Chemical Physics Letters (v.378, #5-6)

Spin–orbit induced predissociation of the B2Σg and 22Σg + states of the O2 + ion by A. Mitrushenkov; P. Palmieri; G. Chambaud; P. Rosmus (463-469).
Highly correlated MRCI electronic wavefunctions are used to calculate the potential energy functions, the non-adiabatic and the spin–orbit coupling elements for the B2Σg and neighboring states of the O2 + ion. The results are employed in calculations of the spectroscopic constants and the spin–orbit induced predissociation lifetimes using the complex scaling method. The dominant predissociation path correlating with the O + O+ atoms in their ground states is found to proceed via the f4Πg state. The path leading to the O(1D) + O+(4S) asymptote involves couplings with the 22Πg and 24Πg states. The weak vibrational progression embedded in the progression of the B-state is assigned to the 22Σg + state. The predissociative lifetimes are calculated to be distinctly longer than the apparent experimental lifetimes reported previously.

Low defect density and planar patterned SOI materials by masked SIMOX by Yemin Dong; Xi Wang; Xiang Wang; Meng Chen; Jing Chen (470-473).
Patterned silicon-on-insulator (SOI) materials have been fabricated by masked separation by implantation of oxygen (SIMOX) technique. The formed SOI structure was analyzed by cross-sectional transmission electron microscopy (XTEM). The patterned SOI materials prepared with low-dose and low-energy SIMOX technique exhibit high quality featured by defect free transition of SOI and bulk silicon region and high degree of surface planarity. Furthermore, the buried oxide (BOX) layer is of high integrity at such a low-dose ion implantation. The fabricated excellent quality patterned SOI materials will be the desirable substrates for system-on-a-chip (SOC) applications.

Irradiation of CW-CO2 laser on a powder target. Formation of fullerene film from graphite powder by Seisuke Kano; Masamichi Kohno; Kaname Sakiyama; Shinya Sasaki; Nobuhiro Aya; Hirofumi Shimura (474-480).
A process was investigated to irradiate a continuous-wave CO2 laser (4.5 kW) on graphite powder with a mean diameter of 5 μm in a flow of Ar or He gas at 30 or 200 Torr. The film produced on a Ni substrate in the Ar flow at 200 Torr showed the Raman bands of C60 and C70, and the C60/C70 ratio was estimated to be 5/1 by MALDI-TOF mass spectrometry. The film formation process is discussed on the basis of an analysis of laser-irradiated powder, which included 0.01 wt% of C60, by UV–Vis absorption, and by observations of the plumes and SEM images.

Porphyrins are functional dyes with a variety of unique physical (photophysical), chemical and biological properties. We report on the first finding that porphyrin molecules dissolve single-walled carbon nanotubes (SWNTs) in organic solutions and that the SWNTs–porphyrin hybrid nanomaterials can be separated from the solutions. Atomic force microscope investigation revealed that both individually dissolved nanotubes and bundled nanotubes coexisted in the solution. The solid purified SWNTs (p-SWNTs)–porphyrin nanomaterials were readily separated from the p-SWNTs–porphyrin solution, and this nanomaterial was re-dissolvable in DMF. Measurements of its fluorescence spectrum gave evidence for the interaction between the nanotube sidewall and porphyrin molecules in solution.

Collision energy dependence of the Penning ionization cross-section for the Kr(3P) + (CH3)3CBr → Kr + (CH3)3CBr+  + e reaction was determined under orientation-angle resolved condition using an oriented (CH3)3CBr beam and time-of-flight measurements. A remarkable resonance-type structure was found in the energy dependence of orientation-angle resolved Penning ionization cross-section. This resonance-type structure could be interpreted as a new-type ‘nuclear-excited Feshbach resonance’ in the formation of vibrational excited Rydberg states leading to a competitive dissociative exit channel.

Time-resolved Fourier transform infrared study of the 193 nm photolysis of SO2 by Yuchuan Gong; Vladimir I. Makarov; Brad R. Weiner (493-502).
The 193 nm photolysis of SO2 has been studied by monitoring the infrared emission of the vibrationally excited species using time-resolved FTIR emission spectroscopy. Evidence for a direct electronic transition from the C state to the ground state of SO2 was found in addition to photodissociation following the 193 nm excitation. Relaxation rate constants for the observed vibrationally excited species, SO, SO2(ν 1), and SO2(ν 3), were calculated from the experimental decays.

A method for an accurate treatment of the electronic correlation in multi-reference systems using localized orbitals has been recently proposed. However, for some systems, the physics impose that they cannot be described only from local contributions. In that case, it is impossible to use only localized orbitals. The particular example presented here concerns the study of a chain of transition metal atoms (nickel and copper). It appears that the d orbitals may be kept local, while the s electrons are described by orbitals that are delocalized on the whole shape of the molecule.

The effect of a substituent group in dimeric complexes of benzene with ethylene and its four derivatives on the intermolecular CH/π interaction was studied theoretically. The hydrogen-bonding nature of the CH/π interaction is confirmed by the bond critical point analysis within the atoms-in-molecules (AIM) theory. It has been found that the CH⋯π access angle depends on the exchange repulsion and the induction effect from the substituent group. The effect of the charge transfer interaction by the substituent group is energetically insignificant. However, the charge density can be varied by substitution, and this contribution cannot be ignored.

Efficient production of B-substituted single-wall carbon nanotubes by E. Borowiak-Palen; T. Pichler; G.G. Fuentes; A. Graff; R.J. Kalenczuk; M. Knupfer; J. Fink (516-520).
We present an efficient method to achieve high-purity boron doped single-walled carbon nanotubes (SWCNT) using an adapted substitution reaction. We observe that around 15 at.% of the carbon atoms are substituted by boron, with local concentrations of up to 20 at.% (higher than previous report). The as-prepared material was characterised by local scale method: transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) as well as by bulk sensitive methods like optical spectroscopy in the infrared energy range (IR), X-ray photoemission spectroscopy (XPS) and bulk sensitive high-resolution EELS.

Adsorption states of nitrogen on tungsten pentamers supported on a highly oriented pyrolytic graphite surface have been investigated using X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy. An adsorption state that is not seen on a bulk polycrystalline tungsten surface was observed at temperatures below room temperature. It has been found that the adsorption state is a molecular state and is a precursor to dissociation of N2 on the tungsten nanocluster.

Activation of Au nanoparticles on oxide surfaces: reaction of SO2 with Au/MgO(1 0 0) by J.A. Rodriguez; M. Pérez; T. Jirsak; J. Evans; J. Hrbek; L. González (526-532).
Photoemission, thermal desorption spectroscopy and density-functional calculations were used to study the chemistry of SO2 on Au nanoparticles supported on MgO(1 0 0). The heat of adsorption of the molecule on the Au nanoparticles is ∼15 kcal/mol compared to 8 kcal for SO2 on Au(1 1 1). corner sites in the Au nanoparticles are responsible for this difference in reactivity. The dissociation of SO2 on Au/MgO(1 0 0) is very limited due to weak Au ↔ MgO interactions. A comparison of the behavior of SO2 on Au/MgO(1 0 0) and Au/TiO2(1 1 0) shows how important can be the effects of the oxide support for the activation of Au nanoparticles.

The quantum interference pattern in the double-slit experiment is qualitatively reproduced by independent classical trajectories evolving on an effective potential obtained in the lowest order quantized Hamilton dynamics approach. The quantum effects appear due to a simple modification of the original classical potential that is sufficient to obtain a diffraction maximum in the middle between the two slits and an auxiliary diffraction structure on the sides of the central maximum.

The equilibrium structure of the negatively charged water dimer (H2O)2 has been studied using the path-integral molecular dynamics simulation. All the atomic motions as well as the excess electron were treated quantum mechanically, employing a semi-empirical model combining a water–water interatomic potential with an electron–water pseudopotential. It is demonstrated that the molecular structure of (H2O)2 is more flexible than that of (H2O)2; both the donor switching and donor–acceptor interchange can more effectively occur in (H2O)2 than in (H2O)2. We conclude that this floppy character is a result of the breakdown of the adiabatic Born–Oppenheimer picture.

Lifetime lengthening and the Renner–Teller effect in the HCF ( A ̃ 1 A″← X ̃ 1 A ) system by Haiyan Fan; Ionela Ionescu; Chris Annesley; Scott A Reid (548-552).
We report new measurements of fluorescence excitation spectra and single rovibronic level fluorescence lifetimes for (0,υ 2 ,0)←(0,0,0) bands with υ 2 =0–6 in the HCF A ̃ 1 A″← X ̃ 1 A system. Due primarily to the Renner–Teller interaction, upper state levels with K a⩾1 display an obvious lengthening of lifetime with increasing energy which is strongly correlated with the presence of vibronic and rovibronic perturbations, and increases dramatically as the barrier to linearity is approached. This system provides a textbook example of the small molecule limit of radiationless transitions.

Photoinduced birefringence in bulk azodye-doped hybrid inorganic–organic materials by a femtosecond laser by Jiayu Guo; Jinhai Si; Guodong Qian; Jianrong Qiu; Minquan Wang; Kazuyuki Hirao (553-558).
The photoinduced birefringence induced by a femtosecond laser through two-photon absorption was studied in bulk azodye-doped hybrid inorganic–organic materials. The probe transmittance for the induced birefringence was estimated to be 92% after irradiating disperse red 1-doped silica materials with the femtosecond writing beam. The relaxation characteristic of the induced birefringence was also investigated. Although the signal decay process was fast, these photoinduced birefringence materials might have wide applications in optical image storage.

Line shapes and photon echoes within a generalized Kubo model by J.R Schmidt; N Sundlass; J.L Skinner (559-566).
Kubo introduced a stochastic model of frequency fluctuations that naturally leads to an understanding of homogeneous and inhomogeneous broadening in condensed phase spectroscopy. In this Letter we discuss a generalized Kubo model that has two different timescales. The model is relevant for a variety of different physical situations, and in certain limits describes combined homogeneous and inhomogeneous broadening. We explore the limits of applicability of this homogeneous/inhomogeneous phenomenology for both the line shape and the integrated two-pulse photon echo intensity. Our results have implications for non-linear ultrafast liquid-state spectroscopy.

Experimental and theoretical study of spin evolution ‘freezing’ of the radical ion pair in MARY spectroscopy by V.N Verkhovlyuk; V.A Morozov; D.V Stass; A.B Doktorov; Yu.N Molin (567-575).
The Letter presents a method of describing the process of single charge transfer in the partner of a radical ion pair, accompanied by changes in its hyperfine structure. In the particular case of ‘freezing’ of spin evolution in the pair due to collapse of hyperfine interactions upon single charge transfer and in the approximation of exponential recombination kinetics the suggested model describes MARY spectra for three important classes of processes in nonpolar solutions: chemical decay of the pair partner, ‘dark capture’ of the partner, and slowing down of spin evolution in the partner upon abrupt narrowing of its ESR spectrum.

Enhanced entanglement of two atoms confined in a multi-mode optical cavity by Takashi Taneichi; Takayoshi Kobayashi (576-581).
A numerical calculation of entanglement dynamics between two non-resonant atoms interacting with a multi-mode optical cavity field has shown that the field enhances the degree of entanglement between the two atoms. Using the position of one atom of the pair as a controlling parameter, entanglement dynamics is analyzed in terms of the entanglement of formation, making a comparison with the dynamics in a single-mode cavity field. It is shown that the degree of entanglement of the present system is enhanced by the usage of multi-mode cavity and by positioning the atom closely at the node of the field mode.

Absolute Raman cross-sections are reported for fourteen normal modes of betaine-30 dye in CH3OH, CH3OD, and CD3OD solution, at five different excitation frequencies within the visible absorption band. Consistent with previous results from our lab, the Raman intensities of betaine-30 in CD3OD solution are generally stronger than in CH3OH solution. For most of the betaine-30 normal modes, however, the resonance Raman cross-sections are similar in CH3OD and CH3OH solution. The results suggest that methyl group motion, more so than hydroxyl group motion, is responsible for dephasing of the resonant electronic transition of betaine-30 in methanol.

Molecular orbital analysis based on fragment molecular orbital scheme by Hideo Sekino; Yasuo Sengoku; Shinichiro Sugiki; Noriyuki Kurita (589-597).
Dipole and quadrupole moments computed in the fragment molecular orbital (FMO) scheme reproduce the results from the full molecular orbital (MO) theory within a few percent error. It is also shown that the FMO molecular orbitals for creating the FMO density matrix of each fragment provide qualitatively correct information on the chemical active sites of molecular aggregates in comparison with the full MO counterpart. The FMO also provides correct HOMO for single strand DNA, while the ordering of the LUMO among the fragments is not correct.

π-Molecular orbitals in fullerenes and the free electron model by Naomi Mizorogi; Masaki Kiuchi; Kumiko Tanaka; Rika Sekine; Jun-ichi Aihara (598-602).
π-Molecular orbitals (π-MOs) in fullerenes were characterized by reference to energy levels of the electron gas bound on the surface of a sphere. We plotted all π-MOs in many fullerene isomers on a single energy scale and obtained the quasi density-of-state (DOS) curve for the fullerene. The quasi DOS curve on the low-energy side consists of several distinct peaks, which were found to correspond to the electronic shells of the free electron model. The quasi DOS near the non-bonding level is small like that of a graphite sheet, indicating that fullerenes have a relatively large HOMO–LUMO gap energy.

A new local control theory applicable to tracking control of wave packet dynamics is developed. In order to achieve the wave packet tracking, a new performance index is introduced with multiple moving target wave packets, each of which evolves under the field-free condition and satisfies intermediate/final conditions associated to the tracking point. The present theory is applied to the one-dimensional model system of hydrogen fluoride.

The complex permittivity of multi-walled carbon nanotube–polystyrene composite films in X-band by P.C.P. Watts; D.R. Ponnampalam; W.K. Hsu; A. Barnes; B. Chambers (609-614).
The complex permittivity of various carbon nanotubes incorporated into polystyrene films is studied within X-band (8–12 GHz). Films containing defective carbon nanotubes exhibit higher real permittivity than values obtained from graphitic nanotube composites.

A wavepacket study of the dynamics of H2 formation through a collinear Eley Rideal mechanism that explicitly handles the substrate relaxation effects is presented. The substrate used is a planar polycyclic aromatic hydrocarbon (PAH) which exhibits some strong similarities with a perfect graphite (0 0 0 1) surface. The collision energy range investigated lies between 0.4 meV (T=5 K) and 0.46 eV. The reaction probability is large, except at very low collision energy. Most of the available energy goes into H2 vibration. The substrate relaxation facilitates the reaction.

A new interatomic potential for nanoscale silica by E Flikkema; S.T Bromley (622-629).
A new interatomic potential, tuned to calculate the energies and structures of nanoscale silica, is presented. The potential parameters for Si–O, O–O, and Si–Si interactions were fitted using a training set consisting of a number of (SiO2) n (n⩽5) nanocluster structures calculated using density functional theory (DFT). Testing of the potential was performed using 25 different (SiO2)7 nanoclusters. For nanocluster energies and structures, the new potential out-performs other similar potentials widely used for bulk silica and also performs favourably compared with semi-empirical methods. In addition, using a genetic algorithm with the new potential, two novel (SiO2)7 ground state structures are proposed.

Femtosecond two-photon-absorption-resonant four-wave mixing for time-resolved studies of photochromism in three dimensions by S.O Konorov; D.A Sidorov-Biryukov; I Bugar; D Chorvat; D Chorvat; A.M Zheltikov (630-637).
Four-wave mixing (FWM) with two-photon absorption (TPA) resonances is shown to allow time-resolved studies of ultrafast processes involved in photochromism in three-dimensional spiropyran/PMMA samples on the femtosecond time scale. An FWM signal was produced using 130-fs pulses of 800-nm Ti:sapphire-laser radiation as a TPA-resonant pump and frequency-tunable pulses of an optical parametric amplifier as a time-delayed probe. Two-photon resonances allow strong one-photon absorption to be avoided, giving an access to any point inside a 3D photochromic sample. An ultrafast dynamics of photochromic transformations is revealed by comparing the time-resolved FWM signal with the cross-correlation of pump and probe pulses.

Molecular theory of an electrochemical double layer in a nanoporous carbon supercapacitor by Ayumi Tanimura; Andriy Kovalenko; Fumio Hirata (638-646).
We develop the replica RISM theory of electrolyte solution sorbed in a nanoporous carbon electrode. The model comprises carbon nanospheres forming a disordered network with the porosity, pores sizes and surface area fitted to carbonized polyvinylidene chloride (PVDC) material, and to activated carbon. We obtained the huge capacitance comparable to that achieved in supercapacitors, and found the higher capacitance per pores surface for carbonized PVDC material with uniform nanoporous texture than for activated carbon with micro- as well as nanopores. Unlike a planar electrochemical double layer with the voltage dominated by the inner layer, the nanoporous supercapacitor voltage is driven by the solvation chemical potentials of the sorbed ions.

Rydberg states: sensitive probes of molecular structure by Narayanan Kuthirummal; Peter M Weber (647-653).
Using the isomers naphthalene and azulene, and isomeric forms of the cyclohexadiene/hexatriene system, we demonstrate that the energies of Rydberg states are sensitive to the geometrical structure of the molecular ion core. Rydberg states with low quantum numbers are conveniently accessed using multi-photon excitation via valence states, providing spectra with intensity distributions that depend sensitively on the molecular isomeric form. This discovery opens up the possibility of using Rydberg states to fingerprint the shapes of molecules. Because of the large size of the Rydberg orbitals, the Rydberg fingerprint methodology can have applications in the characterization of biological and nanoscale structures.

Photoelectron and intervalence absorption spectra of a cofacially stacked μ-oxo silicon phthalocyanine dimer are interpreted as arising from one of two possible sinusoidal forms of the inter-ring electronic coupling J as a function of rotation about the μ-oxo linkage. Of these, only one corresponds qualitatively to the ‘theoretical predictions of Pietro, Marks and Ratner, with J having extremum values of 0.32 eV (eclipsed) and −0.16 eV (staggered). The eclipsed inter-ring coupling is found to be insensitive to the form of the individual molecular orbitals involved, and spectra for the corresponding trimer are successfully interpreted in terms of an exciton model.

A simple method to synthesize gallium oxide nanosheets and nanobelts by Xu Xiang; Chuan-Bao Cao; Ya-Jun Guo; He-Sun Zhu (660-664).
β-Ga2O3 nanosheets and nanobelts have been synthesized by evaporating metallic gallium source in a flow of argon gas. X-ray powder diffraction and transmission electron microscopy investigations revealed that the as-synthesized products are pure, single-crystalline β-Ga2O3 with monoclinic structure. Energy dispersive X-ray analysis indicated the products are free from any other elements in addition to gallium and oxygen. The nanosheets of Ga2O3 had rectangular geometric shape. The nanobelts had uniform width along its entire long axis, and were in the range of 100–400 nm widths and several tens of micrometer in lengths. The growth process of β-Ga2O3 nanosheets and nanobelts may be dominated by vapor–solid mechanism as well as kinetics.

Inelastic neutron scattering (INS) spectrum of tetracyanoquinodimethane (TCNQ) by A. Pawlukojć; I. Natkaniec; G. Bator; L. Sobczyk; E. Grech (665-672).
The vibrational spectra of tetracyanoquinodimethane (TCNQ) in the solid state were studied using the inelastic neutron scattering (INS) technique. The detailed analysis of INS spectra showed advantages of this technique, particularly when analyzing the low frequency (below 1200 cm−1) region. The INS as well as Raman and infra-red (IR) spectra were compared with simulated ones using the Gaussian 98, Gamess and auntieClimax programs. All internal low frequency modes predicted by the calculations were well manifested. One observed an excellent agreement between experimental and calculated INS band intensities. The problem of the frequency scaling factor and an appearance of the summation frequencies was discussed.

Author Index (673-684).