Static anisotropic and isotropic electric dipole polarizabilities of Na n F n-1 clusters

Recently, electric susceptibilities of Na_nF_n-1 clusters have been measured by deflecting a molecular beam in an inhomogeneous electric field. The analysis of the deflection of a cluster by the electric field needs the calculation of the electric properties. We present the calculation of the static anisotropic and average dipolar polarizabilities within a model in which the Na_nF_n-1 clusters ( 2 n 23) are treated as one electron embedded in the field of n ions Na⁺ and of n - 1 ions F^-. The accuracy of the results is evaluated and discussed on small clusters (n 5) in comparison with ab initio calculations. The relationship between the polarizabilities, the electronic localization and the geometric structure is discussed. Received 10 September 2002 Published online 3 July 2003     

Superconductivity of Rb 3 C 60 : breakdown of the Migdal-Eliashberg theory

In this paper, through an exhaustive analysis within the Migdal-Eliashberg theory, we show the incompatibility of experimental data of Rb₃C₆₀ with the basic assumptions of the standard theory of superconductivity. For different models of the electron-phonon spectral function α ² F (Ω) we solve numerically the Eliashberg equations to find which values of the electron-phonon coupling λ, of the logarithmic phonon frequency and of the Coulomb pseudopotential μ ^* reproduce the experimental data of Rb₃C₆₀. We find that the solutions are essentially independent of the particular shape of α ² F (Ω) and that, to explain the experimental data of Rb₃C₆₀, one has to resort to extremely large couplings: λ = 3.0±0.8. This results differs from the usual partial analyses reported up to now and we claim that this value exceeds the maximum allowed λ compatible with the crystal lattice stability. Moreover, we show quantitatively that the obtained values of λ and strongly violate Migdal's theorem and consequently are incompatible with the Migdal-Eliashberg theory. One has therefore to consider the generalization of the theory of superconductivity in the nonadiabatic regime to account for the experimental properties of fullerides. Received 30 March 2001     

Electric dipole moments and conformations of isolated peptides

The electric dipole moments of the isolated amino acid tryptophan and small glycine-based peptides (WG_n, n = 1-5, W = tryptophan, G = glycine) have been measured by deflection of a molecular beam in an inhomogeneous electric field. The measurements are compared to the results of ab initio calculations and Monte-Carlo simulations. The conformation and the flexibility of the peptides, at different temperatures, are discussed. The WG_n peptides are much more floppy than an isolated tryptophan, even a single glycine is enough to make the peptide floppy on the timescale of the electric deflection measurements. Received 4 January 2002 Published online 13 September 2002     

A study of Peierls instabilities for a two-dimensional t-t' model

In this paper we study Peierls instabilities for a half-filled two-dimensional tight-binding model with nearest-neighbour hopping t and next nearest-neighbour hopping t' at zero and finite temperatures. Two dimerization patterns corresponding to the same phonon vector (π,π) are considered to be realizations of Peierls states. The effect of imperfect nesting introduced by t' on the Peierls instability, the properties of the dimerized ground state, as well as the competition between two dimerized states for each t' and temperature T, are investigated. It is found: (i). The Peierls instability will be frustrated by t' for each of the dimerized states. The Peierls transition itself, as well as its suppression by t', may be of second- or first-order. (ii). When the two dimerized states are considered jointly, one of them will dominate the other depending on parameters t' and T. Two successive Peierls transitions, that is, the system passing from the uniform state to one dimerized state and then to the other may take place with decrease of temperature. Implications of our results to real materials are discussed. Received 31 July 2001     

Nonlinear Optical Properties of Novel C60 Derivatives under Picosecond Laser Excitation

We investigate the third-order nonlinear optical properties of six novel fullerene derivatives under picosecond laser excitation by Z-scan technique. The experimental results reveal that all the derivatives have very large nonlinear absorption coefficient under 532 nm pulses excitation and great third-order nonlinear refraction index under 1064 nm pulses excitation. The molecular second hyperpolarizabilities are obtained from the experimental results.     

Solubility of 13 nonpolar gases in deuterium oxide at 15–45°C and 101.325 kPa. Thermodynamics of transfer of nonpolar gases from H2O to D2O

Using a computer controlled version of the Ben-Naim and Baer apparatus, the solubilities of He, Ne, Ar, Kr, H₂, D₂, N₂, O₂, CH₄, C₂H₆, C₃H₈, CF₄, and SF₆ in D₂O (and the solubility of CF₄ in H₂O) were determined at four temperatures in the range 288 to 318 K and at a partial pressure of gas of 101.325 kPa. The precision of the measurements was 0.3–1.0%. The experimental data were processed using rigorous thermodynamic methods and were fitted to the Clarke-Glew-Weiss equation. Changes in the thermodynamic properties on solution were calculated from the smoothing equations. Scaled-particle theory (SPT) was used to determine the Lennard-Jones (6,12) pair potential parameters for D₂O: =0.275 nm and /k=83.4 K. Experimental mole fraction solubilities and thermodynamic functions at 298.15 K were compared with results calculated using the SPT. The thermodynamic transfer functions of gases from H₂O to D₂O were calculated. The changes induced by the solvation process on the structure of water were estimated from the Gibbs energy of transfer and the difference in the hydrogen bond energies for D₂O and H₂O.     

Electron-LO-phonon interaction in wurtzite GaN quantum wells under a magnetic field

We calculate the electron-LO-phonon relaxation rates in wurtzite GaN quantum wells in the presence of a magnetic field parallel to the growth direction. Using the dielectric continuum model (DCM), we are able to include contributions from both the interface and the quasi-confined phonon modes. The relaxation rate expression takes the phonon dispersion into account, and is applicable to all phonon modes. We find that the relaxation rates show strong oscillations as a function of the applied magnetic field. In relatively wide (8 nm) quantum wells, the inclusion of interface phonon mode decreases this oscillation amplitude. But in thin wells (5 nm), the interface phonon mode is of the same importance as the quasi-confined mode, and it strongly modifies the oscillation behavior.     

Isotopic effect in the vibronic spectra of europium compounds

The effect of the kinematic factor on vibronic spectra of europium compounds and Eu³⁺-doped lanthanide compounds was examined experimentally. It was demonstrated that isotopic or quasi-isotopic substitution of the ions of the crystal lattice gives rise not only to the changes of the vibration frequencies but also to alteration of the value of electron-phonon interaction. The latter displays in changing the relative integral intensity of vibronic sidebands of electronic transitions of Eu³⁺ ion. Eu³⁺ vibronic spectra of a number of pairs of natural and isotopically or quasi-isotopically substituted compounds: nitrates, halides, formates, acetates, oxalates, β-diketonates, etc., were studied. In most cases the substitution of deuterium for hydrogen was applied. Decrease of the electron-phonon interaction with the increase of the isotopic mass depends on different structural characteristics. It was found that a factor of decreasing the relative intensity of the vibronic sideband of electronic transition of Eu³⁺ ion lies within the range ∼1.2 and ∼7 for pairs of compounds under investigation. The largest change of the intensity of vibronic sidebands was observed in a pair of formates Eu(HCOO)₃ and Eu(DCOO)₃ having the tridentate-bridging coordination of the formate anions and a three-dimensional frame structure. One should take into consideration both decreasing the vibration frequencies and diminishing the value of electron-phonon interaction at introduction of heavy isotope or quasi-isotope in the crystal lattice of lanthanide compounds to reduce the multiphonon quenching of luminescence.     

Optical addressing in dye-doped cholesteric liquid crystals

This study investigates a method of optical addressing in dye-doped cholesteric liquid crystals (DDCLCs). Photo-induced randomly adsorbed dyes can change the CLC textures from planar to focal conic. Such patterning can be adopted to develop a display that is initially invisible, but becomes visible upon heating above the clearing temperature, followed by cooling to room temperature. The display can also become visible upon the application of a suitable voltage, and its rapid release. Additionally, the display is thermally erasable, optically rewritable and electrically switchable. It can be applied for use as a smart card.     

单电离态Kr原子强跃迁能级振子强度的研究

阐述了用束-箔方法通过测量单电离态Kr原子强跃迁谱线的能级寿命来 确定振子强度f-值的方法,并对确定出的强跃迁谱线的能级吸收振子强度进行简要讨 论。