Chemical Physics Letters (v.323, #3-4)

The adsorption of isobutane on a Pt(533) stepped surface has been investigated using temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Isobutane exists in two monolayer states and one multilayer phase on the Pt(533) surface. The most stable desorption state is caused by isobutane desorbing from the terrace planes. The orientation of isobutane in this state was determined from RAIRS measurements. The second monolayer phase probably belongs to isobutane molecules with only two hydrogen atoms in direct contact with the surface. RAIRS spectra also showed there was little bond weakening due to metal adsorbate interaction.

Self-assembled monolayer of normal hexatriacontane (n-C36H74) is formed at the interface between a solution of n-C36H74 in dodecane and a reconstructed Au(111) surface. Two kinds of lamellae have been observed, in which the long axis of n-C36H74 molecule is either perpendicular to or tilted 63.5° to the trough of the lamellae. The packing structures in both kinds of the lamellae are different. Theoretical calculations based on molecular mechanics have been performed to elucidate the ways of the molecular packing in the self-assembled monolayers on gold.

Diffusion flame synthesis of single-walled carbon nanotubes by Randall L. Vander Wal; Thomas M. Ticich; Valerie E. Curtis (217-223).
Flame synthesis is demonstrated for the synthesis of single-wall nanotubes via a simple laboratory-scale diffusion flame. Results using different hydrocarbon reagents, the effects of dilution with an inert, the role of hydrogen and processes likely accountable for the deactivation of the catalyst particles are illustrated and discussed. Finally, a plot of the integrated residence time–temperature history of a fluid parcel along the flame axis indicates carbon nanotube growth occurring within 20 ms, thus demonstrating the great potential of flame synthesis for large-scale commercial production of nanotubes.

Growth of amorphous silicon nanowires via a solid–liquid–solid mechanism by H.F Yan; Y.J Xing; Q.L Hang; D.P Yu; Y.P Wang; J Xu; Z.H Xi; S.Q Feng (224-228).
Amorphous silicon nanowires (a-SiNW) with an average diameter of ca. 20 nm were synthesized at about 950°C under an Ar/H2 atmosphere on a large area of a (111) Si substrate without supplying any gaseous or liquid Si sources. The Si substrate, deposited with a layer of Ni (ca. 40 nm thick), served itself as a silicon source for the growth of the a-SiNWs. In contrast to the well-known vapor–liquid–solid (VLS) for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid–liquid–solid (SLS) mechanism, which is analogous to the VLS model.

Energetics of carbon clusters C20 from all-electron quantum Monte Carlo calculations by Svetlana Sokolova; Arne Lüchow; James B. Anderson (229-233).
The energetics of the ring, bowl, and cage isomers of C20 were determined in all-electron fixed-node quantum Monte Carlo calculations. In good agreement with results from valence-only quantum Monte Carlo calculations and from Møller–Plesset calculations with large basis sets, the calculations predict the lowest-energy isomer to be the bowl isomer. The energies of the ring and cage, relative to the bowl, are 1.1±0.5 and 2.1±0.5 eV, respectively.

2-(4'-N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) exhibits dual emission in sodium dodecylsulphate (SDS), cetyltrimethylammonium bromide (CTAB) and triton X-100 (TX-100) micelles. Normal and twisted intramolecular charge transfer (TICT) emissions are blue-shifted by 15–18 and 5–10 nm, respectively. In both cases, the DMAPPI molecule is located at the same site of the micelles. The greater stabilization in the TICT state has been attributed to the hydrogen bonding at the electron acceptor group. The enhancement in TICT emission is nearly the same in all the micelles, whereas the enhancement in normal emission is different in different micelles.

Magnetic properties of triangulo cobalt–hydride cluster [H3Co3(μ 2-CO)3(PMe3)6] by Roman Boča; Hans-Friedrich Klein; Andreas Schmidt; Marián Valko; Wolfgang Linert (243-248).
Transformation of [HCo(CO)(PMe3)3] in tetrahydrofuran under argon at 90°C yields a solid cobalt–hydrido cluster [H3(Co(CO)(PMe3)2)3]. The magnetic susceptibility down to 3.8 K confirms an antiferromagnetic coupling within the Co3 ring and the Co–H interaction is of the antiferromagnetic nature as well: J Co–Co=−192.4 cm−1, J H–Co=−570.0 cm−1, and g Co=2.402.

Exciton migration dynamics in a dendritic molecular aggregate by M. Nakano; M. Takahata; H. Fujita; S. Kiribayashi; K. Yamaguchi (249-256).
We investigate exciton migration dynamics in two types of dendritic molecular aggregate models, i.e., Bethe lattice, with mutually distinct intermolecular interaction between adjacent linear legs at the branching points. Each monomer molecule is assumed to be a dipole unit (a two-state model) coupled with each other by the dipole–dipole interaction. The generation of one exciton in the aggregates by the incident laser field and its subsequent dynamical behavior are investigated in a numerically exact manner. The two types of models show different exciton energy structures and distribution. It is also found that relaxation effects in the exciton states are essential for exciton migration from the periphery to the core.

Which carbon oxide is more soluble? Ab initio study on carbon monoxide and dioxide in aqueous solution by Hirofumi Sato; Nobuyuki Matubayasi; Masaru Nakahara; Fumio Hirata (257-262).
In disagreement with an intuitive prediction on the basis of the molecular size and dipole moment, it is observed empirically that the solubility of carbon dioxide (CO2) in water is larger than that of carbon monoxide (CO). In order to shed light on this puzzling behavior, the solvation free energy of CO and CO2 in aqueous solution is studied by means of the RISM-SCF/MCSCF method, a combined ab initio molecular orbital theory and statistical mechanics theory of molecular liquids. It is shown that the specific hydrogen bonding between oxygen atoms in CO2 and water molecules makes CO2 more soluble.

We study the singlet and triplet states of HF2 + at the B3LYP, BHandH, BHandHLYP, MP2, MP3, MP4 and QCISD(T) levels of theory with the 6-311++G(2DF,2PD) basis set. We report geometries, vibrational frequencies and vertical excitation energies. We find that the ground state of HF2 + is the C s HFF+ singlet with r(F–F) = 1.444, r(H–F) = 0.980 Å and α=101.96°; ω 1=857, ω 2=1145 and ω 3=3320 cm−1. For the triplet state, there are two stable structures: the C s HFF+ triplet (T e =0.79 eV) with r(F–F) = 1.799, r(H–F) = 0.962 Å and α=102.53°; ω 1=543, ω 2=734 and ω 3=3530 cm−1 and the Cv FHF+ triplet (T e =0.92 eV) with r(H–F) = 1.179 Å; ω 1=544, ω 2=900 and ω 3=1418 cm−1.

Spectrally resolved three pulse photon echoes in the vibrational infrared by M.C Asplund; M Lim; R.M Hochstrasser (269-277).
Spectrally resolved vibrational photon echo measurements of the N3 ion in D2O and carbonmonoxy hemoglobin in D2O have been made using 100 fs infrared pulses in the 1800–2100 cm−1 region. These measurements allow the separation of contributions from different Feynman pathways to the echo signal, allowing comparisons of the time dependence and correlation functions for the 1–0 and 2–1 coherence terms. The spectra show good agreement with a theoretical description of the data fitted to previously determined multiexponential correlation functions. Fluctuations in the anharmonicity are found to be negligible for HbCO when the harmonic vibrational relaxation model is used.

Theoretical studies of structures, thermodynamics, infrared spectra, and nature of interactions are presented for carbonium ions shellvated by molecular hydrogen, CH+ 3(H2) n (n=1, 9). The shell structure of the studied clusters has been determined based on the geometry, energetics, normal vibrations, and the nature of the interactions.

In this Letter, the reflection and transmission of plane waves are examined from a `true' complex potential. A simple and exact numerical procedure has been developed for treating the absorbing potential problem. Studying sample complex absorbing potentials, we could compare our exact step potential method and the grid-based time-dependent wavepacket approach. The results indicate convincingly that the step potential scheme works fast, produces accurate numbers and performs in good agreement with the grid-based wavepacket method.

The electronic structure of a single monatomic gold wire has been calculated with density-functional calculations taking into account relativistic effects. For stretched structures in the experimentally accessible range the conduction band is exactly half-filled, whereas the band structures are more complex for the optimized structure. Moreover, for these structures the system can lower its total energy by letting the bond lengths alternate leading to more or less separated dimers with bond lengths of about 2.5 Å, largely independent of the stretching. Finally, finite Au2, Au4, and Au6 chains possess electronic properties very similar to those of the infinite chain.

Characterization of dynamic optical nonlinearities in ytterbium bis-phthalocyanine solution by C.R Mendonça; L Gaffo; L Misoguti; W.C Moreira; O.N Oliveira Jr; S.C Zilio (300-304).
Dynamic optical nonlinearities of ytterbium bis-phthalocyanine (YbPc2) dissolved in chloroform were characterized using the Z-scan technique with pulse trains. Two contributions of opposite signs were observed for the nonlinear refraction: a fast process related to the singlet population, and a slow accumulative contribution arising from the triplet population and thermal lensing. As the fluence increases, the nonlinear absorption first presents a weak saturation, followed by a reverse saturable absorption process. We used a six-energy-level diagram to explain these results and to obtain spectroscopic parameters such as the excited state cross-sections and the intersystem crossing lifetime.

We have measured an ultraviolet action spectrum following excitation of cyanoacetylene between 212 and 264 nm. The low yield emission, principally to the red of 290 nm, is shown to be the product of multiphoton processes. Below 230 nm, secondary photolysis of metastable HC3N yields electronically excited C3N. Metastable HC3N can be monitored by separately exciting the metastables at 193 nm and then photodissociatively exciting them at 220 nm. The process leading to emission above 240 nm is more complex involving formation of a stable intermediate followed by its photodissociative excitation.

Femtosecond relaxation of photoexcited para-nitroaniline: solvation, charge transfer, internal conversion and cooling by S.A. Kovalenko; R. Schanz; V.M. Farztdinov; H. Hennig; N.P. Ernsting (312-322).
The ultrafast relaxation of p-nitroaniline (PNA) in water and acetonitrile is studied experimentally and theoretically. Transient absorption spectra are measured by the pump–supercontinuum probe technique (PSCP) after 50 fs excitation at 400 nm. The relaxation includes several stages with distinct time scales: solvation, intramolecular charge transfer (CT), internal conversion and cooling. The spectral evolution before 100 fs reflects mainly solvation with dynamic Stokes shift of 3500 cm−1 in acetonitrile and 4000 cm−1 in water. CT and internal conversion are governed by twisting of the –NO2 group and proceed in water with 120 and 250 fs, respectively. A hot ground state upon internal conversion is characterized by an initial temperature of 1400 K. The subsequent solute–solvent energy transfer is characterized by exponential behavior between 1 and 3 ps and by a nonexponential decay at longer delays, the solute cooling time lies in the range 0.85–1.3 ps.

Near-infrared laser-induced breakdown of liquid benzene by Kei Toyota; Satoru Nakashima; Tadashi Okada (323-328).
The reaction of liquid benzene induced by Nd3+:YAG fundamental pulsed laser irradiation was investigated. The emission spectrum of the breakdown was measured. The spectrum showed the emission to be assigned to d  3Πg – a  3Πu of C2 and the A  2Δ – X  2Π transition of CH radicals accompanied with the broad continuum emission. Picosecond transient absorption spectra did not show the absorption bands due to the electronically excited state of benzene. The breakdown threshold of various aromatic liquids is explained with the microwave breakdown theory. Considering these results, we concluded that the reaction is induced by the strong electric field of the laser.

Electron paramagnetic resonance and electron nuclear double resonance spectroscopy of a heme protein maquette by M. Fahnenschmidt; R. Bittl; H.K. Rau; W. Haehnel; W. Lubitz (329-339).
A de novo synthesized heme protein is characterized with EPR (electron paramagnetic resonance) and ENDOR (electron nuclear double resonance) spectroscopy. It consists of a four-helix bundle providing bis-histidine binding pockets for the paramagnetic cofactor heme. The EPR spectrum shows Fe3+ low-spin signals that were simulated on the basis of a g-strain line broadening mechanism. Pulsed-ENDOR spectra revealed proton and nitrogen resonances of axial histidine ligands. Simulation of histidine proton ENDOR signals yielded detailed structural information about the heme binding situation complementing the EPR spectroscopic observations.

Beneficial effect of heating on the morphology and second-order nonlinear optical efficiency of anisotropic thin films by S Van Elshocht; T Verbiest; G de Schaetzen; L Hellemans; K.E.S Phillips; C Nuckolls; T.J Katz; A Persoons (340-344).
In this Letter we describe the effect of heating on the quality, morphology and nonlinear optical efficiency of anisotropic Langmuir–Blodgett films composed of a chiral helicenebisquinone derivative. The samples were studied with second harmonic generation (SHG), UV–Vis absorption spectroscopy and atomic force microscopy (AFM). Heating a Langmuir–Blodgett film of a helicenebisquinone derivative improves the film quality (homogeneity, surface smoothness). In addition, it increases the film's anisotropy and, as a consequence doubles its nonlinear optical efficiency.

When a pulsed gas dynamically cooled supersonic molecular flow interacts with solid surface a cold shock wave is formed in front of it, non-equilibrium conditions in which may be `reverse' to those in the incident (unperturbed) flow. Isotopically selective infrared multiphoton dissociation of SF6 in the cold shock wave was studied. Anomalously a large increase (more than one order of magnitude) of the yield of products was found, as compared with the case of excitation of SF6 in unperturbed flow, without essential decrease of the selectivity of process.

Dual fluorescence and fast intramolecular charge transfer with 4-(diisopropylamino)benzonitrile in alkane solvents by Attila Demeter; Sergey Druzhinin; Mathew George; Edwin Haselbach; Jean-Luc Roulin; Klaas A. Zachariasse (351-360).
Dual fluorescence and fast intramolecular charge transfer (ICT) is observed with 4-(diisopropylamino)benzonitrile (DIABN) in alkane solvents. The rate constant k a for the reaction from the locally excited (LE) to the ICT state has a value of 3.4×1011 s−1 in n-hexane at 25°C, with an activation energy E a of 6 kJ mol−1. Efficient intersystem crossing with a yield of 0.94 takes place from the ICT state. With 4-(dimethylamino)benzonitrile, in contrast, dual fluorescence is not observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap ΔE(S 1,S 2), in accordance with the PICT model for ICT in aminobenzonitriles.

The Fourier transform infrared (IR) spectrum of the ν 6 band of methylene fluoride-d2 (CD2F2) has been recorded with an unapodized resolution of 0.006 cm−1 in the frequency range 2220–2340 cm−1. A total of 1111 IR transitions of ν 6 were used with Watson's A- and S-reduced Hamiltonians in the I r representation to derive rovibrational constants for the upper state (ν 6=1) up to four quartic terms. This relatively unperturbed band was found to be of C-type with an unperturbed band centre at 2283.73996±0.00007 cm−1.

The internal conversions of trans- and cis-1,3,5-hexatriene in cyclohexane solution studied with sub-50 fs UV pulses by Neil A Anderson; Charles G Durfee; Margaret M Murnane; Henry C Kapteyn; Roseanne J Sension (365-371).
Subpicosecond internal conversion dynamics of cis- and trans-1,3,5-hexatriene in cyclohexane were studied using sub 50 fs ultraviolet pulses. This time resolution allows direct observation of the 1B (S2) and 2A (S1) lifetimes. The 1B lifetime is 55±20 fs for trans-hexatriene, and ⩽50 fs for cis-hexatriene. The subsequent 2A lifetime of trans-hexatriene is found to be 190±30 fs, and is controlled by the rate of intramolecular vibrational energy redistribution. The 2A lifetime in cis-hexatriene is 250±30 fs, and is affected by modification of the excited state potential energy surface from coupling with the solvent.

Photo-response of the chemical oscillator composed of BrO3 , SO3 2− and Ru(bpy)3 2+ in the acidic aqueous solution was examined in a flow system. After establishing the dark state diagram, state diagrams have been determined taking P (the illumination light power) as one of the external parameters. In the P versus [SO3 2−] plane, the bifurcation between the reduced steady state and the oscillatory or oxidized steady state was found to be independent of P, occurring at a fixed critical concentration of SO3 2−. Qualitative discussions are given for this and other characteristic features in the system containing SO3 2− as a reductant.