Chemical Physics Letters (v.339, #5-6)

Impact of catalyst coarsening on the formation of single-wall carbon nanotubes by O. Jost; A.A. Gorbunov; J. Möller; W. Pompe; A. Graff; R. Friedlein; X. Liu; M.S. Golden; J. Fink (297-304).
The single-wall carbon nanotube (SWCN) yield as a function of the gas flow velocity for different catalyst contents in a furnace-based pulsed laser evaporation method is shown to depend sensitively on the size distribution and growth conditions of the condensed catalyst nanoparticles in the gas phase. In particular, accelerated particle coarsening should be avoided. Consequently, a high number density of small catalyst nanoparticles leads to a high nanotube yield within the timescale of a few hundred milliseconds. Hence, the attainment of enhanced particle growth control will enable a high yield evaporation-based synthesis of high-quality SWCNT.

Low-frequency Raman modes in Cs- and Rb-doped single wall carbon nanotubes by N. Bendiab; A. Righi; E. Anglaret; J.L. Sauvajol; L. Duclaux; F. Béguin (305-310).
Single wall carbon nanotubes (SWNT) doped at saturation by alkali-metals are investigated by Raman scattering. The Raman profiles in the frequency range of the tangential modes (TM) are close for Cs and Rb doping and only weakly dependent on the laser energy. This loss of resonance is assigned to a loss of Van Hove singularities in the electronic density of states. The most striking intrinsic features of the doped compounds are two low-frequency peaks assigned to modes involving both radial motions of tubes and alkali-atoms vibrations.

Room temperature filling of single-wall carbon nanotubes with chromium oxide in open air by Jagjiwan Mittal; Marc Monthioux; Hatem Allouche; Odile Stephan (311-318).
A very easy and simple way of filling single-wall carbon nanotubes (SWNTs) at room temperature in open air was discovered by soaking as-prepared SWNT materials in a mixture of a transition metal oxide in an inorganic acid (CrO3 in HCl). The filling of SWNTs was found to start just after three hours of treatment and to increase with the treatment time up to two days. Both X-ray energy-dispersive spectroscopy (X-EDS) and electron energy loss spectroscopy (EELS) suggest that the filling compound is a chromium-containing material, probably CrO3. Filled SWNTs are stable in vacuum under electron beam (using conventional imaging conditions), in air and in polar solvents like water, alcohol, and acetone.

Photoluminescence studies of SiC nanocrystals embedded in a SiO2 matrix by Y.P. Guo; J.C. Zheng; A.T.S. Wee; C.H.A. Huan; K. Li; J.S. Pan; Z.C. Feng; S.J. Chua (319-322).
The dependence of the photoluminescence (PL) from SiC nanocrystals embedded in a SiO2 matrix on annealing is presented. Blue-green PL has been observed at room temperature from annealed SiC–SiO2 composite films. The intensity of the single emission band at 460 nm (2.7 eV) shows a strong dependence on the annealing temperature. The combination of high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) transmission spectra and PL results suggest that SiC nanocrystals have been incorporated into the SiO2 matrix and O-deficient defects were formed. The origin of luminescence is attributed to the creation of defects in silicon oxide.

Acceleration of neutral silicon cluster isomers in a helium gas expansion by Michael Rosemeyer; Rolf Schäfer; Jörg August Becker (323-330).
The acceleration of isolated neutral silicon cluster isomers in a helium gas expansion has been studied within a molecular beam experiment. The molecular beam flows through a nozzle tube that can be heated, expands into vacuum and the velocity slip of the clusters against the helium is investigated by a time-of-flight method. The measured mean cluster velocities can be analyzed by use of a simple acceleration model and give evidence for two structural classes of cluster isomers. Temperature depending investigations indicate that the cluster isomer class with an enhanced collision cross-section is metastable and transforms into the stable one if the nozzle tube is heated.

Energy transfer dynamics in polyfluorene-based polymer blends by A.R. Buckley; M.D. Rahn; J. Hill; J. Cabanillas-Gonzalez; A.M. Fox; D.D.C. Bradley (331-336).
The Förster energy transfer mechanisms in polyfluorene-based polymer blends have been investigated by picosecond time-resolved photoluminescence spectroscopy. Blends of poly(9,9-dioctylfluorene), (F8), and poly(9,9-dioctylfluorene-co-benzothiadiazole), (BT), ranging from 50% to 0.01% BT have been investigated from room temperature down to 12 K. In high concentration BT/F8 films, nearest neighbour transfers from F8 to BT, characterised by an F8 fluorescence lifetime of 12 ps, totally dominate. In low concentration BT/F8 films energy transfer between F8 and BT still occurs at nearest neighbour sites, but exciton migration to these sites amongst F8 chain segments now also contributes to the measured dynamics.

Observation of resonant two-photon photodetachment of water cluster anions via femtosecond photoelectron spectroscopy by J.M. Weber; J. Kim; E.A. Woronowicz; G.H. Weddle; I. Becker; O. Cheshnovsky; M.A. Johnson (337-342).
Photoexcitation of the (H2O) n (n=20–100) clusters with 100 fs pulses at 800 nm results in an increasing propensity for two-photon electron photoejection with increasing cluster size. This increase correlates with the size range (n≈30) where the first excited electronic state drops below the electron continuum, and the electronic absorption band approaches the energy of the 800 nm pump photon. No above-threshold, two-photon detachment is observed for n=20. Differences in the shape of the resonant two-photon photoelectron spectrum compared to that arising from direct (high energy) photodetachment are interpreted in terms of the vibrational state selection created in the resonant step.

The excitation spectra of terthiophene and terthiophene-S,S-dioxide are investigated by time-dependent density-functional theory (TD-DFT) calculated data are compared with recent experimental results. The second singlet–triplet excitation energy (T 2) is calculated above the first singlet–singlet (S 1) one. In terthiophene-S,S-dioxide the formation of a bonding interaction in the lowest unoccupied molecular orbital decreases its kinetic energy and explains the red-shift of the excitation spectrum. The inter-system crossing (ISC) rate in terthiophene-S,S-dioxide is expected to be lower than in terthiophene due to the increased T 2S 1 energy gap, which is also found to not decrease with inter-ring torsion as in terthiophene.

The effect of ion–ion interactions on the structure and dynamics of the sulfate ion in aqueous MgSO4 solution has been studied by polarization-resolved CARS spectroscopy. Depolarization ratio of the totally symmetric S–O stretch band of the SO4 2− ion was determined accurately for three different MgSO4 concentrations, 0.8, 1.3, 2.5 mol dm −3 . At 0.8 mol dm −3 , the observed depolarization ratio was zero as expected from Td symmetry. On the other hand, small but meaningful non-zero depolarization ratios were obtained at higher concentrations. These non-zero depolarization ratios are interpreted in terms of symmetry lowering of the sulfate ion, which is due to dynamic ion pair formation between Mg2+ and SO4 2− ions.

Light has been known to suppress wave activity in the vast majority of studies of excitable photosensitive Belousov–Zhabotinsky (BZ) media. In this report, we uncover that light perturbation can induce pattern formation when the dynamics of the BZ system is close to a bifurcation point, though light causes an increase of bromide concentration. The minimal light intensity for initiating pattern formation increases rapidly while the system departs from the bifurcation point. Backfiring behavior was also observed when a global light perturbation was applied to propagating waves. This study was carried out with a three-variable Oregonator model, modified to describe photosensitivity.

We regard the excitation of a diatomic molecule with a chirped ultrashort laser pulse. In this way, nuclear wave packets are prepared which, depending on the chirp parameter, exhibit different vibrational dynamics. Using the potassium dimer as a numerical example, we show that time-resolved photoelectron spectroscopy is able to map the characteristics of the nuclear probability density and, in particular, to distinguish densities obtained from excitations using differently chirped pulses.

A topological analysis of charge density in complexes between derivatives of squaric acid and ammonium cation by David Quiñonero; Antonio Frontera; Pau Ballester; Carolina Garau; Antoni Costa; Pere M. Deyà (369-374).
A topological analysis of the electron charge density in complexes between derivatives of squaric acid and ammonium cation has been carried out. There are excellent relationships between either the charge density or its associated Laplacian at the ring critical point originated upon complexation and both geometric and energetic parameters. There is a remarkable influence of the substituents attached to the four-membered ring on the charge density at the ring critical point.

High-level self-consistent-field calculations have been performed on amines NH3−n R n (with R suitable substituent and n=1,2,3) to study the influence of the substituent on the proton affinity. Corrected for zero-point vibrational energies, calculations agree fairly well with available experimental data. The amine proton affinities are not correlated with nitrogen charge, irrespective of the way used to calculate the charge.

The optical rotations (ORs) of chiral organic molecules have been calculated by time-dependent density functional response theory (TDDFT) employing the B3LYP hybrid functional. For selected examples, the origin as well as the frequency dependence of the OR has been investigated. The theoretical data for 20 organic molecules including large systems as e.g., octahelicene compare favourably with experimental observations even if small valence basis augmentented with diffuse basis functions are employed. The inclusion of the frequency dependence in the theoretical treatment is recommended to obtain reliable predictions for measured OR values.

Density-functional calculations on a single, isolated, infinite, periodic chain of poly(cyanoacetylene) have been performed. Thereby more different structures have been considered, partly differing in bond-length-alternation patterns and partly differing in whether the system has a polyacetylene-like or a polyacene-like structure. It is found that all structures with a bond-length alternation are stable, that the systems are semiconductors, and that the polyacene-like structures are the stablest ones. Finally, the consequences of these findings for excitation and transport processes are discussed, and an interesting charge separation for the polyacene-like structures is found.

Anisotropic hyperfine interaction-induced spin relaxation in a low magnetic field by M.V. Fedin; P.A. Purtov; E.G. Bagryanskaya (395-404).
Spin relaxation caused by modulation of an anisotropic hyperfine interaction (HFI) in a low magnetic field has been correctly considered for a radical with one magnetic nucleus in a framework of Redfield relaxation theory. The analysis of the results obtained revealed that HFI-induced relaxation is very different in a low magnetic field from that in a high field; and that the use of traditional high-field expressions for calculation of T 1 and T 2 is not correct. It has been shown that taking correct account of the role of HFI-induced relaxation is crucial for the calculations of time-resolved CIDEP and the line width of EPR in a low magnetic field. In calculations of low-field CIDNP and MARY, taking correct account of the relaxation mainly influences the amplitude of the field dependence.

The observation of fluorescence from excited states of NF2 and NF following the photodissociation of NF3 in the 11–30 eV range by Dominic P. Seccombe; Richard P. Tuckett; Hans-Werner Jochims; Helmut Baumgärtel (405-412).
A study of the vacuum-ultraviolet (VUV) fluorescence spectroscopy of NF3 in the 11–30 eV range is presented. Synchrotron radiation (SR) from BESSY1, Berlin provides the VUV excitation source. Emission due to NF b   1Σ+–X   3Σ at 528.8 nm is observed between 11.5 and 21.0 eV. Other emissions, which are believed to be due to transitions between highly excited states of NF2, are observed at higher energies, 14.8–24.5 eV. Lifetimes associated with two of these bands at 325 and 410 nm are determined to be 12.9 and 13.0 ns, respectively. The similarity of the measured values implies that the two transitions originate from a common upper state.

Collisional quantum interference effect on rotational energy transfer in an atom–diatom system by Mengtao Sun; Guohe Sha; Shulin Cong; Fengcai Ma; Jinchun Xie; Cunhao Zhang (413-420).
The theoretical model of collisional quantum interference (CQI) in intramolecular rotational energy transfer is described in an atom–diatom system, based on the first Born approximation of time-dependent perturbation theory and considering a long-range interaction potential. The relation between differential and integral interference angles is obtained. For the CO A 1Π(v=0)/e 3Σ(v=1)–He collision system, the calculated integral interference angles are consistent with the experimental values. The physical significance of interference angle and the essential factors it depends on as well as the influence of the short-range interaction on CQI are discussed.

For small radicals in mobile liquids, the anisotropy of the g-value and hyperfine tensors is normally averaged out by molecular rotations. In the very strong magnetic fields (1–30 T) now in use for chemical studies, this may not be true: Larmor precession is so fast that even small differences due to differences in orientation can have a significant effect. The `reversion' or reversal of direction of magnetic field effects on some radical pair reactions above 1 T may be due in part to this effect. The proposal is supported by numerical calculations.

The interaction potential of the lowest triplet excited state, a   3Σ+ u , for Li2 has been calculated using the ab initio method, QCISD(T) and the correlation-consistent valence polarised quintuple zeta basis set, cc-pV5Z. Equilibrium constants and vibrational levels for 7 Li 2 are in good agreement with experimental determinations. The a   3Σ+ u energy curve for Li2 is characterised at the QCISD(T, full)/cc-pV5Z level of theory by the parameters D e, D o, R e, ω e and ω e x e which are found to be 334.145 cm−1, 301.989 cm−1, 4.1686 Å, 65.400 cm−1 and 3.208 cm−1, respectively.

Spin-unrestricted time-dependent density functional theory (TDDFT) calculations for excited states of VO and MoO molecules have been undertaken to validate its applicability to highly open-shell systems. Equilibrium geometries, vibrational frequencies and excitation energies are compared with experimental data and ΔSCF DFT calculations where available. Overall good performance of TDDFT for intricate spectroscopic properties of transition metal (TM) oxides is found. Examples where discrepancies between experiment and theory could be expected are spotted and discussed.

Author index (439-446).