Chemical Physics Letters (v.319, #1-2)

Photoluminescence (PL), time resolved PL and optical absorption spectra near the band gap energy and in the range of IR active vibration modes of a novel molecular complex of fullerene, C26H18Te2·C60·CS2 (BTX C60·CS2), have been investigated. The new absorption band in the range 1100 cm−1, and a novel broad ESR signal have been found, indicating the optically excited charge transfer transition in the complex. The shift of the PL spectrum by 0.32 Ev towards lower energies (with respect to one in the C60 crystal), together with a long time decay of all PL lines, allows us to assume that the recombination is also accompanied by a charge transfer.

Two kinds of covalently tethered fullerene monolayers, which exhibit the photoelectric conversion property, are reported. The monolayers were fabricated on hydrophilic indium–tin oxide (ITO), quartz, and mica by an esterification reaction and characterized by contact angle, AFM, UV spectrum, and cyclic voltammetry. The thicknesses of the films calculated by the MM2 molecular mechanics method were in good agreement with the results of AFM, indicating the formation of monolayers. The photoelectric conversion properties of both monolayers were studied.

Scaling behavior of cross-sections for electron-impact multiple-ionization of negatively-charged fullerene ions by D. Hathiramani; P. Scheier; K. Aichele; W. Arnold; K. Huber; E. Salzborn (13-19).
Absolute cross-sections for the electron-impact multiple-ionization of mass-selected negatively charged fullerene ions C n C n q+ (n=60, 70, 84; q=1, 2, 3) have been measured for the first time. A scaling law has been observed for the magnitude of the cross-sections that is a function of the fullerene size n and the charge state q of the product ion. The data indicate that different mechanisms account for the detachment of the extra electron from the negatively charged fullerene and the formation of a positively charged ion, respectively. Moreover, the multiple-ionization of a fullerene anion is found to be a sequential process.

α-S8 is the most stable crystalline form, at ambient pressure and temperature (STP), of elemental sulfur. In this Letter, we analyze the zero pressure low temperature part of the phase diagram of this crystal, in order to test a simple and flexible model molecule. The calculations consist in a series of molecular dynamics (MD) simulations, performed in the constant pressure–constant temperature ensemble. Our calculations show that this model, that gives good results for three crystalline phases at STP and T≳300 K, fails at low temperatures, predicting a structural phase transition at 200 K where there should be none.

Semiclassical wavefunction propagation for chaotic systems suffer from numerical difficulties due to the chaotic nature of classical trajectories, resulting in reduced accuracy for standard initial value representation (IVR) methods. We compare four recent IVR methods developed to overcome these difficulties (Herman–Kluk with trajectory truncation; stationary-phase Herman–Kluk (SPHK); cellularized frozen Gaussian approximation; stationary-phase Monte Carlo) by computing the Franck–Condon spectrum of the 2-dimensional Henon–Heiles and quartic oscillator systems. The SPHK is found to be the most successful of the four methods. The SPHK is then used to determine the spectrum for collinear CO2 photodissociation.

An algebraic Hamiltonian for H2O, D2O, H2S, CH2 of CH2Br2 and CD2 of CD2Br2 containing anharmonicities and resonances is proposed with coefficients elucidated from the fit with the experimentally observed levels. It is demonstrated that most often the anharmonic parts show clues of action (energy) localization. Resonances are shown full of periodic, quasiperiodic and chaotic motion, while the intersection of the various resonances is full of chaotic motion. The barrier in intramolecular vibrational relaxation with extreme mutual irrational ratios of resonance frequencies is shown full of periodic or quasiperiodic motion, leading to the dynamical bottlenecks.

Ab initio study on the photoisomers of a nitro-substituted spiropyran by Grazia Cottone; Rosina Noto; Gianfranco La Manna; Sandro L. Fornili (51-59).
Structural and spectroscopic properties of the photoisomers of a nitro-substituted spiropyran have been investigated by performing ab initio molecular orbital (MO) calculations both in vacuo and in hexafluoro-2-propanol solution. Full geometry optimisation of the closed form and of the transoid conformations of the open form has been carried out. Dipole moments of both photoisomers have been determined, the ratio of which agrees with recent experimental results. Net atomic charges have also been determined according to three different approaches.

We present a simple and efficient molecular dynamics based method for determining Henry's constant of gases dissolved in liquids. The method is an extension of an algorithm we presented earlier to study osmosis and reverse osmosis in liquid and gaseous solutions/mixtures. It is based on separating a gaseous compartment in the molecular dynamics system from the solvent using a semi-permeable membrane, permeable only to the gas molecules. The system is then allowed to come to equilibrium at the desired density and temperature and, at equilibrium, Henry's constant can be easily determined using simple thermodynamics. Since particle insertions or deletions are not needed in this method, it is free of any limitations in the high-density regime. We have confirmed the accuracy of the proposed method by comparing it with the existing Monte Carlo results for moderate density and showed that it can be easily used at high densities as well where insertion/deletion schemes may become quite inefficient.

Evaluation of the iterative simulated annealing technique in conformational search of peptides by Francesc J. Corcho; Marta Filizola; Juan Jesús Pérez (65-70).
Characterization of the subset of low energy minima of a peptide is hampered by the multiple minima problem associated to the roughness of its potential energy surface. The iterative simulated annealing procedure was recently proposed as an effective procedure to overcome these difficulties. In the present work results of a thorough exploration of the conformational space of the peptide Ac–Cys–Val–Tic–Met performed by means of the simulated annealing procedure is compared to the results of a random search. Profile differences in the two sets of low energy conformations obtained are analyzed. The results are also discussed in terms of the rotational isomeric model and its usefulness in assessing the degree of completeness of the conformational search.

A nonlinear dynamical analysis is made for the generation of potential spikes in response to the change in the current in the electrochemical oxidation of formic acid on Pt, based on a mechanistic model. This model assumes the potential-dependent adsorption, desorption, and reaction of the adsorbed water and carbon monoxide. The model is found to reproduce the spikes observed experimentally in a previous report. The trajectory of the state point in the state space shows how and why the system generates spikes.

An electron momentum spectroscopy study of the highest occupied molecular orbital of chlorotrifluoromethane by X.J. Chen; C.C. Jia; C.K. Xu; G. Ouyang; L.L. Peng; S.X. Tian; K.Z. Xu (76-80).
The highest occupied molecular orbital (HOMO) of chlorotrifluoromethane (CF3Cl) has been studied by binary (e, 2e) electron momentum spectroscopy (EMS) for the first time. Impact energy of 1200 eV plus binding energy and symmetric non-coplanar geometry are employed. The experimental momentum profile of HOMO is compared with Hartree–Fock (HF) and density functional theory (DFT-B3LYP) calculations. An outer-valence Green's function (OVGF) calculation of ionization potentials of outer-valence orbitals has also been carried out and compared with previous MS Xα calculations and photoelectron spectroscopy (PES) measurements.

Low-frequency Raman spectra of amorphous ices by Y Suzuki; Y Takasaki; Y Tominaga; O Mishima (81-84).
Low-frequency Raman spectroscopy measurements are performed at atmospheric pressure on low (LDA)- and high (HDA)-density amorphous ices. The HDA is annealed at 15 kbar, up to 160 K. It is found that the low-frequency Raman profile of the HDA is similar to that of liquid water, but is different from that of crystalline ices. This result indicates that the structure of the hydrogen-bonded networks in HDA is liquid-like.

A He I photoelectron spectrum of the (CH3CH2CH2)2N radical by Xinjiang Zhu; Maofa Ge; Chunhua Qiao; Zheng Sun; Dianxun Wang (85-88).
The He I photoelectron spectrum (PES) of the (CH3CH2CH2)2N radical is obtained by pyrolysis of di-propylnitrosamine (CH3CH2CH2)2NNO at 615±0.5°C. Except for obvious PES bands of the NO species, at least four new PES bands emerge in the PE spectrum of the pyrolysis products of the (CH3CH2CH2)2NNO compound. In order to assign the PES bands of (CH3CH2CH2)2N, an improved density functional theory calculation has been carried out. A sharp peak with the lowest ionization energy at 8.36±0.02 eV comes from electron ionization of the HOMO(4b1) of the (CH3CH2CH2)2N radical, corresponding to the ionization of (CH3CH2CH2)2N(X   2 B 1) to (CH3CH2CH2)2N+ (X   1 A 1).

Measurement of third-order optical nonlinearity of C60M2 (M=Pd, Pt, and Sm) organometallic compounds by the femtosecond optically heterodyned optical Kerr effect by Wei Qian; Li Lin; Zongju Xia; Yinghua Zou; Shixiong Qian; Guohong Ma; Yanghui Lin; Ruifang Cai; Yu Chen; Zu-En Huang (89-94).
We have measured the real components of the third-order optical nonlinearity of organometallic compound films of C60M2 (M=Pd, Pt, and Sm) by using the femtosecond optically heterodyned optical Kerr effect at 647.0 nm. The results show that there is an enhancement of the real components of the third-order optical nonlinearity of C60M2 films compared to that of pristine C60 film. We attribute the origin of this enhancement to the charge transfer from the metal atoms to the C60 molecules.

Dissociation energy of the PdO molecule by D.L Hildenbrand; K.H Lau (95-98).
Mass spectrometric studies of the gaseous equilibrium Pd+O ⇌ PdO yield the dissociation energy D 0 o(PdO)=56±3 kcal mol−1. This new value is significantly lower than one recommended in the most recent review, D 0 o(PdO)=90±20 kcal mol−1, where an earlier value near 55 kcal mol−1 was rejected because of higher values ranging from 90 to 98 kcal mol−1 for neighboring metal monoxides. It is shown that the lower value reported here is consistent with the unique closed-shell structure of the Pd atom.

Thermal rate constants over thirty orders of magnitude for the I+H2 reaction by J.V Michael; S.S Kumaran; M.-C Su; K.P Lim (99-106).
I-atom atomic resonance absorption spectrometry has been used to measure thermal rate constants for the I+H2  → HI+H reaction between 1755 and 2605 K in reflected shock waves. Measured rate constants can then be expressed by k 1=(3.92±1.15)×10−9  exp(−21 398±658  K/T) cm3 molecule−1 s−1. Combining these results with earlier values, the rate behavior over the combined temperature range, 230 K⩽T⩽2605 K, is: k 1=(4.52±0.34)×10−10  exp(−17 070±34  K/T) cm3 molecule−1 s−1. This result holds over ∼30 orders of magnitude and is discussed in terms of bimolecular rate theory.

Second-harmonic generation and absorption spectra of platinum organometallic complexes incorporated in PMMA films by M. Casalboni; F. Sarcinelli; R. Pizzoferrato; R. D'Amato; A. Furlani; M.V. Russo (107-112).
Pt-acetylides with different end-side groups have been synthesized and incorporated in poly(methylmethacrylate) (PMMA) films for second-order nonlinear optics through high-temperature corona-poling. By using two complementary optical techniques, i.e. in-situ second-harmonic generation (SHG) and angle-resolved spectroscopy, the order parameter and the molecular hyperpolarizability β of the chromophores have been characterized inside the polymer matrix. The relaxation of orientational after order poling fits a bi-exponential equation in agreement with the Debye model for guest/host systems. A comparison with the azo-dye Disperse Red 1 (DR1) is discussed within the two-level model and the role of the platinum moiety for conjugated molecules.

Photoluminescence intensity and anisotropy decays in amorphous carbon by M.N. Berberan-Santos; A. Fedorov; J.P. Conde; C. Godet; T. Heitz; J.E. Bourée (113-118).
The decays of intensity and anisotropy of UV-excited photoluminescence (PL) in hydrogenated amorphous carbon have been investigated in the ps–ns time range. For emission energies E em between 1.8 and 3.5 eV, anisotropy decreases within 100 ps and reaches a plateau within 1 ns. The emission anisotropy plateau value increases (from 0.02 to 0.12) and the decay time of PL intensity decreases (from 1 ns to 40 ps) as E em increases. The exponential increase of time-averaged anisotropy as E em increases is explained by a competition between exciton decay and randomization of polarization due to electronic excitation energy transfer (Förster mechanism) between chromophores.

Liquid–liquid coexistence curves of the system (tetrahydrofuran+water) and the pseudo-binary system (tetrahydrofuran+deuterated tetrahydrofuran+heavy water) were measured in the reduced temperature range 3×10−5<τ<7×10−2 from their lower critical points. The composition of the deuterated system was adjusted to get the critical temperature close to that of the normal binary mixtures. This ensures that the effect of the upper consolute temperatures is the same in both cases, thus facilitating data interpretation. The refractive index and the Lorentz–Lorenz function were chosen as composition variables for analyzing the order parameter. For τ<10−3 both mixtures exhibit pure Ising behavior. The leading critical amplitude increases in the deuterated mixture. The crossover behavior is found to be sensitive to changes in deuteration.

OH detection by absorption of frequency-doubled diode laser radiation at 308 nm by Hugh R Barry; Ben Bakowski; Laura Corner; Tim Freegarde; Oliver T.W Hawkins; Gus Hancock; Robert M.J Jacobs; Robert Peverall; Grant A.D Ritchie (125-130).
Radiation at 308 nm has been obtained by frequency doubling the output of a commercial diode laser cooled to 165 K. A single pass through a crystal of LiIO3 converted 1 mW of 616 nm radiation to 50 pW of UV, and this was used to detect the OH radical in absorption in a flow tube. Possible extensions of the method for detection of OH in the atmosphere are discussed.

We use a rapidly swept optical cavity in a novel approach to cavity ringdown spectroscopy (CRDS) with a continuous-wave (cw) laser. This simplifies cw-CRDS by eliminating the need for a fast optical switch and is demonstrated by recording weak combination-band spectra of carbon dioxide diluted in nitrogen, using a cw single-mode tunable diode laser (TDL) in the 1.55 μm near-infrared region. We employ a simultaneous combination of CRDS and cavity-enhanced absorption spectroscopy, to compensate for nonlinear CRDS response that is observed for stronger absorption features. Experimental results and performance characteristics are presented, together with numerical simulations.

Photoreduction of methylene blue on cadmium sulfide powder by E.L Quitevis; J Martorell; Y Chang; T.W Scott (138-144).
The relaxation of photogenerated charge carriers in cadmium sulfide powder and the reduction of surface bound methylene blue have been investigated using both picosecond multiple light scattering transients and transient band gap emission measurements. A comparison of the bleaching kinetics for adsorbed dye with near-IR absorption and band gap emission transients from the semiconductor indicate that interfacial electron transfer is driven by mobile conduction band carriers with little participation by the longer-lived weakly trapped carriers. In addition, adsorbate reduction is simulated using the diffusion equation with a surface reaction.

Introduction of a 60 fs deactivation channel in the photosynthetic antenna LH1 by Ni-bacteriopheophytin a by L Fiedor; H Scheer; C.N Hunter; F Tschirschwitz; B Voigt; J Ehlert; E Nibbering; D Leupold; T Elsaesser (145-152).
Forty femtosecond pump and probe investigations in the 870 nm absorption band of the reconstituted core antenna, LH1, from Rhodobacter sphaeroides, in which varying amounts of Ni– bacteriopheophytin replace part of the native bacteriochlorophyll, show a tremendous shortening of the ground state recovery time with increasing amount of exchanged pigments. In the Ni–bacteriopheophytin containing antenna, a 60 fs deactivation channel has been found, which originates from a one-exciton state delocalized over the whole LH1 of about 20 pigment molecules. The 60 fs channel is interpreted as internal conversion in Ni–bacteriopheophytin.

Isolation and characterization of two Pr@C82 isomers by Takeshi Akasaka; Shingo Okubo; Masahiro Kondo; Yutaka Maeda; Takatsugu Wakahara; Tatsuhisa Kato; Toshiyasu Suzuki; Kazunori Yamamoto; Kaoru Kobayashi; Shigeru Nagase (153-156).
The second isomer of Pr@C82 (Pr@C82-II) has been isolated by a two-step HPLC method, separately from the known major isomer (Pr@C82-I). Visible and near-IR absorption spectra, as well as cyclic and differential pulse voltammograms show that the characteristic features of Pr@C82-II are significantly different from those of Pr@C82-I. The absorption peaks of Pr@C82-II have blue-shifts and the redox potentials have negative shifts, relative to those of Pr@C82-I. Chemical derivatization of both isomers with disilirane have also been achieved.

S1 excited state relaxation processes of [2,2′-bipyridyl]-3,3′-diol are investigated at room temperature in solution. After excitation at 355–360 nm, the transient UV–visible spectra are investigated in the 350–650 nm region where both transient absorption and stimulated emission occur. The signal is measured at various probe wavelengths as a function of time up to delays of 150 ps. The spectral features appear with times shorter than the instrumental resolution (<200 fs) and evolve with slower dynamics in the 10–20 ps range. A single-exponential on top of the instantaneous rise of the stimulated emission is observed with time constants of 8–10 ps depending on the solvent. This confirms the coexistence of two proton-transfer mechanisms, one directly forming the di-keto, the other involving the formation of the transient mono-keto tautomer. The transient absorption spectrum intensity decreases following a biexponential law, the fast component coinciding with that of the stimulated emission increase. The slow component is attributed to the decay of the (n, π) formed because of its proximity to the (π, π) state.

The spectroscopic properties of the trans isomer of four 4-R-4′-nitrostilbenes (I–R, R: H, OMe, NH2, NMe2) in solution at 25°C were studied by time-resolved absorption techniques. An absorption increase within 10 ps and the subsequent decay in the visible range are attributed to the excited singlet state (S1→S n transition). The lifetime for this decay (τ S) varies for trans-I–NMe2 from 1–3 ns in solvents of low polarity to ≅10 ps in acetonitrile. Also for trans-I–NH2 and trans-I–OMe, τ S is sensitive to the solvent polarity. In those cases where the quantum yield of intersystem crossing is substantial, the transient absorption due to the T1→T n transition at an appropriate wavelength, increases with almost the same lifetime and then remains constant.

In this Letter, we study the nonequilibrium effects which appear in a thermally activated exothermic reaction A+A→products using the molecular dynamics for reactive hard spheres. We have found that the rate constant is reduced with respect to its equilibrium value and the relative decrease of rate constant is similar to that observed for a thermoneutral reaction with the same activation energy. It is demonstrated that a simple phenomenology, which assumes a Maxwellian distribution of the energetic states for the reactant and for the system as a whole, gives a good estimation of the nonequilibrium rate constant.

Excitation of vibrational levels of HI up to v=8 by electron impact by Anne-Christelle Sergenton; Michael Allan (179-183).
Relative differential cross-sections for the excitation of vibrational levels of HI up to v=8 by slow electron impact have been measured as a function of the incident electron energy in the range 0–3 eV. The results confirm the shapes of the theoretical cross-sections of Domcke and Nakamura [Z. Phys. D 42 (1997) 181], in particular the prediction that no threshold peaks are found in HI, in contrast to the lighter hydrogen halides. The spectra up to v=0→6 have structures at vibrational thresholds, in agreement with the theoretical predictions.