Chemical Physics Letters (v.333, #3-4)

Fluorescence stability of 8-hydroxyquinoline aluminum by Gui Yu; Dezhen Shen; Yunqi Liu; Daoben Zhu (207-211).
Physical and chemical characteristics of 8-hydroxyquinoline aluminum (Alq3) have been investigated in this work. The fluorescence stabilities of Alq3 films were compared before and after UV irradiation. They were analyzed by using a UV-Vis absorption spectrophotometer, an in situ fluorescence spectrometer, attenuated X-ray photoelectron spectroscopy, and a nanosecond fluorescence spectrophotometer. Degradation in photoluminescence properties was significant in the presence of humid air (at an 85% relative humidity). These results indicate that water and oxygen aid the photodegradation, and the degradation mechanism of Alq3 in the presence of H2O and O2 is discussed in terms of chemical reactions.

Thin films of tris-(8-hydroxyquinoline) aluminum (Alq3) were exposed to trace amounts of O2, CO2, H2O, or to ambient air. Evolution of electronic structures of Alq3 films with increasing gas exposure was measured using ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). The vacuum energy level, the highest occupied molecular orbital, and XPS core levels of the constituting elements in Alq3 shifted according to the kind of gas exposure. Chemical reaction between oxygen and the Alq3 films was observed upon oxygen exposure. Moreover, it was found that the dominant influence of ambient conditions on the electronic structures of the Alq3 films was from H2O.

Behavior of a hydrophobic fluid containing a hydrophilic component at low concentration, which is confined between macroparticles separated by distance L, is analyzed using the reference interaction site model and hypernetted-chain theory. When the hydrophilicity of the macroparticle surface is sufficiently high, the following is observed. A layer within which particles of the hydrophilic component are enriched is formed around each macroparticle. As L decreases the two enriched layers within the confined domain continue to grow, eventually leading to a phase transition. The range of the macroparticle interaction induced can be far longer than the molecular scale.

Pressure-induced dimerization of fullerene C60: a kinetic study by V.A Davydov; L.S Kashevarova; A.V Rakhmanina; V.M Senyavin; O.P Pronina; N.N Oleynikov; V Agafonov; R Céolin; H Allouchi; H Szwarc (224-229).
The kinetics of pressure-induced dimerization of fullerite C60 at 1.5 GPa in the 373–473 K temperature interval was studied by X-ray diffraction, infrared and Raman spectroscopy. Kinetic curves of the dimerization reaction in the fcc and sc phases of C60 were obtained by monitoring the dimeric (C60)2 IR line at 796 cm−1. The value of the dimerization activation energy was determined to be E a(dim)=134±6  kJ mol −1 , assuming the second order irreversible reaction. The peculiarities of the dimerization processes in the fcc and sc phases of C60 fullerite are also discussed.

The infrared spectra of co-condensed nitrous oxide and acetylene are reported. It is shown, from these spectra that, in addition to crystalline nitrous oxide and acetylene, two further crystalline phases, each having the stoichiometry N2O·C2H2, are formed. When cooling rates are high the less stable of these phases can be the only phase formed: however, at slower rates of cooling the more thermodynamically stable phase predominates. It is argued that the two phases differ in the relative orientations of the dipole moments of the constituent nitrous oxide molecules.

Long-lasting phosphorescence in oxygen-deficient Ge-doped silica glasses at room temperature by Jianrong Qiu; Alexander L Gaeta; Kazuyuki Hirao (236-241).
We report on a novel phenomenon in oxygen-deficient Ge-doped silica glasses at room temperature. Irradiation of focused 120 fs laser pulses at 800 nm induced long-lasting phosphorescence with peaks at 290 and 390 nm for oxygen-deficient Ge-doped silica glass. The phosphorescence persisted for not less than 1 h after the removal of the irradiating light. The intensity of the phosphorescence at 390 nm increased with an increase in the concentration of oxygen-deficiency associated with Ge ions. Based on the time dependence of the intensity of the phosphorescence, the long-lasting phosphorescence in these glasses is considered to be due to the thermally activated electron–hole recombination at shallow traps.

Fluorescence-detected magnetic resonance and photoconductivity-detected magnetic resonance are simultaneously applied to the system of a radical-ion pair formed in the photoinduced electron transfer reaction between pyrene and 1,3-dicyanobenzene. The former monitors the exciplex fluorescence via the singlet radical-ion pair, while the latter monitors free ionic radicals escaping from the radical-ion pair. Both spectra show good coincidence, which shows that the population of the singlet state is proportional to the total radical-ion pair one. The data obtained from the photoconductivity-detected magnetic resonance method suggest the faster quenching reaction of the triplet state of the radical-ion pair than that of the singlet state.

Acetic acid dimers in liquid CCl4 have been studied by picosecond infrared double resonance spectroscopy in the 2000 to 3800 cm−1 region. A subpicosecond redistribution of the vibrational energy among the various modes within the broad OH band of the hydrogen-bonded dimer ring was found; an effective vibrational lifetime of 4.0±1.5 ps was measured for the whole ensemble. The additionally observed slower decay component of 15 ps is attributed to nanoscopic thermal diffusion and the reaction of the hydrogen bond system to the elevated temperature.

Conformational rearrangements in and twisting of a single molecule by J Hofkens; T Vosch; M Maus; F Köhn; M Cotlet; T Weil; A Herrmann; K Müllen; F.C De Schryver (255-263).
Single molecule spectroscopy is used to obtain detailed information on the photophysical properties of immobilized perylenediimide-based molecules, substituted in the bay positions. The fluorescence spectra recorded for numerous single molecules show a clear bimodal distribution of the peak position. Within the low energy component of the distribution, two different vibronic shapes of the emission spectrum can be seen, which can be correlated to different decay times. We show that former observation can be explained by conformational changes of the bay substituents while the latter are related to twisting of the single molecule around the central perylenediimide long axis.

Pressure-induced transformations of cristobalite by L.S Dubrovinsky; N.A Dubrovinskaia; S.K Saxena; F Tutti; S Rekhi; T Le Bihan; Guoyin Shen; J Hu (264-270).
X-ray in situ studies in electrically and laser-heated diamond anvil cells (DACs) at pressures over 80 GPa and temperatures above 2500 K were used to determine stable silica phase at extreme conditions. We demonstrate that so far unidentified phases obtained on the compression of α-cristobalite, new dense silica polymorph in the Martian Shergotty meteorite, and controversial post-stishovite phase is α-PbO2-like silica.

Kinetics of photogenerated electrons in platinized TiO2 (P-25) particles were observed by time-resolved infrared (IR) absorption spectroscopy with AC-coupled detection of cw probe light. The sample irradiated by UV pulse (355 nm and 10 ns) gave transient absorption which monotonously increased in intensity with decreasing wavenumber from 3000 to 900  cm −1 . Photogenerated electrons trapped in shallow mid-gap states were proposed to originate the transient absorption. The decay kinetics of the trapped electrons were fitted to a multi-exponential formula with six lifetimes ranging from 1×10−7 to 3×10−1 s.

We have performed kinetic Monte Carlo simulations for diffusion of methane, perfluoromethane and 2-methylhexane in silicalite to study the inter-relations between self-, jump- and transport-diffusivities. Both the self- and jump-diffusivities were found to decrease with occupancy, or loading, within the zeolite matrix. Correlation effects cause the self-diffusivity to be lower in value than the jump-diffusivity. Using the Maxwell–Stefan theory for diffusion we derive a simple formula to relate the self- and jump-diffusivities.

Cavity-enhanced absorption spectroscopy of methane at 1.73 μm by H.R Barry; L Corner; G Hancock; R Peverall; G.A.D Ritchie (285-289).
The integrated cavity output spectroscopy technique has been applied to the study of methane near 1.73 μm using a tunable diode laser. We have shown that this simple approach produces accurate high resolution spectra and have achieved a detection sensitivity of 1.8×10−7 cm−1 for mirror reflectivities of 99.84%.

Hydration of methanol in water. A DFT-based molecular dynamics study by Titus S. van Erp; Evert Jan Meijer (290-296).
We studied the hydration of a single methanol molecule in aqueous solution by first-principle DFT-based molecular dynamics simulation. The calculations show that the local structural and short-time dynamical properties of the water molecules remain almost unchanged by the presence of the methanol, confirming the observation from recent experimental structural data for dilute solutions. We also see, in accordance with this experimental work, a distinct shell of water molecules that consists of about 15 molecules. We found no evidence for a strong tangential ordering of the water molecules in the first hydration shell.

Two-photon spectroscopy is used to measure the frequency of the Kekulé vibrational frequency of several benzene derivatives in the first excited state (S1). It is found that the frequency is always increased with respect to the ground state, and that it is essentially constant. This finding is in line with a model presented by the authors, in which the ground and first excited states of benzene are treated as `twin states'. The physical nature of this vibration is discussed in view of quantum chemical calculations.

Low energy deposition of size-selected Si clusters onto graphite by R Neuendorf; R.E Palmer; R Smith (304-307).
Molecular Dynamics simulations have been performed to describe the deposition of size-selected silicon clusters onto a graphite surface. The cluster sizes range from N=9 to N=200 atoms per cluster, deposited with kinetic energies from E=0.5  eV to E=2.0  eV per atom. We find that the clusters remain mainly intact on top of the graphite substrate after deposition. The degree of cluster deformation, i.e., the shape of the cluster on the surface, can be controlled via the deposition energy, from the preservation of a 3D cluster morphology to surface wetting, i.e., formation of commensurate 2D islands.

Quantum mechanical model of localized electrons on the surface of polymer nanospheres by Stephen K. Gray; Bobby G. Sumpter; Donald W. Noid; Michael D. Barnes (308-313).
We examine the quantum mechanics of a simple model for charged dielectric nanospheres called quantum drops. The model, previously introduced in semiclassical quantization work, involves one electron moving in an effective potential field due to N−1 other electrons on the surface of a nanosphere. We discuss how electron confinement occurs and how energy levels exhibit scaling properties similar to those expected for a particle moving in a restricted, two-dimensional area.

The Rydberg states of alkali/rare-gas diatomic molecules show complex potential energy curves due to two effects: (1) avoided crossings between two close atomic configurations, and (2) undulations originating from the atomic orbital structures of the alkali Rydberg states. This work reports the potential energy curves for ten 2Σ+ states correlated to the atomic states from Na(3s) to Na(6s) obtained from accurate ab initio calculations. We give the spectroscopic constants and present a complete analysis for these potential curves.

Wiener index revisited by Sonja Nikolić; Nenad Trinajstić; Milan Randić (319-321).
We introduce a modification of the Wiener index. This modified Wiener index has a structural interpretation in terms of the greater weights of outer than inner bonds in a saturated hydrocarbon. Both the Wiener index and its modification together with the molecular polarity index lead to comparable structure-property models.

It has recently been proposed by Kanno, Tomikawa and Mishima that novel Raman spectra for amorphous ices show that the low-density form (LDA) resembles ice whilst the high-density form (HDA) resembles lithium chloride glasses. The purpose of this Letter is to stress that the differences between LDA and HDA are not this great. In particular, a key difference is that LiCl glasses contain a large number of vacancies, whilst there are none in any of the amorphous forms of water.

To clarify the misunderstanding in the statements made in the above paper, it is shown that Raman OD stretching spectra of glassy aqueous tetraalkylammonium chloride are very similar to those of low-density amorphous D2O ice (LDA) despite the structural differences between LDA and glassy aqueous tetraalkylammonium chloride solutions.