摘要:

A general method is described for calculating the vibronic energy levels associated with two Born-Oppenheimer states of a triatomic molecule that become degenerate in the linear limit. The method combines Renner's matrix treatment of orbital angular momentum in linear molecules with Hougen, Bunker and Johns' formalism for large amplitude bending motions, and allows for the effects of electron spins; it applies equally well to linear and bent molecules. The effects of the non-integral nature of the matrix elements ofLzin bent molecules are included, and the calculation of relative band intensities in optical and photoelectron spectra is described.

ISSN:0026-8976    

DOI:10.1080/00268978000101291

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The vibrational andK-type rotational levels of the[Xtilde]2B1andÃ2A1states of NH2and H2O+have been fitted by least squares to give sets of Born-Oppenheimer potential curves for the combining electronic states. The model used allows a simultaneous fit to all the observed vibronic levels for each of the two molecules, using a large amplitude formalism and making no approximations other than to neglect Fermi (anharmonic) resonance effects. An accurate description of the effects of orbital angular momentum on the level positions in the two molecules has been obtained, and relative transition moments have been calculated for the vibronic bands of theÃ2A1-[Xtilde]2B1transitions. For NH2the relative transition moments permit the calculation of the emission lifetimes (with the absolute value of the electronic transition moment taken from theab initiocalculations of Peyerimhoff and Buenker); the calculated lifetimes are found to agree very well with the experimental values of Halpernet al., and Donnellyet al. Relative transition moments for the photoelectron spectrum of H2O(H2O+,Ã2A1and[Xtilde]2B1← H2O,[Xtilde]1A1) have also been calculated. TheÃ2A1state of H2O+is confirmed as being linear at equilibrium, with the potential function having an unusual wide flat minimum.

ISSN:0026-8976    

DOI:10.1080/00268978000101301

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The effects of orbital angular momentum on the details of the spin and rotational fine structure of the[Xtilde]2B1andÃ2A1states of NH2and H2O+are considered. It is found that the erratic spin-orbit splittings and asymmetry parameters of theÃ2A1states can be reproduced with good accuracy by calculations that also account for the regular spin and rotational structures of the ground states; the only input parameters are the shapes of the Born-Oppenheimer potential curves, the bond lengths and the spin-orbit coupling constants. The same calculations give an almost quantitative explanation of various perturbations in theÃ2A1state of NH2. These perturbations are caused by the ground state and result from the presence of orbital angular momentum. Three types have been documented; one is direct spin-orbit interaction between theÃand[Xtilde]states, and the others involve rotational asymmetry, which is found to be an important mechanism for causing perturbations between electronic states.

ISSN:0026-8976    

DOI:10.1080/00268978000101311

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The electronic orbital angular momentum effects in theÃ1IIustate of C3have been considered. A very good fit to the observed vibronic energy levels and rotational constants has been obtained using a full matrix treatment of the orbital angular momentum in conjunction with a large amplitude formalism for the bending motion. The importance of the variation of the expectation value <Lz> with bond angle is emphasized; the quality of the final fit is very much improved when this is included. Franck-Condon overlap integrals have been calculated for bands of theÃ1IIu-[Xtilde]1Σg+transition, which is the carrier of the 4050 Å group of bands prominent in cometary spectra. A comparison has been made between the vibronic energy levels of linear and quasi-linear molecules in orbitally degenerate electronic states.

ISSN:0026-8976    

DOI:10.1080/00268978000101321

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The structure and dynamics of liquid CCl4are analysed on the basis of a molecular dynamics experiment involving 64 particles and lasting 10 ps. CCl4molecules are modelled as rigid tetrahedra interacting by a 4CLJ potential. The resulting spatial and time correlation functions are compared to X-ray, neutron and light scattering data. Selected expansion coefficients ofg(r, Θ1, Θ2) are used in a critical examination of various orientational models.

ISSN:0026-8976    

DOI:10.1080/00268978000101331

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

We have simulated the properties of an ensemble of biaxial particles interacting via an anisotropic pair potential, truncated at the second rank terms, using the Monte Carlo technique. The particles are confined to the sites of a face centred cubic lattice and the interactions are restricted to nearest neighbours. The system exhibits three phases in addition to the biaxial crystalline phase at absolute zero. There is a rotationally disordered (isotropic) phase followed by a uniaxial rotationally ordered (nematic) phase which in turn becomes biaxial. The isotropic-nematic transition is found to be first order while the uniaxial-biaxial nematic transition is almost certainly second order. These observations confirm therefore the qualitative predictions of approximate theories of nematics formed by non-cylindrically symmetric particles. The quantitative agreement with certain of these theories for quantities, such as the order parameter and the transition temperature, is quite good.

ISSN:0026-8976    

DOI:10.1080/00268978000101341

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

We present an original method for integrating numerically a scalar generalized Langevin Equation (GLE) for an arbitrary gaussian ‘random force’ process. Our procedure rests on the approximation of this last stochastic process by a vectorial gaussian-Markov one which reproduces the original autocorrelation function within a given accuracy. This is obtained by truncating at some suitablenth order the Mori continued fraction representation of the original autocorrelation function. In this way, the integrodifferential GLE is shown to be equivalent to a set ofnfirst order differential equations with an additional gaussian white noise source. This system can then be integrated with the usual algorithms. The method is illustrated by the generalized brownian motion of a spherical particle in a medium at equilibrium. Assuming a gaussian velocity autocorrelation function, we compare Markov approximations of increasing order to the original function and we perform a stochastic dynamics of the process in the 10th order Markov approximation. Various time averages computed on the resulting trajectory are shown to be in good agreement with the known exact results.

ISSN:0026-8976    

DOI:10.1080/00268978000101351

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

We present a theory to calculate, first, the correlation function of the physical quantity (angular momentum for spin-rotation interaction, second order spherical harmonics for the dipolar interaction), involved in magnetic spin-lattice relaxation and then the relaxation times. The time behaviour of the physical quantityh(t) is described by a generalized Langevin equation whose memory function,K(t), is assumed to be proportional to the density fluctuation correlation function,N(t). The time-independent proportionality function, α, is obtained by assuming that the relaxation time τh(for h(t) to reach local equilibrium) is equal to the correlation time, τn, of the density fluctuations. The density fluctuation correlation function is calculated by using the classical hydrodynamics form of the structure factorS(q, ω) [1, 2]. The definition chosen forN(t) forces us to consider only temperatures above the critical temperature, since we do not know the particle density of the liquid below the critical temperature.

ISSN:0026-8976    

DOI:10.1080/00268978000101361

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The results of a previous simulation for a fluid of rigid heteronuclear diatomic molecules using a shifted force, site-site potential [1] are extended to evaluate several time correlation functions for four state points for the orthobaric liquid. Time autocorrelation functions are reported for the centre of mass velocity, the force, the torque, the angular velocity and the orientational functionsP1(cos ϑ) andP2(cos ϑ). We also report the self and distinct parts of the Van Hove time dependent pair correlation function. The orientational correlation times and functions are compared with experimental results from nuclear magnetic resonance, Raman scattering and far infrared absorption and also with two popular simple models of molecular reorientation; the simulation results are in only moderate agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268978000101371

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Measurements of the slow neutron scattering cross-section at 0·5 Å for nitrogen gas and oxygen gas at room temperature and low density are reported. The results are interpreted in terms of a new quantity, the structural second virial coefficient,B2(Q, T), whereB2(O, T) is the conventional second virial coefficient. It is shown that for nitrogen the observed function, or the ‘difference’ cross-section, corresponds quite well to that calculated for an isotropic Lennard-Jones potential and less well for recently proposed anisotropic potentials. The method is promising for the determination of interaction potentials for molecules.

ISSN:0026-8976    

DOI:10.1080/00268978000101381

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor