Theoretical simulations of the He79Br2 B, v=8←X, v″=0 excitation spectrum: Spectroscopic manifestation of a linear isomer? (original) (raw)
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The Journal of Chemical Physics, 2002
The B←X rovibronic excitation spectrum of the HeBr 2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology ͑linear and T-shaped wells͒ for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of the HeBr 2 (X) complex. The complete assignment of the spectrum in terms of individual rovibronic transitions is presented.
Physical Review B, 1992
We consider a-filled, two-band discrete tight-binding Peierls-Hubbard. model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or MX chain). W'e have employed the adiabatic approximation in which the quantum Buctuations associated with phonons are implicitly treated as an external field for the electrons, and treat electron-electron eÃects in the Hartree-Fock approximation. We investigate ground states as functions of the model parameters and doping-induced and photoinduced excitationskinks, polarons, bipolarons, and excitons. Results for several experimental observables, including the lattice distortion, the excess charge and spin densities of defects, and the optical absorption, are compiled. For the ground state, we find that the bond-order-wave (BOW) portion of the one-band phase diagram is eliminated from the two-band phase diagram, in agreement with the lack of real materials in the pure BOW phase. The extent of electron-hole asymmetry and of spatial localization or delocalization of defects is explored. Two separate solitons or polarons are compared with corresponding bipolarons. We demonstrate explicitly the need to employ the two-band model for a realistic modeling of the Mx systems, focusing on three specific systems: (a) highly distorted, valence-localized (strongly charge-disproportionated) PtC1, (b) moderately distorted PtBr, and (c) weakly distorted, valence-delocalized (weak chargedensity wave) PtI. The compilation of resu1ts reported here constitutes a reference resource against which the rapidly expanding experimental data can be compared.
Structural Chemistry, 2012
ABSTRACT Spectroscopy, interaction energy, and dissociation of linear and T-shaped isomers of HeClF, NeClF, and ArClF van der Waals complexes in their ground state have been studied in detail using MP2 and CCSD(T) methods in conjunction with correlation consistent valence triple and quadruple zeta basis sets. A method, called potential method, has been developed to remove the discrepancy between theoretical and experimental values for the depth of the potential well and dissociation energies for these complexes. This is also supported by the supermolecular approach. Most of the structural and spectroscopic properties of these complexes are first reported and the rest agree very well with the experimental and theoretical values wherever available. Two local minima corresponding to linear and asymmetric T-shaped are found for RgClF complexes. For NeClF complex, the predicted values for the equilibrium bond length and well depth are R NeCl = 3.096 Å and $ D_{\text{e}}^{\text{p}} $ = 161.50 cm−1 for the linear isomer and R NeCl = 3.503 Å and $ D_{\text{e}}^{\text{p}} $ = 126.10 cm−1 for the T-shaped isomer. Various dissociation channels are also investigated in detail.
Excitation spectra of van der Waals molecules
The present contribution offers an educated guess of the electronic excitation spectrum of van der Waals complexes, which can be used to set up initial experimental conditions in laser induced fluorescence spectroscopy experiments. In particular, we present bound-state calculations and predicted vertical excitation energies of Li-CO, CO-Ar and CO-H2 van der Waals molecules, for which none or insufficient experimental data exist on their electronic excitation spectra. A review the experimental techniques and ab initio theoretical methods used to study interaction energies of van der Waals complexes is also presented.
The Journal of Chemical Physics, 2008
Valence excitation spectra for the linear isomers of He–, Ne–, and Ar–Br2 are reported and compared to a two-dimensional simulation using the currently available potential energy surfaces. Excitation spectra from the ground electronic state to the region of the inner turning point of the Rg–Br2 (B,ν′) stretching coordinate are recorded while probing the asymptotic Br2 (B,ν′) state. Each spectrum is a broad continuum extending over hundreds of wavenumbers, becoming broader and more blueshifted as the rare gas atom is changed from He to Ne to Ar. In the case of Ne–Br2, the threshold for producing the asymptotic product state reveals the X-state linear isomer bond energy to be 71±3cm−1. The qualitative agreement between experiment and theory shows that the spectra can be correctly regarded as revealing the one-atom solvent shifts and also provides new insight into the one-atom cage effect on the halogen vibrational relaxation. The measured spectra provide data to test future ab initio ...
Chem Phys, 1997
We report the investigation of the 3s←2p transition in the Bar2 cluster. In a supersonic expansion of B atoms entrained in Ar, at high beam source backing pressures we observe several features in the fluorescence excitation spectrum which cannot be assigned to the Bar diatom. Using Bar(X,B) potential energy curves which reproduce our experimental observations on this molecule and an Ar-Ar interaction potential, we employ a pairwise additive model, along with variational and diffusion Monte Carlo treatments of the nuclear motion, to determine the lowest vibrational state of the Bar2 cluster. A subsequent simulation of the fluorescence excitation spectrum reproduces nearly quantitatively the strongest feature in our experimental spectrum not assignable to Bar. Because of the barrier in the Bar(B 2Σ+) potential energy curve, the 3s←2p transition in the Bar2 is predicted to have an asymmetric profile, as is found experimentally.