Anharmonic potential function of 1,1-dichloroethylene

A simple anharmonic potential containing 18 quadratic, 6 cubic, and 6 quartic constants is proposed to fit 124 vibrational frequencies of 1,1-dichloroethylene and its ³⁷Cl isotope in the region from 10 000 to 350 cm^?1. The level patterns in the first and the second overtone regions of the CH₂ stretching vibrations are explained by the absence of large higher-order interaction terms between the two CH stretching coordinates.     

Anharmonic potential function and equilibrium structure of methane

The vibration-rotation constants of methane-d₂ reported by Deroche [Ph.D. Thesis, Paris (1977)] were analyzed by a method of Hoy et al. [Mol. Phys. 24, 1265–1290 (1972)]. All the observed α constants were well reproduced by using 13 third-order anharmonic potential constants except for α₇^A and α₄^A; the discrepancies were ascribed to a large Coriolis interaction between ν₄ and ν₇. Of the 13 third-order potential constants thus derived only f¹¹¹ (1 means one of the CH stretching coordinates) was significant and others were determined either with large uncertainties or not uniquely because of redundancy. The observed rotational constants in the ground state were corrected for the centrifugal distortion effects, the electron “slipping” effects, and the contributions from the zero-point vibrations, to derive the equilibrium rotational constants. The r_e(CH) length thus obtained is 1.087₀ ± 0.0007 Å. This result was compared with those from an ab initio calculation and from recent electron-diffraction experiments.     

Anharmonic relaxation of phonons

Using the Ziman scheme of considering a drifting Planck’s distribution, as the eigenvector of the linearized phonon collision operator, the anharmonic relaxation of phonons is discussed. The earlier arbitrariness in the phonon-phonon coupling parameters is removed by formulating explicit expressions for different allowed processes in terms of measurable quantities. Low and high temperature approximations of relaxation rates are also discussed: the results differ from earlier calculations. At low temperatures superthermal or high frequency phonons, which have temperature-independent and equal N- and U-relaxation rates, play important roles in thermal conduction in pure insulators. Presently working as University of Wales Fellow in the Applied Physics Department, UWIST Cathays Park, Cardiff CF1 3NU, U.K.     

Anharmonic properties of TmTe

Data on elastic constants and associated properties at high temperature for TmTe crystal are presented and discussed starting from primary physical parameters viz. nearest neighbour distance and hardness parameter assuming long- and short-range potentials. When the values of the higher order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher order elastic constants and related properties are computed upto 1000 K for TmTe. The first-order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures. The results thus obtained are compared with other available data and found in well agreement with present values.     

Anharmonic constants of methyl chloride

Eighteen anharmonic constants of methyl chloride have been deduced from the measurements of band centers found in the literature.     

Anharmonic force field of acetylene

The harmonic and anharmonic force field of acetylene has been determined in a least-squares calculation from recently determined data on the spectroscopic constants of various isotopic species (including the vibrational l-doubling constant). A general quadratic and cubic force field was used, but a constrained quartic force field containing only 8 of the 23 possible quartic constants. The results are discussed and compared with earlier work.     

Algebraic Description of Anharmonic Stretching Vibrations

A U(2) algebraic model is presented to describe stretching vibrations of XYn (n = 2, 3, and 4) systems, where anharmonic interactions between the bond modes are considered. This model in a limit corresponds to an anharmonically coupled local-mode model. As an example, the model for a molecule XY4 is applied to recently observed spectra of methane in both gas and liquid phases, and the results obtained are in good agreement with the experiments. Copyright 1999 Academic Press.     

Anharmonic Parameters of Vibrationally Excited Molecules

An algebraic method of perturbation theory for eigenvalues and eigenvectors is used to examine the intramolecular parameters of excited vibrational states. Based on this method, formulas for higher-order coefficients of the matrix of vibrational modes, changes in the Cartesian coordinates of vibrating atoms, and anharmonic elements of the tensor of inertia moment are derived.     

Anharmonic force field for FCP

It is shown that, whereas a simple harmonic force field suggests that the FC stretching constant exceeds that for CP, a fuller anharmonic treatment suggests values of 806 and 946 N m^?1, respectively. Equilibrium bond lengths are 127.17 and 154.81 pm respectively.     

Anharmonic stacking in supercoiled DNA

Multistep denaturation in a short circular DNA molecule is analyzed by a mesoscopic Hamiltonian model which accounts for the helicoidal geometry. Computation of melting profiles by the path integral method suggests that stacking anharmonicity stabilizes the double helix against thermal disruption of the hydrogen bonds. Twisting is essential in the model to capture the importance of nonlinear effects on the thermodynamical properties. In a ladder model with zero twist, anharmonic stacking scarcely affects the thermodynamics. Moderately untwisted helices, with respect to the equilibrium conformation, show an energetic advantage against the overtwisted ones. Accordingly moderately untwisted helices better sustain local fluctuational openings and make more unlikely the thermally driven complete strand separation.     

Anharmonic force field for methanol

An ab initio quartic anharmonic force field for methanol has been calculated at the equilibrium position using the CCSD(T) method for the structure and the harmonic potential energy surface, and the MP4(SDQ) method for the anharmonic part of the surface. A triple zeta basis set was employed with symmetrized curvilinear internal valence coordinates in all calculations. The internal coordinate force field constants have been transformed into force constants in the dimensionless normal coordinate representation for various isotopomers. Vibrational term values for CH₃OH, CH₃OD, CD₃OH, and CD₃OD have been obtained using second order perturbation theory. Particular care has been devoted to the inclusion of Fermi resonance interactions between different vibrational states. A good accuracy has been achieved in the calculation of the fundamentals for all the isotopomers, the mean absolute error being 5.8 cm^?1.     

Anharmonic properties of rocksalt structure solids

An effort has been made for obtaining the anharmonic properties of rocksalt structure solids starting from primary physical parameters viz. nearest-neighbor distance and hardness parameter assuming long- and short-range potentials at elevated temperatures. The elastic energy density for a deformed crystal can be expanded as power series of strains for obtaining coefficients of quadratic, cubic and quartic terms which are known as the second-, third- and fourth-order elastic constants, respectively. When the values of the higher-order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher-order elastic constants are computed up to their melting temperature for rocksalt structure solids, which are alkali cyanides, sodium and potassium halides. The first order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures for these substances. The results thus obtained are compared with experimental data and found in well agreement with present values.     

Anharmonic effect of adiabatic quantum pumping

Based on the scattering matrix approach.we systematically investigate the anharmonic effect of the pumped current in double-barrier strructures with adiabatic time-modulation of two sinusoidal AC driven potential heights. The pumped currem as a functionof the pha.se difference bet ween the two driven potentials looks like to be sinusoidal, but actually it contains sine functions of double and more phase difference, it is found that this kind of anharmonic effect of the pumped current is determined combinedly by the Berry curvature and parameter va.riation loop trajectory. Therefore small ratio of the driving amplitude and the static amplitude is not necessary for harmonic pattern in the pmnped era＇rent, to dominate for smooth Berry curvatureon the surface within the parameter variation loop.