Chemical Physics Letters (v.355, #1-2)

Sterically hindered phenols are extremely efficient light quenchers by Ortwin Brede; Sergej Naumov; Ralf Hermann (1-7).
By means of the direct quantitative determination of the quantum yields of all deactivation channels of the first excited singlet state of selected alkyl substituted phenols it is shown, that sterically hindered (2,6-di-tert-butyl substituted) phenols (SHP) have extremely short singlet lifetimes and relax dominantly by radiationless processes. A quantum-chemical molecular analysis (vibration analysis, steric factors) gives evidence for an intense coupling interaction of the phenol group with the bulky substituents.

Study of excited states of furan and pyrrole by time-dependent density functional theory by Rudolf Burcl; Roger D. Amos; Nicholas C. Handy (8-18).
Excited states of furan and pyrrole were studied by time-dependent density functional theory. The effect of basis set and density functional on the vertical excitation energies was investigated. Energy gradients and dipole moments were evaluated analytically. Stationary points on the lowest excited states were determined. Harmonic frequencies and (v =0←v=0) excitation energies were evaluated. Many of the results agree well with the experimental values available as well as most recent theoretical ab initio values, but there remain discrepancies in the valence states. The dipole moments of many excited states show a large variation with the basis set and functional; this is due to the fact that the states have an extremely large polarisability.

We report on persistent spectral hole-burning studies of thiazine and oxazine dyes encapsulated in an AlPO4-5 aluminophosphate molecular sieve. A non-photochemical hole-burning mechanism is provided by small additional solvent molecules, e.g. glassy ethanol or amorphous water, in the pore system. Hole-burning efficiency and spontaneous hole-filling are in the typical range of glassy bulk systems. However, spectral diffusion is reduced drastically in the solvent-filled molecular sieve compared to the corresponding bulk glass. Based on previous work by the Fayer group [J. Chem. Phys. 92 (1990) 4125; Chem. Phys. Lett. 168 (1990) 371] this finding is discussed as an example of spatial restrictions to the solvation shell around the dye molecules.

The non-linear dynamics of electron excitation in diatomic molecules is studied for ultra-short laser pulse with λ=147 nm, τ=1 fs and peak intensities I=1013–1016   W/cm 2 . The electromagnetic wave frequency is in resonance to an electronic transition in the molecule. The molecule is modeled by a system of two active 1D electrons moving in the field of a frozen core. The time dependences of electron density distribution, electron energy, and one-electron/two-electron ionization probability are calculated and show the effect of giant above-threshold absorption in the pulse of intensity I∼1016   W/cm 2 .

The SiCO4 molecule has been produced by reaction between silicon dioxide and carbon dioxide molecules in solid argon. Silicon dioxide molecules were prepared by reactions of laser-ablated silicon atoms with oxygen in excess argon. When carbon dioxide molecules were added in the reagent gas, new product absorptions at 1934.9, 1326.1, 1070.2 and 866.0  cm −1 were produced spontaneously on annealing. Based on isotopic substitution and density functional theoretical calculations, these absorptions are assigned to the vibrational fundamentals of the SiCO4 molecule that has C2v symmetry with two terminal O atoms and two bridging O atoms.

A remarkable enhancement of methane adsorptivity of activated carbon fiber (ACF) with dispersion of ultra fine NiO particles was observed. The Ni K-edge X-ray adsorption near-edge structures (XANES) and extended X-ray adsorption fine structures (EXAFS) spectra of dispersed NiO on ACF elucidated that the dispersed NiO showing the most remarkable enhancement effect has a specific structure whose nearest-neighbor atomic distance of NiO is larger than that of powdered NiO. The X-ray photoelectron spectroscopy (XPS) spectra suggested that the electrons of the ligand valence band in the dispersed NiO are tightly bound than those of the powdered NiO.

Effect of electric field upon the ZnO growth on sapphire (0 0 0 1) by atomic layer epitaxy method by C.H. Liu; Min Yan; Xiang Liu; Eric Seelig; R.P.H. Chang (43-47).
This paper investigates the effect of electric field upon the atomic layer-controlled growth of ZnO on the (0 0 0 1) sapphire substrates. The results indicated, for the first time, that electric field could make remarkable growth difference for the ZnO films with this method. With a proper electric field, higher quality of epitaxial films has been obtained. Comprehensive analyses revealed that electric field-induced increase of the hydroxyl packing density on the substrate surface could account for the results.

First observation of the triplet state of fluorobenzene by Tetsuo Okutsu; Kouhei Ichiyanagi Hiroshi Haneda; Hiroshi Hiratsuka (48-52).
A triplet–triplet (T–T) absorption spectrum of fluorobenzene was observed for the first time. The broad absorption in the region of 330–550 nm with lifetime of τ=0.7  μs was ascribed to the T–T absorption spectrum by the T–T energy transfer experiment using naphthalene as an acceptor. The spectrum reported previously as the T–T absorption was elucidated to be the spectrum of radical. Quantum yield of the intersystem crossing φ ISC was determined to be 0.27 by use of the time-resolved thermal lensing (TRTL) experiment.

Effect of the in situ Cs treatment on field emission of a multi-walled carbon nanotube by Do-Hyung Kim; Hyeong-Rag Lee; Man-Woo Lee; Jin-Ho Lee; Yoon-Ho Song; Jong-Gi Jee; Sang-Yun Lee (53-58).
The effect of an in situ Cs treatment on the field emission of a multi-walled carbon nanotube (MWNT) was investigated. The field emission of as-grown MWNT shows two Fowler–Nordheim (F–N) slopes in current–voltage characteristics. Both slopes in the two regions (high voltage and low voltage) were lowered as the result of the Cs treatment. The turn-on voltage was significantly decreased by a factor of about 1.3 and the total emission current was increased by over one order of magnitude after Cs deposition. In addition, the emission current was decreased beyond the optimum Cs deposition time. A stable emission current was observed after Cs deposition and during the no-Cs-deposition state. A decrease in work function is the major cause of the electron field enhancement as the result of the formation of a surface dipole layer induced by Cs deposition.

REMPI spectroscopy of cytosine by E. Nir; M. Müller; L.I. Grace; M.S. de Vries (59-64).
We report resonant two-photon ionization spectra of laser desorbed, jet cooled, cytosine, 1-methyl cytosine, 5-methyl cytosine, and dimers of these. Unlike other pyrimidine bases, cytosine exhibits vibronic spectra with sharp features in two spectral regions, separated by about 5000  cm −1 . We interpret these as being due to two tautomeric forms, one keto and one enol. The dimers absorb at wavelengths that are intermediate between those of the two monomer forms. By UV–UV hole burning we determined the numbers of isomers contributing to each spectrum and by delayed two color ionization we determined triplet lifetimes. We observed hydrogen transfer between bases both in collisions between monomers and after photo-excitation in clusters.

Intramolecular excimer and exciplex emission of 1,4-dipyrenyl substituted cyclohexasilane by Cornelis A. van Walree; Veronica E.M. Kaats-Richters; Leonardus W. Jenneskens; René M. Williams; Ivo H.M. van Stokkum (65-70).
Intramolecular excimer emission is observed for cis-1,4-di(1-pyrenyl)decamethylcyclohexasilane in nonpolar solvents. Time-resolved fluorescence spectroscopy and kinetic modelling indicate that the driving force of excimer formation is very small, and that the process is governed by the flexibility of the silicon ring. In the polar solvent acetonitrile, photoinduced electron transfer occurs, with the cyclohexasilane ring acting as electron donor and the pyrenyl group as electron acceptor.

Zero-phonon linewidths for single dibenzo-anthanthrene molecules in solid Xe by Jerzy Sepioł; Robert Kołos; Alois Renn; Urs P. Wild (71-76).
The properties of zero-phonon lines (ZPLs) for individual dibenzo-anthanthrene (DBATT) molecules in solid Xe have been studied. Systematic differences in the widths of ZPLs for molecules occupying different matrix sites were observed. For the majority of samples a monotonic increase of the average linewidth with temperature was found and satisfactorily described within a model, which combined (i) the line broadening caused by the interaction with two-level systems and (ii) dephasing due to the coupling with a local mode. For about 30% of studied samples, however, a surprisingly flat dependence of ZPL width on temperature was found.

The steric opacity functions for the title Penning ionization reactions were determined. It was found that the most reactive site shifts from the collinear X-end toward the sideways direction, as the X change from Cl to I. The main feature in the experimental steric opacity function can be well explained by the electron exchange mechanism, which is controlled by the shift of center-of-mass and the electron density distributions of the HOMO of CH3X molecule.

Ultraviolet absorption spectra of shock-heated carbon dioxide and water between 900 and 3050 K by C. Schulz; J.D. Koch; D.F. Davidson; J.B. Jeffries; R.K. Hanson (82-88).
Spectrally resolved UV absorption cross-sections between 190 and 320 nm were measured in shock-heated CO2 between 880 and 3050 K and H2O between 1230 and 2860 K. Absorption spectra were acquired with 10  μs time resolution using a unique kinetic spectrograph, thereby enabling comparisons with time-dependent chemical kinetic modeling of post-shock thermal decomposition and chemical reactions. Although room temperature CO2 is transparent (σ<10−22   cm 2) at wavelengths longer than 200 nm, hot CO2 has significant absorption (σ>10−20   cm 2) extending to wavelengths longer than 300 nm. The temperature dependence of CO2 absorption strongly suggests sharply increased transition probabilities from excited vibrational levels.

We report theoretical studies on the optical absorption in 2-methoxy-5-(2-ethyl-hexyloxy)-distyryl benzene (MEH-DSB). We first produce the major absorption features existing in experimental results, in particular a weak band at 5.1 eV, which has not been observed in poly(p-phenylene vinylene) (PPV). We find that this band can appear only when the strength of intersite Coulomb interactions becomes smaller, in comparison with that in five-ring PPV oligomer with no substituents. Detailed analysis of wavefunctions related to the corresponding absorption bands will give correct assignments.

A method enabling a visual and numerical recovery of the change of fluorophore–matrix aligning interactions upon the electronic excitation of fluorophores, in planar ordered media (e.g. planar membranes or Langmuir–Blodgett films), is discussed. The introduced method, based on two particular experimental arrangements for the time-resolved polarized fluorescence measurements and the necessary parameterization, is exemplified by a series of the synthetic data and their numerical analyses.

Formation of hydrogen-capped polyynes by laser ablation of graphite particles suspended in solution by Masaharu Tsuji; Takeshi Tsuji; Shingo Kuboyama; Seong-Ho Yoon; Yozo Korai; Teppei Tsujimoto; Kanji Kubo; Akira Mori; Isao Mochida (101-108).
Laser ablation of graphite particles suspended in benzene, toluene, or hexane solution has been studied using a Nd:YAG laser (355, 532, and 1064 nm). Product analyses using HPLC coupled with UV and visible absorption spectroscopy and GC/MS showed that linear hydrogen-capped polyynes (C n H2: n=10, 12, 14, and 16) were formed in benzene and toluene, while those of n=8, 10, 12, and 14 were produced in hexane. The formation rates of polyynes increased with increasing particle concentration from 0.5 to 4 mg/ml, then decreased above that until 10 mg/ml. The formation rates of polyynes decreased with increasing wavelength of the Nd:YAG laser, 355  nm>532  nm>1064  nm .

The infrared spectra of the NH2 stretching vibrations of aniline+–Ar n (n=1–21) cluster cations have been measured by the depletion method. Aniline+–Ar n clusters were prepared by near-resonant resonance-enhanced multiphoton ionization (REMPI) of larger aniline–Ar m clusters. The red shifts of the NH stretching vibrations were 17–25  cm −1 , depending on the cluster size. The spectra of small clusters showed that there were hydrogen-bond-type (NH–Ar) and dispersion-type (Ar–π) isomers and that the NH–Ar isomer was more stable. The red shifts of the NH2 stretching vibrations varied with the number of Ar atoms and showed a maximum at n=12 and a local minimum at n≈19.

Laser induced fluorescence spectroscopy of the HC6S radical by Masakazu Nakajima; Yoshihiro Sumiyoshi; Yasuki Endo (116-122).
New vibronic bands were observed in the 17 000–17 600  cm −1 region by laser induced fluorescence (LIF) spectroscopy in the discharged products of a mixture of C2H2 and CS2 in a supersonic jet. The effective rotational constants and the position of the vibronic band origin have been determined to be B″ eff =0.01910(2)  cm −1,B eff =0.01891(2)  cm −1 , and T0=16961.5161(5)  cm −1 , respectively. The spectral carrier of the bands has been confirmed to be the HC6S radical from the chemical behavior of the products and the determined rotational constant.

An ab initio CASPT2//CASSCF study of the ground 1 A″ potential energy surface (PES) for the title reactions has been carried out. The lowest energy barrier heights correspond to cis-arrangements in both reactive channels. Crossings with the first excited 1 A″ PES are located, being complicated by spurious symmetry breaking of the complete active space self-consistent field (CASSCF) wave function. The rate constant values have been calculated employing transition state theory (TST) in the temperature range 500–5000 K. The main contribution to the total rate constant stems from the barrierless ground 1 A PES, while the 1 A″ PES is only relevant at the highest temperatures.

The extension of the laser vaporization method to composite fullerene-containing carbon targets is reported. High-yield synthesis of the recently discovered multishell fullerenes, which represent a novel variety of carbon nanoclusters, has been achieved. The content of the multishell fullerenes in the deposits may reach up to 90%. The multishell fullerenes such as double-shell C60@C240 or triple-shell C60@C240@C560 are the smallest among other multishell carbon clusters.

One-dimensional and frequency-selected vibrational echo experiments on the CO stretching mode of a metalloporphyrin carbonyl compound (model heme) in poly-methylmethacrylate (PMMA) and in 2-methylteterahydrofuran (2-MTHF) are compared. Spectrally resolving the vibrational echo signal and observing the decay at selected wavelengths, permits the independent detection of the 0–1 and 1–2 vibrational level dephasing dynamics, eliminating cross terms and anharmonic beats. In the high temperature 2-MTHF liquid, 0–1 and 1–2 dephasing dynamics are identical within experimental error. The decays are non-exponential, demonstrating that the CO stretching mode absorption spectrum is not a completely homogeneous, motionally narrowed line.

β-SiC nanowires has been grown from SiO x thin films deposited on Si (1 0 0) substrate at 1300 °C . A plate of graphite was used as the only carbon source. Argon was the only gas fed into the system. Structural and optical properties of the SiC nanowires were investigated using scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. It was found that the as-grown SiC nanowires are nearly free from undesirable thick oxide shell typically found on SiC nanowires synthesized by other methods. The present approach has also the potential advantage of highly selective growth on patterned substrate.

From the assignment of more than 30 vibronic transitions from the X ̃ 1Σ+ g (0,0,0) ground state to the 1 B 1,K=1(ν 1,ν 2,ν 3) excited states, the vibrational frequencies ω 1,ω 2,ω 3, the anharmonicity constants x 11 and x 22, and the mode-coupling constant x 23 of the lowest excited 1 B 1  (K=1) state of 7 Li 16 O 7 Li have been determined.

Mechanisms for the growth of polycyclic aromatic hydrocarbon (PAH) cations by Charles W. Bauschlicher; Alessandra Ricca; Marzio Rosi (159-163).
The barriers and heats of reaction for the conversion of benzene cation to naphthalene cation by acetylene additions using the B3LYP/6-31G* approach are reported. The barrierless path we previously reported, using the B3LYP/4-31G approach, is shown to be incorrect. New paths for the ring formation are shown to have low barriers.

Using Maxwell-displacement-current (MDC) measurement for Langmuir monolayer, dipole moment of azobenzene dendrimer on the water surface was examined, and analyzed using a simple two-layer model. The dipole moment along molecular long axis of the azobenzene dendrimer molecule increases as the number of constituent azobenzene groups increases, and it also increases with the number of alkyl groups. The dipole moment obtained in the MDC experiments had a similar tendency to the results estimated using molecular orbital (MO) calculation.

Photo-induced reverse valence tautomerism in a metastable Co compound by Osamu Sato; Shinya Hayami; Zhong-ze Gu; Kazuyuki Takahashi; Rie Nakajima; Akira Fujishima (169-174).
In our preceding paper, we have reported that a Co complex, [CoIII-LS(tmeda)(3,5-DBSQ)(3,5-DBCat)]·0.5C6H5CH3, exhibits a photo-induced valence tautomerism. The photo-induced metastable state, [CoII-HS(tmeda)(3,5-DBSQ)2]·0.5C6H5CH3, has a back electron transfer band from CoII-HS to 3,5-DBSQ at around 800 nm, indicating that the valence tautomerism should be photo-reversible. In fact, we have found that the metastable state could be photo-pumped back to the original state by exciting the back electron transfer band. The process can be expressed as [Co II-HS (tmeda)(3,5-DBSQ)2]·0.5C 6 H 5 CH 3(metastable state)→[Co III-LS (tmeda)(3,5-DBSQ)(3,5-DBCat)]·0.5C 6 H 5 CH 3 (ground state).

Resonance enhancement of two-photon absorption in porphyrins by Mikhail Drobizhev; Aliaksandr Karotki; Mikalai Kruk; Aleksander Rebane (175-182).
Two-photon absorption spectra in absolute cross-section values are presented for a number of octaethyl-, tetraphenyl-, and tetrabenzo-substituted porphyrins. In excitation spectral region of 710–810 nm, the two-photon absorption efficiency is resonantly enhanced due to nearby Q(0–0) one-photon transition. The cross-section value calculated directly from the ground- and excited-state absorption parameters in a single intermediate level approximation, agrees well with that measured in experiment.

Optimization of MCSCF excited states using directions of negative curvature by Mark R. Hoffmann; C. David Sherrill; Matthew L. Leininger; Henry F. Schaefer III (183-192).
A line search method that uses directions of negative curvature for the optimization of ground and excited state multiconfigurational self-consistent field (MCSCF) wavefunctions is suggested. The method is applicable to general MCSCF wavefunctions and not restricted to specific classes of model spaces, such as CASSCF functions. It is shown that the approach can be implemented with Newton or quasi-Newton methods for determination of descent directions. Thus, the method is particularly promising for cases in which it is inconvenient or costly to calculate exact curvature matrices. We demonstrate the viability of the approach by numerical example on the difficult BeO problem.

Mechanism and dynamics of the CH2OH+O2 reaction by Theodore S. Dibble (193-200).
Quantum mechanical calculations using B3LYP functional and the CBS-QB3 approach are used to determine the critical points on the potential energy surface for the reaction of CH 2 OH with O2, and RRKM-Master Equation calculations are used to follow the time history of the intermediates. The HOCH2OO radical is bound by 35.8 kcal/mole and the barrier for the stabilized adduct forming a CH2OHO2 complex is only 13.6 kcal/mole. At 298 K in 760 Torr of N2, the chemically activated HOCH2OO that does not promptly fall apart to reactants will be entirely transformed to CH2O+HO2 within 50 ps.

Quantum resonances in chaotic scattering by Kevin K. Lin; Maciej Zworski (201-205).
This Letter summarizes numerical results from [J. Comp. Phys. (to appear)] which show that in quantum systems with chaotic classical dynamics, the number of scattering resonances near an energy E scales like ℏ−(D KE +1)/2 . Here, K E denotes the subset of the classical energy surface H=E which stays bounded for all time under the flow of H and D KE denotes its fractal dimension. Since the number of bound states in a quantum system with n degrees of freedom scales like ℏn , this suggests that the quantity (D KE +1)/2 represents the effective number of degrees of freedom in chaotic scattering problems.