New Experimental and Theoretical Results for Argon Broadening and Shift of HCO+ Rotational Lines

An experimental and theoretical study of the pressure broadening and the pressure shift of three HCO⁺ rotational lines (j=4←3, 5←4 and 6←5) perturbed by collisions with Ar is presented. The measurements are carried out at 77 K and are compared to close‐coupling calculations performed on an accurate potential energy surface for the Ar–HCO⁺ interaction extending from small to very large separations between the ion and the perturber. For the pressure broadening, agreement between experiment and theory is satisfactory for both close‐coupling and semiclassical calculations. For the pressure shift, however, close‐coupling calculations are superior. The results agree with experiment in sign and order of magnitude, while semiclassical calculations are inaccurate for the shift of the presently studied lines because they neglect the contribution of strong collisions.     

An ab initio study of the potential surface for the reaction H2COH → HCO + H2

The potential surface for the reaction H₂CO⁺H → HCO⁺ + H₂ has been studied by ab initio SCF calculations, using gaussian-type basis functions. A saddle point on the surface has been found, and a reaction path is proposed to explain the observed release of kinetic energy. The energy of activation and ΔE for the reaction have been estimated.     

On the sudden approximation of cross: computational tests and factorization of cross sections and related scattering phenomena

A sudden approximation recently derived by Cross using a semiclassical treatment of the orbital motion is recast into a form which permits factorization of differential and integral degeneracy averaged cross sections, opacities as a function of final angular momentum quantum number, the scattering amplitude, and the phenomenological cross section which describes spectral line broadening. Calculations are done using an average of initial and final orbital angular momentum quantum numbers for the partial wave parameter for ArN₂, ArTIF, H⁺H₂ and Li⁺H₂. The results indicate that the method is a good approximation for integral cross sections and opacities when the energy sudden approximation is valid and when the coupling of the orbital motion is important.     

Spectroscopic study of the Σ-potential of the system Li3D+He at large separations

We have studied the Li3D+He interaction in the Σ molecular state by measurements of the absorption line wing profiles of the upper Li transition 2P–3D, λ 610 nm. Measurements of absolute cross sections normalized to the integrated line absorption have been performed in a two step laser pump and probe scheme in the detuning range ±300 cm^?1 to both sides of the line center. The analysis of the blue wing on the basis of the quantummechanical theory of the collisional line broadening shows, that the experimental 3DΣ potential energy curve is less repulsive in the asymptotic rangeR≈15–20 a.u. than predicted from semiempirical model calculations of Pascale.     

Theoretical study of single-electron capture in the N5+ + He and O6+ + He collisions by means of ab initio methods

Partial cross sections of single-electron capture on the n = 3 levels have been determined theoretically for the N⁵⁺ + He and O⁶⁺ + He collisions by means of a semiclassical method using ab initio potential energy curves and radial and rotational coupling matrix elements. The different behavior of these two isoelectronic systems is fairly well reproduced by our calculations.     

Collisional broadening of Rydberg atoms perturbed by ground-state sodium atoms

Collisional broadening ofnS Rydberg states of alkali-metal atoms by sodium atoms is calculated using the adiabatic approach for the³ quasimolecular state, and impulse approximation for all other contributions to the collisional width. We obtain oscillatory dependence of the collisional width on the principal quantum number which was observed earlier for heavy alkalies as perturbers. However, the width and the amplitude of the oscillation are much lower than in the case of heavy alkalies. We present a comparison between the results for broadening of heavy alkalies by sodium and for broadening of sodium atoms by heavy alkalies.     

Efficient evaluation of the accuracy of molecular quantum dynamics on an approximate analytical or interpolated ab initio potential energy surface

Ab initio electronic structure methods have reached a satisfactory accuracy for the calculation of static properties, but remain too expensive for quantum dynamical calculations. Recently, an efficient semiclassical method was proposed to evaluate the accuracy of quantum dynamics on an approximate potential without having to perform the expensive quantum dynamics on the accurate potential. Here, this method is applied for the first time to evaluate the accuracy of quantum dynamics on an approximate analytical or interpolated potential in comparison to the quantum dynamics on an accurate potential obtained by an ab initio electronic structure method. Specifically, the vibrational dynamics of H₂ on a Morse potential is compared with that on the full CI potential, and the photodissociation dynamics of CO₂ on a LEPS potential with that on the excited ¹Π surface computed at the EOM‐CCSD/aug‐cc‐pVDZ level of theory. Finally, the effect of discretization of a potential energy surface on the quantum dynamics is evaluated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2426–2435, 2010     

Quantum and quasiclassical study of the collinear reaction O(3P) + H2 → OH + H using a LEPS and fitted AB initio potential energy surface

Accurate quantum calculations of reaction probabilities P^T_ν′←ν have been carried out for the collinear reaction O(³P) + H₂ (ν = 0,1) → OH(ν′) + H using a LEPS and fitted ab initio potential energy surface. The energy dependence of the P^T_{ν′ ← ν} is similar for both surfaces. Collinear quasiclassical trajectory calculations have also been carried out, for comparison, on the LEPS surface for ν = 0, 1 and 2.     

CAS SCF/CI calculations of potential energy surfaces of He3+ and He2+

Complete active space MC SCF (CAS SCF) calculations followed by second-order configuration interaction (SOCI) calculations are carried out on the potential energy surfaces (bending surface, linear surfaces) of the ²Σ_g⁺ ground state of He₃⁺. The potential minimum for the ²Σ_g⁺ state occurs at a linear geometry with HeHe bond length of 1.248 Å. The binding energy of He₃⁺ with respect to He + He⁺ + He was calculated to be 2.47 eV at the SOCI level. The energy required to dissociate He₃⁺ (²Σ_g⁺) into He₂⁺ (²Σ_u⁺) and He(¹S) is calculated to be 0.14 eV. The same level of SOCI calculations of He₂⁺ yield a D_e value of 2.36 eV.     

Experimental determination of level shift and broadening by predissociation of the B3Π0· state of ICl

From the semiclassical analysis of the coupling between discrete molecular levels and continuum levels simple relations for the level shift and the level broadening are known. Laser spectroscopy on the transition is B³Π₀·?X¹Σ⁺ is applied to demonstrate the overall validity of these relations in the rotational ladder of the vibrational state υ′ = 3 of B³Π₀·. Deviations for low rotational quantum numbers might be attributed to the still unresolved hyperfine structure and the possible hyperfine predissociation and for very large rotational quantum numbers to the distortion through a coupling with a second continuum.     

Rotational energy transfer and anisotropic intermolecular potentials : confrontation of experiment with ab-initio calculations for CO-H2 and CO-He

A tunable diode lase in the 4.7 μm region has been used to perform measurements of pressure-broadening in infrared lines of CO(v = 1 ←0) perturbed by H₂ and He at liquid nitrogen and room temperatures. For the COH₂ system the experimental results are compared to previous double resonance data and to ab-initio quantum calculations. The COHe data make a confrontation of the two available potential energy surfaces feasible.     

Stimulated radiative dissociation and gain measurements of Xe2Cl in solid xenon

The molecular charge transfer states of Cl-doped solid xenon form an ideal four-level laser system. UV excitation on the molecular XeCl (Bi ← X) pair potentials leads to the formation of the diatomic exciplex which relaxes with a nearly unity quantum yield to the triatomic Xe⁺₂ Cl⁻ (4²Γ) state. The radiative dissociation of the triatomic exciplex can be stimulated to provide monoenergetic atoms ≈ 1 eV above ground. While these systems are characterized by large gain coefficients, scattering losses predominate in samples prepared by standard matrix isolation techniques. Scattering losses are due to the inability of the lattice to accommodate the excess kinetic energy released in the bound to repulsive transition of the triatomic exciplex.     

The potential barrier of theB 1Γ u state of Na2

With a semiclassical model the measured energy and width of quasibound levels are described for theB ¹Γ _u state of Na₂. A reliable complete potential function of this electronic state is derived including the potential hump which results from the competition between the electron exchange energy and the long range dipole-dipole interaction. The proposed computation of molecular potentials can be used in the general case and shows some advantages over RKR determinations.     

An ab initio calculation of symmetric bending and stretching vibrational states of the H3O+ and D3O+ ions

Calculations are reported for the symmetric bending and stretching vibrational states of H₃O⁺ and D₃O⁺ including coupling between these two modes. The calculations were carried out by using a potential surface calculated by the SCF CI method and expressed in terms of symmetric internal coordinates. The transition energy of the ν₂ (1^? ← 0⁺) inversion mode is found to be 985 cm^?1, which is comparable to the experimental value of 954.417 cm^?1 observed by Haese and Oka. The calculated inversion doubling of the lowest state is 51 cm^?1.     

Energetics and Structures of Charged Helium Clusters: Comparing Stabilities of Dimer and Trimer Cationic Cores

We present accurate ab initio calculations of the most stable structures of He_n⁺ clusters in order to determine the more likely ionic core arrangements existing after reaching structural equilibrium of the clusters. Two potential energy surfaces are presented: one for the He₂⁺ and the other with the He₃⁺ linear ion, both interacting with one He atom. The two computed potentials are in turn employed within a classical structure optimization where the overall interaction forces are obtained within the sum‐of‐ potentials approximation described in the main text. Because of the presence of many‐body effects within the ionic core, we find that the arrangements with He₃⁺ as a core turn out to be energetically preferred, leading to the formation of He₃⁺(He)_n?3 stable aggregates. Nanoscopic considerations about the relative stability of clusters with the two different cores are shown to give us new information on the dynamical processes observed in the impact ionization experiments of pure helium clusters and the importance of pre‐equilibrium evaporation of the ionic dimers in the ionized clusters.     

Stark broadening of Mg I and Mg II spectral lines and Debye shielding effect in laser induced plasma

We report Stark broadening parameters for three Mg I lines and one Mg II line in the electron number density range (0.67–1.09) · 10¹⁷ cm^− 3 and electron temperature interval (6200–6500) K. The electron density is determined from the half width of hydrogen impurity line, the H_α, while the electron temperature is measured from relative intensities of Mg I or Al II lines using Boltzmann plot technique. The plasma source was induced by Nd:YAG laser radiation at 1.06 μm having pulse width 15 ns and pulse energy 50 mJ. Laser induced plasma is generated in front of a solid state surface. High speed photography is used to determine time of plasma decay with good homogeneity and then applied line self-absorption test and Abel inversion procedure. The details of data acquisition and data processing are described and illustrated with typical examples. The experimental results are compared with two sets of semiclassical calculations and the results of this comparison for Mg I lines are not unambiguous while for Mg II 448.1 nm line, the results of Dimitrijević and Sahal-Bréchot calculations agree well with our and other experimental results in the temperature range (5000–12,000) K and these theoretical results are recommended for plasma diagnostic purposes. The study of line shapes within Mg I 383.53 nm multiplet shows that the use of Debye shielding correction improves the agreement between theoretical and experimental Stark broadening parameters.     

Bridge‐Localized HOMO‐Binding Character of Divinylanthracene‐Bridged Dinuclear Ruthenium Carbonyl Complexes: Spectroscopic,Spectroelectrochemical, and Computational Studies

The electronic properties of four divinylanthracene‐bridged diruthenium carbonyl complexes [{RuCl(CO)(PMe₃)₃}₂(μ? CH?CHArCH?CH)] (Ar=9,10‐anthracene ( 1 ), 1,5‐anthracene ( 2 ), 2,6‐anthracene ( 3 ), 1,8‐anthracene ( 4 )) obtained by molecular spectroscopic methods (IR, UV/Vis/near‐IR, and EPR spectroscopy) and DFT calculations are reported. IR spectroelectrochemical studies have revealed that these complexes are first oxidized at the noninnocent bridging ligand, which is in line with the very small ν(C?O) wavenumber shift that accompanies this process and also supported by DFT calculations. Because of poor conjugation in complex 1 , except oxidized 1⁺ , the electronic absorption spectra of complexes 2⁺ , 3⁺ , and 4⁺ all display the characteristic near‐IR band envelopes that have been deconvoluted into three Gaussian sub‐bands. Two of the sub‐bands belong mainly to metal‐to‐ligand charge‐transfer (MLCT) transitions according to results from time‐dependent DFT calculations. EPR spectroscopy of chemically generated 1⁺ – 4⁺ proves largely ligand‐centered spin density, again in accordance with IR spectra and DFT calculations results.     

A comparative study of the effects of different buffer gases on the line shape of water vapour rovibronic transition

The line shape parameters of water vapour rovibrational transition at 12145.444 cm⁻¹ belonging to the (2ν₁ + ν₂ + ν₃) overtone band in the presence of three buffer gases He, N₂ and CO₂ are studied by high resolution near infrared (NIR) diode laser spectrometer. The wavelength modulation spectroscopy (WMS) is adopted to probe buffer gas broadened first derivative (1f) signal of water vapour transition. The simulation of the experimental signals based on Voigt profile provides the collisional broadening coefficients and line strength parameters. The comparison is made between the broadening effects on water vapour transition induced by He, N₂ and CO₂.     

On the origin of the dynamical threshold for collision-induced dissociation processes

Collision-induced dissociation from the ground vibration-rotation state of the reactant diatom is studied in the systems He + H₂, H + H₂, and He + H⁺₂ by quasiclassical trajectory calculations, using ab initio potential energy surfaces. The dependence of the dynamical threshold values on the shape of the potential energy surface is discussed.