Allan L. Smith

Orientational disorder in solvent-free solid c70.

The high-temperature structure of solvent-free C70 has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

Structural phase transitions and orientational ordering in C70

Abstract The thermal behavior of solid C70 has been studied by synchroton X-ray powder diffraction and differential scanning calorimetry. The equilibrium solid state structures formed by C70 were solved by full profile refinement techniques in which orientational and packing disorder were explicitly accounted for. Above 345 K, C70 forms a plastic crystal, with an equilibrium face-centered cubic structure. At lower temperatures, orientational freezing occurs in two stages. Between 295 and 345 K, disorder persists only about the long axis of the molecule, and the lattice undergoes a rhombohedral distortion. Below 295 K, the rhombohedral lattice undergoes a further distortion, resulting in a previously unobserved monoclinic structure, in which the molecules are presumed to be essentially static. At all temperatures, however, the structure of C70 retains an ABC packing sequence.

Comparison of the ultraviolet absorption cross section of buckminsterfullerene in the gas phase and in hexane solution

Five previously reported measurements of the gas-phase absorption cross section for in the ultraviolet region are reviewed in the light of recent improvements in the determination of vapour pressure. An improved gas-phase absorption cross section at is presented and compared with a cross section derived from absorbance measurements in hexane solution by applying the (refractive index-dependent) Chako correction. The agreement between the measured gas-phase absorption cross section and that derived from the solution phase absorbences is very good. The highest energy absorption band (band G, 209 nm) is suppressed in intensity and shifted lower in energy more than the other two bands, E (254 nm) and G (326 nm).

Ultraviolet molar absorptivities of aqueous hydrogen peroxide and hydroperoxyl ion

Abstract Molar absorptivities are measured at 11 wavelengths between 200 and 300 nm for H 2 O 2 (aq) with an average error of 8.5% (95% confidence limit), and at 10 wavelengths in the same range for HO 2 − (aq), from absorbance vs. concentration plots. Results are compared to previous measurements of both aqueous and vapor-phase molar absorptivities.

Flash photolysis absorption spectroscopy of xenon fluoride: Vibrational analysis of the B-X transition

Kinetic spectroscopy of a flash-photolyzed mixture of Xe and UF6 yielded the B-X transition (300–355 nm) of XeF in absorption. Vibrational assignments were made for 22 bands (0 ≤ v′ ≤ 12, 0 ≤ v″ ≤ 3), and were confirmed by computer modeling of the relative absorption intensities. Spectroscopic constants for XeF (B-X) are: Te = 28 813.9 ± 0.5 cm−1, ω′e = 308.23 ± 0.5 cm−1, ωeχ′e = 1.4266 ± 0.03 cm−1, ω″e = 225.06 ± 1.4 cm−1, ωeχ″e = 10.568 ± 0.4 cm−1, and ΔRe = R′e - R″e = 0.35 ± 0.02 A. A linear Birge-Sponer extrapolation yields a ground state XeF dissociation of about 1200 cm−1.

Laser-induced plasma formation in Xe, Ar, N 2 , andO 2 at the first four Nd:YAG harmonics

Irradiance values have been measured for the onset of laser-induced plasma formation in Xe, Ar, N(2), and O(2) at pressures from 760 to 25 Torr at wavelengths of 1.064, 0.532, 0.355, and 0.266 microm. These values have been compared with the results of other workers who used similar experimental setups. There is agreement within a factor of 4 when irradiance values are compared and within a factor of 2 when ratios of irradiance values for different gasses are compared. Comparisons among workers who used widely different pulse lengths indicate that the onset of plasma formation is measured better by energy fluence than by irradiance.

Fluorescence of S2(B–X) excited by fixed frequency ultraviolet lasers

Fluorescence spectra from rotational levels in the B 3Σ−u state of disulfur excited by pumping the ground state with a cw helium–cadmium laser (325 nm) and with a pulsed nitrogen laser (337 nm) have been studied. The He–Cd laser excites the transition (v′ = 3, N′ = 24, J′ = 25) ← (v″ = 3, N″ = 25, J″ = 26), or (3,3)P1(25). A comparison of high resolution spectra of the nitrogen laser and of S2 shows that several transitions in the (2,4) band could be excited: R3(23), P2(13), R2(21), and R3(25).

R‐variation of electronic transition moments from vibrationally crowded spectra: the B–X and D–X transitions of xenon fluoride

Using photographic flash spectroscopy of Xe/F2 mixtures, we have studied the B(1/2)–X(2Σ+) and D(1/2)−X(2Σ+) transition of XeF in absorption between 241 and 358 nm. The spectra exhibit extensive sequence crowding due to overlapping hot bands. To determine the R‐variation of the B–X and D–X electronic transition moments we use band cluster analysis, in which the R centroids of bands contributing to a given cluster are shown to be similar; i.e., the R centroid is a smoothly varying function of bandhead wavelength. Absolute transition moments are derived by normalizing our relative absorption measurements to the measured radiative lifetime of the v′=0 level of the B(1/2) state. The B–X moment is 2.70±0.22 D, independent of R between 2.30 and 2.54 A. The D–X moment varies linearly from 2.255 D at 2.25 A to 2.413 D at 2.50 A (±20%). The computed radiative lifetime of the v′=0 level of the D(1/2) state is 10.0±2.0 nsec.

Tunable laser fluorescence studies of the metastable b1Σ+ state of the ND radical☆

Abstract We have detected the b 1 Σ + state of ND, produced in a pulsed discharge through Ar/ND 3 , by exciting the c 1 Π ← b 1 Σ + 0,0 band with a tunable laser and monitoring c 1 Π → a 1 Δ fluorescence.

Why are Solutions of C60‐Piperazine Purple at pH 11?

Abstract The C60‐piperazine mono adduct was synthesized by the reaction of C60 and piperazine. The saturated C60‐piperazine aqueous solution was colorless when pH is 8 or below. A purple color was developed when pH is around 9 and the pink color is most intense at pH 11. The color of the C60‐piperazine solution fades out when pH is approaching 12 from 11, and the solution remains colorless when pH is 13 or higher. The UV‐Vis spectra of the C60‐piperazine solution were recorded at pH 4, 11 and 14. The mono‐protonated C60‐piperazine was identified to be responsible for the purple color observed. The computational investigation of the un‐protonated, mono‐protonated and di‐protonated C60‐piperazine was conducted at the PM3 and ZINDO(s) levels of theory. Vibronic coupling of the Jahn‐Teller active vibrational mode to the electronic transition was applied to re‐generate the weak absorption between 550–600 nm in the UV‐Vis spectrum of the mono‐protonated C60‐piperazine.