Search

Double-walled carbon nanotubes (DWNTs), synthesized by the catalytic decomposition of methane, were explored by resonance Raman spectroscopy using different energies for laser excitation. Based on the radial breathing mode frequencies, the indices of the two layers of a DWNT were approximately assigned, depending on the interlayer separation of the two coaxial layers of the DWNT, which ranged from 0.34 to 0.40 nm. From the tentatively assigned results, it was found that the two walls of the DWNT are not strongly selected by chirality and diameter. The results, however, suggest that, for the tubes that are resonant with the available laser excitation energies, most of the outer layers of the observed DWNTs in our samples are semiconducting, while the inner layers of the observed DWNTs are either semiconducting or metallic based on the assembled DWNTs. The characteristics of the G, D, and G′ band of the DWNTs are discussed, and a double peak feature in the D and G′ band, originating from the inner and outer layers of the DWNTs, is reported.

A simple time domain method for the observation of polarization-specific Raman responses in electronically nonresonant materials is demonstrated. When a cutoff filter is placed in the probe beam pa...

An analysis of femtosecond optical heterodyne detected (OHD) birefringence and dichroism is presented within the framework of the Z-scan technique previously developed for the measurement of nonlinear refractive indices. In particular, the phase of the effective local oscillator field, in the language of OHD spectroscopy, is shown to depend on the sample position (z) and the spatially detected portion of the signal pulse in the far field for nonresonant pump–probe responses due to focused Gaussian beams. Consequently, this treatment shows how the contributions of OHD birefringence and dichroism, and the corresponding homodyne signals, can best be separated in this simple two-beam experimental configuration and provides a description of the previously reported technique called position-sensitive Kerr lens spectroscopy. The nonresonant third-order polarization electronic response of fused silica illustrates this apertured OHD description. Spatial and frequency filtering results in ultrafast Z-scan responses that are dominated by dichroic and birefringent responses, respectively. An analysis of apertured OHD spectroscopy which incorporates a description of the optical phase fronts of the signal fields is required in order to exploit this relatively simple experimental technique for quantitative determinations of polarization-specific electronic responses and Raman nuclear spectral densities.

Detuning oscillations, previously observed in the dispersed optical heterodyne detected (OHD) birefringent responses of nonresonant liquids when the pump and probe pulses were temporally overlapped, are shown to be inherent to frequency resolved nonresonant P (3) electronic responses. For a Gaussian pump and a one-sided exponential probe pulse, the frequency of these detuning oscillations is equal to the detuning from the probe carrier frequency. This effect is related to the coherence coupling effects well known in resonant pump–probe studies. These detuning oscillations vanish in the corresponding dispersed SHG cross-correlations and dispersed homodyne responses. The influence of near two-photon resonance is considered.

A comparison of the solvation/spectral-response functions obtained by two independent techniques, the single-wavelength and spectral-reconstruction methods, is reported. Determination of the best wavelengths for application of the linear-single-wavelength approximation for the solute coumarin 153 (C153) is achieved using radiative rate data and steady-state emission spectra in a series of 36 different solvents at room temperature. The optimal linear wavelength is found to be 555 nm. (This wavelength, which is on the red side of the spectrum, yields superior results when compared to the more traditional choice of 470--480 nm, on the blue side.) Response functions determined using both 560- and 470-nm observation wavelengths are compared to previously reported spectral-reconstruction results in 24 solvents. A comparison of the characteristic times indicates that the linear-single-wavelength method can be used to predict solvation times with an accuracy of roughly {+-}30--40% (1 standard deviation) using suitably scaled data collected at {approximately}560 nm. Application of a nonlinear version of the single-wavelength method does not provide increased accuracy.

Technical spectra are reported for a series of six secondary emission standards based on the commercially available fluorophors: tryptophan, α-NPO, tetraphenylbutadiene, coumarin 153, DCM, and LDS 751. These spectra cover the wavelength range between 300 and 800 nm with significant spectral overlap and conform to the requirements of secondary emission standards. Standard emission spectra are determined by averaging the technical spectra obtained from three independently calibrated fluorescence spectrometers. A method for generating the wavelength-dependent correction file by using these standards is outlined. Comparison to the National Bureau of Standards (NBS)-reported quinine sulfate spectrum illustrates the accuracy of both the emission standards and the method of generating a complete emission correction file. With the use of the standards and method reported here, the accuracy of the emission correction is estimated to be better than 10% for the entire wavelength range and significantly better over the wavelength region of a typical spectrum.

Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole -dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute -solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10-20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute -solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.

Steady-state and time-resolved emission measurements of the solvatochromic probe coumarin 153 are used to study solvation of a dipolar solute in nondipolar solvents such as benzene and 1,4-dioxane. Contrary to the predictions of dielectric continuum theories, the Stokes shifts (or nuclear reorganization energies) that accompany electronic excitation of this solute are substantial in such solvents (∼1000 cm-1). The magnitudes of the shifts observed in both nondipolar and dipolar solvents can be consistently understood in terms of the relative strength of the interactions between the permanent charge distributions of the solute and solvent molecules. (Information concerning these charge distributions is derived from extensive ab initio calculations on the solute and 31 common solvents.) The dynamics of solvation in nondipolar solvents, as reflected in the time dependence of the Stokes shifts, is qualitatively like that observed in polar solvents. But, whereas the dynamics in polar solvents can be rather sim...

The dynamics of solvation of the probe solute coumarin 153 has been measured with ~100 fs time resolution using the fluorescence upconversion technique. A wide variety of solvents, including polar aprotic solvents, hydrogen bonding solvents, and non-dipolar solvents have been examined. For all solvents of even moderate polarity (49-01), the solvation dynamics observed follows the predictions simple models based on the solvent's bulk dielectric response. For a number of “non-dipolar” solvents such as dioxane and benzene, for which (49-02), we observe surprisingly large time-dependent shifts, which seem to reflect the solute interacting with the large quadrupole (and higher multipole) moments of the solvent molecules. These dynamics represent a solvation distinct from the dipolar solvation dynamics previously studied and they cannot be modeled in terms of any currently available theory.

Time-resolved emission measurements of the solute coumarin 153 (C153) are used to probe the time dependence of solvation in 24 common solvents at room temperature. Significant improvements in experimental time resolution ({approx}100 fs instrument response) as well as corresponding improvements in analysis methods provide confidence that all of the spectral evolution (including both the inertial and the diffusive parts of the response) are observed in these measurements. Extensive data concerning the steady-state solvatochromism of C153, coupled to an examination of the effects of vibrational relaxation, further demonstrate that the spectral dynamics being observed accurately monitor the dynamics of nonspecific solvation. Comparisons to theoretical predictions show that models based on the dielectric response of the pure solvent provide a semiquantitative understanding of the dynamics observed. 156 refs., 26 figs., 5 tabs.

The pressure-volume-temperature (P-V-T) properties of monodisperse alternating poly(ethylene-propylene) (hydrogenated polyisoprene) melts of varying molecular weights are measured at 0.1 MPa < P < 200.1 MPa, and 290 K < T < 510 K in a dilatometer-type apparatus. The Flory-Orwoll-Vrij (FOV), Modified Cell Model (MCM), Sanchez-Lacombe (SL), and the Statistical Associating Fluid Theory (SAFT) equations of state are found to correlate the experimental data with excellent accuracy, except either at low pressures and high temperatures, or at high pressures and low temperatures. On average, MCM and SAFT fit the data slightly better than FOV and SL. The equation-of-state parameters are found to be independent of molecular weight, especially for SAFT, as required by the theory. © 1994 John Wiley & Sons, Inc.

The dispersed optical heterodyne detected (OHD) birefringence and dichroism that result from the electronic response of nonresonant materials are described for a number of ultrafast pulse shapes. The OHD spectrograms that correspond to these dispersed instantaneous electronic responses display detuning oscillations along both the interpulse delay and the probe frequency dimensions. The frequency of these oscillations depends only on the detuning of the selected Fourier component of the probe pulse from the carrier frequency and the pulse shapes. This effect is demonstrated for Gaussian, sech2, and one-sided exponential probe pulses. For each of these pulse shapes the OHD spectrograms exhibit a characteristic dependence on detuning frequency. The effects of linear chirp on these OHD spectrograms are also considered. Experimentally observed nonresonant electronic spectrograms are compared with theory. These OHD spectrograms are also compared with the well-known frequency-resolved optical gating traces. Analysis of the OHD spectrograms may provide a more useful approach for the characterization of ultrafast pulses because of the phase-specific, amplitude level origin of these responses.