Chemical Physics Letters (v.385, #3-4)

Field emission from camphor–pyrolyzed carbon nanotubes by Mukul Kumar; Keita Kakamu; Tsugio Okazaki; Yoshinori Ando (161-165).
Vertically aligned nanotubes, grown on various substrates by thermal decomposition of a botanical hydrocarbon––camphor, were investigated for field emission application. In a parallel plate (∼500 μm apart) field emission device, as-grown nanotubes exhibited a turn-on field as low as 2.6 V/μm, whereas the maximum current density observed was 14 mA/cm2 at an applied field of 7.7 V/μm. Utilizing such an emission efficiency of camphor-grown nanotubes, a model field emission microscope was fabricated with isolated nanotube emitters and informative emission patterns were observed on a fluorescent screen (placed at a working distance of 1–5 cm) at low applied voltages of 300–1000 V. Presence of localized density of states corresponding to pentagonal defects at the emitter tip was identified and discussed.

Inelastic neutron scattering (INS) spectra of NaBH4 and KBH4 are presented to 1500 cm−1. The phonon spectra contain detail unobserved in IR/Raman studies, including combination bands in the phonon and internal modes of KBH4. The BH4 ion vibrational frequencies observed in the solids at 15 K differ significantly from those observed for BH4 in liquid ammonia solution and normal mode frequency calculations for the isolated ion. The INS spectra and ammonia solution frequencies are compared with isolated anion and k=0 periodic DFT calculations to show that the periodic DFT results correctly predict the solid-state molecular vibrational frequencies.

Study of copper nanoparticles formation on supports of different nature by UV–Vis diffuse reflectance spectroscopy by A.N. Pestryakov; V.P. Petranovskii; A. Kryazhov; O. Ozhereliev; N. Pfänder; A. Knop-Gericke (173-176).
Supported copper particles are widely used in heterogeneous catalysis. Influence of supports of different nature on the Cu particles formation is studied by the methods of UV–Vis spectroscopy of diffuse reflectance, XRD and electron microscopy. Different states of supported copper have been identified by UV–Vis spectroscopy: absorption bands at 250 (Cu+), 320–370 and 400–440 (charge transfer bands of O–Cu–O and Cu–O–Cu complexes), 520–580 (Cu n plasmon resonance) and 620–850 nm (d–d transitions in Cu2+ ions). The size of Cu clusters and Cu n /Cu n δ+ ratio strongly depends on the support nature.

Study of the growth of boron nanowires synthesized by laser ablation by Yingjiu Zhang; Hiroki Ago; Motoo Yumura; Satoshi Ohshima; Kunio Uchida; Toshiki Komatsu; Sumio Iijima (177-183).
Boron nanowires with different morphologies and diameters have been fabricated by laser ablation. The effects of the synthesis temperatures, intensities of the laser beams, the types of the metal catalysts and the addition of H3BO3 on the growth of the boron nanowires are studied. The systematic analyses show that the vapor–liquid–solid (VLS) model may play important roles in the growth of the boron nanowires.

Electronic structure of a novel alkylidene fluorene polymer in the pristine state by W. Osikowicz; R. Murdey; M. Giles; M. Heeney; S. Tierney; I. McCulloch; W.R. Salaneck (184-188).
The electronic structure of a novel conjugated polymer, polyalkylidene fluorene has been studied using a combined experimental-theoretical approach. The densities of states in the valence band region of the new derivative, poly(9-(1-decylundecylidene)fluorene), were measured by ultraviolet photoelectron spectroscopy and compared with electronic band-structure calculations performed in the valence effective Hamiltonian framework. The results are compared with those of similar studies on the reference polymer poly(9,9-dioctylfluorene). We report the experimentally determined ionization potential for this new material and discuss the role of substitution in altering the electronic properties of the polymer backbone.

Using a competitive spin-trapping method, we determined the effect of high static pressure on the trapping rate of hydroxyl and phenyl radical in solution. Under various pressures, hydroxyl or phenyl radicals were produced in solution in the presence of two different spin-trapping compounds, and the yield of trapped products was used to calculate relative trapping rates. We found that external pressure increases the phenyl-radical trapping rates by approximately 40% at the pressure of 490 bar, while there was no pressure effect in hydroxyl-radical trapping rates. We interpreted the pressure effect based on the reactivity difference of phenyl and hydroxyl radicals to the spin trap which is mainly caused by the variation in the volume of the group.

Using a Chebyshev wave packet method, initial state specified (v i =0,j i =0) integral cross-section (over 0–0.9 eV) and rate constant (over 0–1500 K) are obtained for the title reaction on a new potential energy surface that has an accurate representation of the asymptotic regions. All partial wave contributions up to J=67 were calculated explicitly within the coupled states approximation. The rate constant confirms the validity of quasi-classical trajectory results, and both are in reasonably good agreement with experiment.

A valence bond study of the σ and π aromatic species Al4 2− by Remco W.A. Havenith; Joop H. van Lenthe (198-201).
Ab initio valence bond calculations were performed to assess the σ and π aromatic character of the all-metallic species Al4 2−. The results show that the σ system is composed from two independent systems (built from the radial and tangential p-orbitals), each containing two delocalised electrons, providing the conduction. The resonance energy of the σ system is significantly higher than that of the π system (123 vs. 40 kcal/mol), and the π resonance energy is substantially lower than that of the π isoelectronic hydrocarbon C4H4 2+ (167 kcal/mol).

Non-equilibrium molecular dynamics (NEMD) simulations were carried out to study the ionic conductivities of 8% yttria-stabilized zirconia (YSZ) at 900, 1273 and 1759 K. The conductivities estimated from NEMD simulations were compared with those calculated from diffusivities obtained in equilibrium molecular dynamics simulation according to the Nernst–Einstein relation. The agreement with equilibrium molecular dynamics results is better at low temperature whereas the agreement with experimental data is better at high temperature.

Functional expansion of non-uniform first-order direct correlation function (DCF) around bulk density is rewritten with the concept of weighted density, it is found that the resulting first-order expansion term is actually the first-order expansion term of the Taylor series expansion of the uniform first-order DCF with weighted density as its density argument, the higher-order terms are incorporated into the first-order expansion approximation by Lagrangian theorem. Based on the functional integration view, the present Letter devises a numerical procedure to predict excess Helmholtz free energy from the approximation for the non-uniform first-order DCF. The physical foundation of the present discussion also applies to electronic density functional theory.

An efficient procedure, based on the CASPT2 method using Dunning’s correlation-consistent bases and extrapolation to the limit of infinite basis, is proposed for studying difficult multiconfigurational species. The procedure is evaluated by calculating the ground state geometry, harmonic wavenumbers and quadratic and cubic force constants of ozone. The best results are obtained with the full-outer-valence (12,9) active space and extrapolation from Dunning’s weighted core-valence basis sets. The quality of these results, obtained using only modest computational resources, is comparable to computationally far more demanding treatments.

The morphology of self-assembled monolayer (SAM) islands of n-octadecyltrichlorosilane (OTS) deposited on chemically and thermally oxidized SiO2 surfaces was studied by dynamic force microscopy (DFM). The shape and size of the islands were found to depend significantly on the hydrophilicity of the SiO2 surface. Langmuir–Blodgett (LB) monolayers of a lipid, dipalmitoylphosphatidylcholine (DPPC), deposited on these SiO2 surfaces covered by OTS-SAM islands have shown that the DPPC monolayer is supported firmly on the SiO2 surface by the hydrophobic islands acting as anchor molecules.

Locality of the reduced-density-matrices: a numerical study by Benoit Bories; Stefano Evangelisti; Thierry Leininger; Daniel Maynau (225-230).
We have considered the expansion of the two-body reduced density matrices (2-RDM) in terms of the two-body cumulant (2-CRDM) which represents pure two-body correlation effects. It possesses also the advantage to be extensive contrarily to the 2-RDM. Thus, the 2-CRDM of a whole system should be well described in a restricted part of the system if one works in a localized basis set of orbitals. We studied the locality of the 2-CRDM, and consequently the correlation energy, for linear chain of hydrogen atoms H N (N=6,…,12) at different geometries of their dimerization. Our results are encouraging for developing low computational cost ab initio methods using the 1- and 2-RDM formalism.

In a recent paper one of us has exposed a link between the idempotent Dirac density matrix built from occupied Kohn–Sham orbitals and the frontier orbitals. An alternative argument is here presented, from the equation of motion of the density matrix generated by any local potential V( r ).

The photoexcitation of (NO)2 at 242–221 nm is studied by photoion and photoelectron imaging. A broad and structureless absorption band starting at 41,300 ± 300 cm−1 is observed. Ionization via the excited state accesses predominantly a dissociative state of (NO)2 +. The broad kinetic energy distribution of the photoelectrons suggests that the excited state has a large valence component in the Franck–Condon region, and that the geometries of the excited neutral state and the dimer ion state differ markedly. We propose that the same ‘bright’ state of mixed valence/Rydberg character is accessed at 242–200 nm, in agreement with preliminary ab initio calculations.

The products from the reaction of laser-ablated Ni atoms with CH4, O2 and NO have been studied via laser-induced and dispersed fluorescence spectroscopies. NiC, NiH and NiO species have been detected between 410 and 510 nm. Twenty NiO bands have been rotationally analyzed for the first time and, in most instances, measurements of the 58NiO/60NiO isotopic shifts have been made. Dispersed fluorescence data have been collected at two excitation wavelengths, corresponding to two prominent LIF bands – near 479.1 and 474.5 nm. Vibrational energies for all ground state levels up to v=9 have been determined.

Mobility of haloforms on ice surfaces by M.L. Grecea; E.H.G. Backus; H.J. Fraser; T. Pradeep; A.W. Kleyn; M. Bonn (244-248).
We have investigated the mobility of bromoform (CHBr3) and chloroform (CHCl3) on amorphous solid water and crystalline ice surfaces, by monitoring their adsorption and desorption behavior using temperature programmed desorption spectroscopy and reflection absorption infrared spectroscopy. Up to its desorption temperature, of 140 K, CHCl3 does not diffuse over the crystalline ice surface, whereas CHBr3 is found to be mobile at temperatures as low as 85 K. The results demonstrate distinct differences between the surface mobility of structurally similar haloform molecules on crystalline ice surfaces, which may have implications to the halocarbon chemistry occurring on atmospheric ice particles.

Extended moment formation and magnetic ordering in the trigonal chain compound Ca3Co2O6 by V. Eyert; C. Laschinger; T. Kopp; R. Frésard (249-254).
The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca3Co2O6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low- and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal–metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d3z2–r2 orbitals of the low-spin cobalt atoms.

Optically induced elasto (piezo-)-optics effect (EOE) in the benzene large-sized nanocrystallites with sizes 80–100 nm incorporated within the polyvinyl alcohol matrices was observed for the first time. A correlation between the number of macrovacancies measured by positron annihilation and the values of EOE coefficient indicates on appearance of enhanced static dipole moments caused by defects. Simultaneously changes of time kinetics of the EOE in picosecond time regime is observed. A substantial role of electron–phonon subsystem in the observed EOE is shown.

Cavity ring-down laser absorption spectroscopy of IrC by Tongmei Ma; J.W.-H Leung; A.S.-C Cheung (259-262).
The absorption spectrum of IrC at wavelength between 445 and 500 nm has been investigated using the technique of laser vaporization/reaction with free-jet expansion and cavity ring-down laser absorption spectroscopy. This wavelength region covers the (0,0), (1,0) and (2,0) bands of the L 2 Φ 7/2–X 2 Δ 5/2 transition. IrC molecules were produced by reacting laser vaporized iridium atoms and methane. Analysis of the spectra gives refined band origins, vibrational and rotational constants for the L 2 Φ 7/2 level.

Upconversion fluorescence spectroscopy of Er3+/Yb3+-codoped lead oxyfluorosilicate glass by Shiqing Xu; Zhongmin Yang; Junjie Zhang; Guonian Wang; Shixun Dai; Lili Hu; Zhonghong Jiang (263-267).
Upconversion fluorescence properties of a new Er3+/Yb3+-codoped lead oxyfluorosilicate glass under 975 nm excitation are investigated. The blue, intense green and red emissions centered at 408, 529, 545, and 667 nm, corresponding to the 2H9/2  →  4I15/2, 2H11/2  →  4I15/2, 4S3/2  →  4I15/2, and 4F9/2  →  4I15/2 transitions of Er3+, respectively, were simultaneously observed at room temperature. The important role of Yb2O3 in upconversion intensity is observed, and the influence of Yb2O3 and PbF2 on blue, green and red emissions is compared and discussed. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are evaluated.

N(2D) in nitrogen afterglow by J. Amorim; V. Kiohara (268-272).
Homogeneous inverse predissociation in the N2(C 3 Π u , v=4) state was investigated, in pure nitrogen afterglow, as a diagnostic tool for N(2D) density measurement. Vibrational distribution of N2(C 3 Π u ) was analysed along the afterglow employing emissions of the second positive system. For nitrogen afterglow with pressure ranging from 3.0 to 9.0 Torr, current between 20 and 50 mA and flux from 300 to 950 sccm, [N(2D)] was found to vary from 3% to 6% of [N(4S)].

The infrared spectroscopy of gaseous ions Fe(CH3OCH3)2 + and Fe(CH3OCH2CH2OCH3) n + (n=1–2) has been studied in the 800–2000 cm−1 energy range using the coupling of the free electron laser CLIO and an FTICR mass spectrometer, and compared to spectra calculated by ab initio quantum chemistry. The match between experimental and theoretical infrared spectra appears to be very good, the experimental spectra corresponding to the most stable structure predicted by the calculations. Characteristic absorption bands of functional groups have been evidenced, allowing this coupling to be a very powerful tool for structure elucidation in the gas phase.

Recently, a new trajectory method for advancing solutions of evolutionary partial differential equations was proposed: the derivative propagation method (DPM) evolves the solution and its spatial derivatives concurrently and eliminates the need for function fitting, finite-differences (FDs), etc. This trajectory method and a stationary lattice FD algorithm are applied in phase space to solve the classical Klein–Kramers and quantum modified Caldeira–Leggett equations for several examples: a double-well oscillator in contact with a thermal bath and the decay of a metastable state. For the latter potential, the trajectory and fixed-grid solutions are compared and any discrepancies are noted.

Jahn–Teller impurities in tetragonal lattices: Why is the ligand octahedron of Cu2+ in layered perovskites compressed? by J.M. Garcı́a-Lastra; J.A. Aramburu; M.T. Barriuso; M. Moreno (286-291).
Cu2+ impurities in tetragonal layered perovskites display a compressed geometry (R eq>R ax) along the crystal c-axis. Searching the microscopic origin of this fact DFT calculations have been performed on clusters simulating K2MgF4:Cu2+. Results for a 37 atoms cluster indicate that the difference, δ ec, between the energies for elongated and compressed minima along the c-axis is around 0.2 eV while R eqR ax⩽0.10 Å. This result is shown to come mainly from the tetragonal electrostatic potential due to the rest of ions on CuF6 4− which provides a gap, Δ, between ∼x 2y 2 and ∼3z 2r 2 orbitals when R eq=R ax. Although δ ec<Δ this gap is shown to play a key role for understanding the equilibrium geometry in tetragonal perovskites.

CCSDT study of the fluoroperoxyl radical, FOO by Pablo A. Denis; Oscar N. Ventura (292-297).
The FOO radical was studied at the coupled-cluster (CC) theoretical level, employing both the perturbative calculation of the contribution of triple excitations, CCSD(T), and the exact evaluation, CCSDT. Two solutions were found, one of them with a large spin contamination. It is shown that spin contamination is a problem at the CCSD(T) level, while it is largely inmaterial at the complete CCSDT level. The full CCSDT level affords an FO distance of 1.632 ± 0.005 Å in reasonable agreement to experiment. The enthalpy of formation at the uncontaminated CCSDT level, Δ f H 0 298(FOO)=6.5±1 kcal/mol, is also in very good agreement with the experimental value of 6.24 ± 0.5 kcal/mol. Contrary to previous studies, CCSDT performs better than CCSD(T) in the calculation of the properties of this radical.

Growth of vertically aligned single-walled carbon nanotube films on quartz substrates and their optical anisotropy by Yoichi Murakami; Shohei Chiashi; Yuhei Miyauchi; Minghui Hu; Masaru Ogura; Tatsuya Okubo; Shigeo Maruyama (298-303).
Films of vertically aligned single-walled carbon nanotubes (SWNTs) with a few micrometer thickness were grown by catalytic chemical vapor deposition (CVD) on quartz substrates. Low-temperature CVD from ethanol was performed by using densely mono-dispersed Co–Mo catalyst of ≈1.0–2.0 nm prepared on quartz substrates by a dip-coating method. Continuous reduction of catalysts with Ar/H2 (3% H2) during CVD was essential for generating dense enough SWNTs with vertical alignment. Vertical alignment was clearly demonstrated by anisotropic optical absorption and transmission characteristics in addition to observations by FE-SEM, TEM and resonance Raman scattering.

A detailed analysis of some vibronic bands of the S1  ← S0 electronic transition of 1,3-benzodioxole was carried out at high resolution. The rotational structures of these vibronic bands were completely assigned using a rigid rotor Hamiltonian model. From the rotational constants obtained by the assignment of the spectra it has been demonstrated that previous assignment of the spectrum was not completely satisfactory. The reassignment of the vibronic spectrum is necessary also for the correct first estimate of the entity of the anomeric effect in the S1 electronic excited state. It is evidenced that the experimental data obtainable with the high resolution electronic spectroscopy are mandatory when assigning vibronic spectra of floppy molecules.

Constrained molecular dynamics techniques were used to investigate the mechanism of the transfer of water and ammonia molecules across the liquid/vapor interface of water. The computed potentials of mean force were nearly constant when the solute was more than several angstrom from the Gibbs dividing surface and decreased with no substantial minimum free energy as they crossed the liquid/vapor interface. The computed solvation free energy for water, estimated from the potential of mean force, was in excellent agreement with the experimental measurement while the corresponding computed solvation free energy for the ammonia molecule somewhat over estimated the experimental value.

Reaction kinetic studies of CCl2 ( X ̃ (0,0,0)) with several simple molecules by Yunzhen Liu; Yao Xin; Linsen Pei; Yang Chen; Congxiang Chen (314-318).
Absolute reaction rate constants have been measured at room temperature for the ground state dichlorocarbene, CCl2 with a series of simple molecules: C2H4, NO, N2O, and halomethanes (T=297 K). The technique of laser-induced fluorescence was employed to obtain these rate constants. The reaction of CCl2  + CH2Cl2 was theoretically studied using a high-level ab initio calculation in order to gain an insight into the mechanism for the reactions CCl2  + halomethanes.

Photocatalytic H2 evolution under visible light irradiation on CdS/ETS-4 composite by Guoqing Guan; Tetsuya Kida; Katsuki Kusakabe; Kunio Kimura; Xiaoming Fang; Tingli Ma; Eiichi Abe; Akira Yoshida (319-322).
The photocatalytic activity of a CdS/ETS-4 composite for hydrogen production from water under visible light irradiation (λ>420 nm) was investigated. It is found that nano-sized CdS particles embedded in ETS-4 zeolite nano-pores showed stable photocatalytic activity in an aqueous solution containing Na2S and Na2SO3 electron donors and the energy conversion efficiency (ECE) was improved by combining CdS with ETS-4. The results suggest that the encapsulation of CdS in ETS-4 zeolite is effective for improving the activity as well as the stability of CdS.

Electronic properties of radial single-walled carbon nanotubes by Yoshinori Sato; Balachandran Jeyadevan; Rikizo Hatakeyama; Atsuo Kasuya; Kazuyuki Tohji (323-328).
We investigated the electronic properties of the radial single-walled carbon nanotubes (SWCNTs) by using the Raman and the UV–Vis–NIR spectroscopy. The radial SWCNTs for the analysis were synthesized by the arc-discharge method with Ce as a catalyst. The yield of the radial SWCNTs with 1.55 nm diameter and 60 nm length was as high as 20% in the soot. From the UV–Vis–NIR spectrum and the tangential mode of Raman spectra, the radial SWCNTs were confirmed to be composed of semiconducting (0.64 and 1.14 eV for Vs 1  → Cs 1 and Vs 2  → Cs 2) and metallic (1.60 eV for Vm 1  → Cm 1) nanotubes.

In a recent Letter by Pakoulev et al. [CPL 371 (2003) 594], time-resolved anti-Stokes Raman measurements of the spectral evolution and relaxation of the OH stretch vibrations of pure H2O were presented. These measurements led the authors to reinterpret the results of previous femtosecond mid-infrared pump-probe studies on water. Here we show that this reinterpretation is incorrect.

Both IR and anti-Stokes Raman measurements on OH stretch vibrations ν OH of water see two time constants, ∼0.3 and ∼0.6 ps. We attribute the former to spectral diffusion and the latter to the ν OH lifetime. This interpretation is evident from our Raman spectra, which show the ν OH spectrum changing shape and the ν OH population decaying. The ν OH decay is accompanied by the rise (∼0.6 ps) and subsequent fall (∼1.4 ps) of population in the δ OH bending vibration. Bakker et al. [Chem. Phys. Lett. 385 (2004) 329] assert their IR data indicates an absence of (>100 fs) spectral diffusion, that the faster time constant is the ν OH lifetime and that the slower is the lifetime of a conjectured intermediate state. The properties of this conjectured intermediate differ significantly from the bending vibration directly observed by us. The intensity analysis by Bakker et al. said to contradict our interpretation relies entirely on ad hoc assumptions about excited-state absorptions. We show these ad hoc assumptions are unfounded and some are unreasonable, and that a plausible explanation for the IR intensities exists that is consistent with our interpretation.