(3)/mol s (1150 K < T < 1250 K)The reaction of p-xylene with molecular oxygen was investigated using the same technique. Mixtures of 0.25-0.5 mol % p-xylene and 2.5-10 mol % oxygen in argon were shock-heated to temperatures between 1130 and 1380 K. We followed the formation of p-methyl-benzyl radicals by time-resolved UV absorption at 265 nm. However, we found a very low yield of p-methyl-benzyl radicals, which we attribute to a fast reaction of these radicals with molecular oxygen. From this low yield, we estimated for ratio k(3)/k(4) Of the rate constants for the reactionsp-CH3C6H4CH3 + O-2 --> p-CH3C6H4CH2 + HO2 (R3)p-CH3C6H4CH2 + O-2 --> p-CH2C6H4CH2 + HO2 (R4)an upper limit of 0.01.

The recombination reactions HO+NO+M⇒HONO+M(1) and HO+NO2+M⇒HNO3+M(2) have been investigated over an extended pressure (1–1000 bar) and temperature (250–400 K) range. HO radicals were generated by laser flash photolysis of suitable precursors and their decays were monitored by saturated laser-induced fluorescence (SLIF) under pseudo-first-order conditions. The measured rate constants were analyzed by constructing falloff curves which provide the high pressure limiting rate constants k∞. In the given temperature range, these rate constants are k1,∞=(3.3±0.5)×10−11×(T/300 K)−(0.3±0.3) and k2,∞=(7.5±2.2)×10−11 cm3 molecule−1 s−1.

The pulsed laser photolysis (PLP), time-resolved saturated laser-induced fluorescence (SLIF) technique has been used to study the reaction CH+CO+M⇒HCCO+M (1) in the temperature range between 300–800 K and at total pressures between 4 and 160 bar helium. The CH radicals have been generated using UV-multiphoton dissociation of CHBr3 at 248 nm. In the investigated temperature and pressure range, the usual falloff behavior of the reaction has been found. Falloff curves have been constructed and a high-pressure limiting rate constant with a weak negative temperature dependence of k1,∞=1.7×10−10(T/300 K)−0.4 cm3 molecule−1 s−1 has been extracted. This high-pressure limiting rate constant has been treated in terms of the statistical adiabatic channel model. Isobar Arrhenius curves have been constructed for pressures between 0.01 and 100 bar, including the formation of C2O+H at high temperatures.

Thermal rate constants of the reaction of CH(((II)-I-2)) with H-2 have been measured at pressures between 1 and 160 bar and temperatures between 185 and 800 K. CH radicals have been generated using multiphoton laser hash photolysis of CHBr3 at 248 nm and detected by saturated laser-induced fluorescence near 430 nm. At low pressures the reaction leads to CH2(B-3) and H, while at high pressures CH3 radicals are produced. S-shaped transition curves have been constructed to describe the pressure dependence of the rate constant. The high-pressure limiting rate constant for the recombination to CH3 has been evaluated to be k(1 infinity) = 2.O X 10(-1O)(T/300 K)(0,15) cm(3) molecule(-1) s(-1). Using experimental low-pressure data from the literature, the rate constant for the second channel could be separated and has been analyzed in terms of SACM theory. A simple kinetic model has been applied to describe the overall rate constant k(1) in an extended temperature and pressure range. Related rate constants for the reaction of CH2(B-3) with H and the unimolecular two-channel dissociation of CH3 have also been analyzed. (C) 1997 American Institute of Physics.

Thermal rate constants of the reaction of CH(2Π) with H2 have been measured at pressures between 1 and 160 bar and temperatures between 185 and 800 K. CH radicals have been generated using multiphoton laser flash photolysis of CHBr3 at 248 nm and detected by saturated laser-induced fluorescence near 430 nm. At low pressures the reaction leads to CH2(3B)and H, while at high pressures CH3 radicals are produced. S-shaped transition curves have been constructed to describe the pressure dependence of the rate constant. The high-pressure limiting rate constant for the recombination to CH3 has been evaluated to be k1,∞=2.0×10−10(T/300 K)0,15 cm3molecule−1 s−1. Using experimental low-pressure data from the literature, the rate constant for the second channel could be separated and has been analyzed in terms of SACM theory. A simple kinetic model has been applied to describe the overall rate constant k1 in an extended temperature and pressure range. Related rate constants for the reaction of CH2(3B) with H and the unimolecular two-channel dissociation of CH3 have also been analyzed.