Bibliothek

Notizen: Diamond powders with grain diameters up to 0.3 μm were obtained by CO2 -laser-induced decomposition of C2H4 at low pressures and temperatures. C2H4 or mixtures of C2H4, H2, and SiH4 were irradiated in a gas-flow reactor with the 10P14 line (10.532 μm) of a grating-tunable CO2 laser with 50-W cw output power. Solid products were produced in a yellow-to-orange colored flame (500–550 °C) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon, and spherical diamond particles. Several diamond particle populations, with mean diameters of 6–120 nm, were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction. Diamond formation is dependent on the residence time in the reaction flame, but relatively independent of the reactant gas compositions. Hydrogen-free pyrolysis of pure ethylene produced diamond of high purity (no diffraction rings of graphite detectable).

Notizen: In this paper we report on the electronic spectroscopy of mass-resolved tetracene⋅Arn (n=1–26) and tetracene⋅Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.The ionization thresholds of the tetracene⋅An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified several isomerization phenomena, i.e., correlated restricted and unrestricted surface motion, adcluster isomerization, surface melting and side crossing, and characterized the size dependence of the temperature onsets of these processes for small and medium sized n=2–20 clusters. These isomerization processes could not be interrogated by the investigation of the size dependence of the spectral shifts and linewidths. The size dependence and the isomer specificity of the spectral shifts are well accounted for by the MC simulations. The homogeneous spectral linewidths of small (n〈8) clusters pertain to the spectroscopy of "static'' isomers, while the line broadening of large (n≥20) clusters manifests inhomogeneous line broadening due to the coexistence of wetting and nonwetting isomers. The temperature dependence of the spectral shifts and inhomogeneous linewidths of large (n≥20) clusters provides means for internal cluster thermometry.

Notizen: Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol⋅(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≈8000 cm−1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol⋅(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity.

Notizen: Efficient excited-state proton transfer in neutral acid–base clusters α-naphthol⋅Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm−1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphthol⋅Bn (or base B) system.

Notizen: The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4-31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for the hydrogen bonding O–H stretch, k(O–Hb), decrease strongly with size, leading to a decrease in the νbridge O–H frequencies; a similar decrease is found for the intramolecular bending force constant k(aitch-theta); (iv) force constants for the free O–H stretching motions k(O–Hf) stay nearly constant, as do the corresponding vibrational frequencies; (v) intermolecular stretch–stretch couplings increase substantially with n and dominate over intramolecular stretch–stretch couplings for n≥3; similarly, intermolecular bend–libration couplings are very important.

Notizen: The irregular vibronic structure resolved in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10-dihydro-9,10[1'2']benzenoanthracene) is assigned in terms of a single-mode E'⊗e' Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe' =47.83 cm−1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≈98% of the observed absorption band intensities over the observable range 0–350 cm−1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry-equivalent minima. Emission spectra from 13 vibronic levels in the excited E' state show extended vibrational progressions with up to 25 members in the analogous e' ground state vibration, which is highly harmonic in the electronic ground state. The Franck–Condon factors of the fluorescence emission spectra calculated with the E' state Jahn–Teller parameters fitted to the absorption spectrum also yield a quantitative fit to observed emission intensities. The eigenvectors of the E' state vibronic levels are hence determined to great precision; the lowest five can be classified as radial oscillator and/or hindered rotor states, while higher levels have mixed character. Several eigenvectors are strongly localized in the upper sheet of the adiabatic Jahn–Teller surface, corresponding to "cone'' states.

Notizen: Van der Waals (vdW) clusters of naphthalene with argon were synthesized in supersonic beam expansions. Mass-selective absorption spectroscopy was carried out by using fragmentation-free two-color resonant two-photon ionization with mass-spectrometric detection. Electronic spectra of naph⋅Arn (n≤6) were recorded at the vicinity of the naphthalene S1←S0 electronic origin (32 020 cm−1 ), and corresponding spectra for the 8¯10 transition up to n=30. For n=3, the spectra due to two different isomers could be separated. Clear evidence for the existence of different cluster isomers was also found for n=6, 8, 9, 12–14, and 25–28. In comparison with most other solvent clusters M⋅Arn (M is the aromatic molecule), the spectral red shifts of 000 and vibronic bands of the naph⋅Arn clusters are very small. Analysis of the size dependence of the electronic spectral shifts indicates that the stepwise solvation of the naphthalene molecule proceeds predominantly on one side of the molecule. Two semicyclic trends in the spectral shifts are interpreted as successive wetting-nonwetting transitions which occur with increasing solvent cluster size. In the proposed mechanism for these transitions, part of the innermost solvent layer "unwets'' the substrate, and moves to the second or third layer, thereby forming a nonwetting dropletlike cluster on one side of the naphthalene substrate.

Notizen: Structures and order–disorder transitions of the rare-gas solvent clusters carbazole⋅Arn were studied for n=1–7 by molecular dynamics and Monte Carlo simulation methods. This study was motivated by the investigation of these clusters by size-specific electronic spectroscopy. Cluster structures corresponding to absolute and local energy minima were obtained by molecular-dynamics slow cooling and annealing. For n=1–3, single minimum-energy structures of Cs symmetry were found, with all n atoms on the same side of the molecular substrate. For n=4–7, the number of energetically low-lying monolayer isomers rises very rapidly with n. Orientational ordering of the adcluster by the substrate is unimportant for n≥4. The isomers for n≥4 differ by (i) rotational/translational displacements of the cluster relative to the substrate, (ii) promotion of atoms to the second solvent layer, and (iii) adsorption of atoms on the second side of the substrate. Order–disorder transitions of the monolayer solvent clusters were studied by MC methods as a function of temperature. Four different quasi-two-dimensional order–disorder transitions were found: the racemization and surface-decoupling transitions reflect the loss of cluster–substrate orientational correlation, while the cluster rigid–fluxional and melting transitions derive from the loss of intracluster bond-orientational order, and increase in bond-length fluctuations. Analogies to order–disorder phase transitions in two-dimensional rare-gas films are discussed.