Dimethylacetylene: the theory required to analyse the infrared and Raman perpendicular bands

In this paper we give a detailed account of the theory required to simulate and analyse the infrared and Raman perpendicular fundamental bands of the dimethylacetylene molecule at high resolution. A summary of this theory has appeared in a previous paper (P.R. Bunker, J.W.C. Johns, A.R.W. McKellar and C. di Lauro, J. Mol. Spectry. 162 (1993) 142) in which an analysis of the infrared methyl rocking fundamental band was given. As well as detailing the effect of various terms in the Hamiltonian we also discuss the Raman selection rules and show that the analysis of the ΔK = 2 component of the perpendicular fundamental bands will lead to a determination of the sign of the torsional barrier. The sign of the barrier (i.e. whether the minimum energy conformation is staggered or eclipsed) cannot be determined from the analysis of the infrared perpendicular bands.     

The potential energy curve of the B 1Πu state of Na2

We have used the experimental energy levels of the B state of Na₂ to determine the shape of the potential curve. The potential curve obtained has been used to determine the Franck—Condon overlap of the continuum vibrational levels in the B state with bound levels in the ground electronic state.     

Trajectory studies of energy transfer: CH3NC with He,Xe, H2 and N2

A trajectory program was used to simulate the collisions of CH₃NC with He, Xe, H₂ and N₂. Calculated energy transfer is in accord with experiment. The pattern of CH₃NC vibrational mode energization is found to be noticeably non-random. The approximate sampling methods used in thermal unimolecular trajectory studies produce a more uniform state distribution.     

Fifteenth International Conference on X-ray Absorption Fine Structure

The 15th International Conference on X-ray Absorption Fine Structure (XAFS15) was held at the Pullman Beijing West Wanda Hotel in Beijing, China, on July 22–28, 2012. The conference was chaired by Ziyu Wu (Chinese Academy of Sciences). In conjunction with XAFS15, the workshop “XAFS theory and nano particles” was held July 18–20, 2012, in Chiba, Japan, and chaired by T. Fujikawa.     

Modeling of pair distribution functions for XAFS in disordered systems

A commonly used expression for modeling the XAFS of highly disordered systems is shown to generate substantial systematic errors in the coordination number in many cases of practical interest. This expression is corrected and generalized. Further, a simpler and more flexible model distribution is proposed, and the corresponding XAFS expression is derived. Comparison with experimental and simulated data show that the new expressions are useful in cases of high disorder for which the cumulant expansion loses its utility, and they explicitly account for the k-dependence of the mean free path.     

A Theoretical Study of ãA2 CH2

The potential energy surface and dipole moment surfaces of the ã⁴A₂ electronic state of CH₂⁺ are calculated ab initio using an augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set, with the incorporation of dynamical correlation using the coupled cluster method with single and double excitations and perturbatively connected triple excitations [CCSD(T)]. We use these surfaces in the MORBID program system to calculate rotation and rotation-vibration term values for ã-state CH₂⁺, CD⁺₂, and CHD⁺ and to simulate the rotation and rotation-vibration absorption spectrum of CH₂⁺ in the ã⁴A₂ electronic state. Our work is motivated by studies of CH₂⁺ that use the Coulomb explosion imaging technique and by the goal of predicting spectra that may be obtained from discharge sources. Although the ã state is the lowest-lying excited state above the X?/Ã ground state pair, it turns out to be relatively high-lying, and we determine that T_e(ã)=30447.5 cm⁻¹. The equilibrium bond angle for ã-state CH₂⁺ is only 77.1°; as a result the asymmetric top κ value is close to 0, and the molecule is equally far from the oblate and prolate symmetric top limits in this electronic state.     

The Renner effect in triatomic molecules with application to CH+, MgNC and NH2

We have developed a computational procedure, based on the variational method, for the calculation of the rovibronic energies of a triatomic molecule in an electronic state that become degenerate at the linear nuclear configuration. In such an electronic state the coupling caused by the electronic orbital angular momentum is very significant and it is called the Renner effect. We include it, and the effect of spin-orbit coupling, in our program. We have developed the procedure to the point where spectral line intensities can be calculated so that absorption and emission spectra can be simulated. In order to gain insight into the nature of the eigenfunctions, we have introduced and calculated the overall bending probability density function f(p) of the states. By projecting the eigenfunctions onto the Born-Oppenheimer basis, we have determined the probability density functions f+(rho) and f-(rho) associated with the individual Born-Oppenheimer states phi(-)elec and phi(+)elec. At a given temperature the Boltzmann averaged value of the f(p) over all the eigenstates gives the bending probability distribution function F(rho), and this can be related to the result of a Coulomb Explosion Imaging (CEI) experiment. We review our work and apply it to the molecules CH2+, MgNC and NH2, all of which are of astrophysical interest.     

The application of the nonrigid bender Hamiltonian to a quasilinear molecule

The form of the nonrigid bender has changes that here we do render. We add, nicely paired, a term to J² and regroup factors that are singular. As a result, the nonrigid bender Hamiltonian can now be set up using only Van Vleck perturbation theory, for any triatomic molecule (linear, quasi-linear, or bent). It can be used to calculate the rotation-vibration energies of the molecule to high $\text{J(?10)}$ from the bending potential energy function and the stretch and stretch-bend force constants.