Experimental and Theoretical Study of Helium Broadening and Shift of HCO+ Rotational Lines

An experimental and theoretical study of pressure broadening and pressure shift of HCO⁺ rotational lines perturbed by collisions with He is presented. Results are reported from measurements at 88 K for the lines j=4←3, 5←4 and 6←5 with frequencies ranging from 0.35 to 0.54 THz. Using a new CCSD(T)/aug‐cc‐pVQZ potential energy surface for the He–HCO⁺ interaction, the collisional line shape parameters are studied from fully quantum and semiclassical calculations. Results from the quantum treatment are in satisfactory agreement with experiments whereas the semiclassical approach can lead to appreciable differences. A study of the dependence of line width Γ and shift s as a function of the translational energy shows the presence of quantum oscillations. Calculations on a previous Hartree–Fock‐based potential energy surface lead to quite similar results for the collisional line shape parameters. Using a simplified version of the potential morphing method it is found that the line width Γ is particularly sensitive to the long‐range part of the potential energy surface. This also explains the success of the first line‐broadening calculations which date back to the 1950s.     

Experimental and Theoretical Study of the Broadening and Shifting of N2H+ Rotational Lines by Helium

Pressure broadening and pressure shift of N₂H⁺ rotational lines perturbed by collisions with He are studied for the first time using experiment and theory. Results are reported from measurements at 88 K for the rotational transitions ${j = 3 \leftarrow 2}$ , ${4 \leftarrow 3}$ , ${5 \leftarrow 4}$ and ${6 \leftarrow 5}$ with frequencies ranging from 0.28 to 0.56 THz. The agreement between experiment and theoretical data derived from close coupling calculations confirms the reliability of a theoretical framework used for state‐to‐state transition rates of interest in the interpretation of spectroscopic data from interstellar molecular clouds. The influence of hyperfine effects on shifts and widths of the rotational lines is discussed in detail. Although in principle possible, experiment and theoretical considerations lead to the conclusion that hyperfine effects only play a minor role.     

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.     

Nonresonant charge transfer: Semiclassical calculation of differential cross sections

Charge transfer and elastic scattering differential cross sections in Li⁺ + Na are described in a two state model by a consequent application of semiclassical methods. The coupling model used is Demkov coupling; coupling parameters and potential curves are taken from calculations of other authors. The theoretical results, which contain no adjustable parameters, are in good agreement with experiment.     

Differential scattering cross sections for collisions of alkali ions and atoms. II. Electronic excitation via rotational coupling in LiK+ and NaK+

Relative differential cross sections, for both direct and charge exchange scattering have been obtained for the Li⁺ + K, Na⁺ + K and K⁺ + Na alkali ion—atom collisions, over the energy range 200–1200 eV and for scattering angles 0–6 mrad (in one case 0–15 mrad)- The experimental results are compared to semiclassical calculations, based upon recent potential energy curves. The charge exchange probabilities are calculated by solving the time dependent Schrödinger equation in a two-state approximation. In the Li⁺+ K experiment diffraction effects are observed, which can be compared to the Fraunhofer diffraction. of an annular diaphragm.     

Toward a Complete Understanding of the Vinyl Fluoride Spectrum in the Atmospheric Region

A deep and comprehensive investigation of the vinyl fluoride (CH₂CHF) spectrum in the atmospheric window around 8.7 μm is presented. At first, the ro‐vibrational patterns are modelled to an effective Hamiltonian, which also takes into account the coupling of the C? F stretching vibration, ν₇, with the neighbouring vibrational combination ν₉+ν₁₂. The obtained Hamiltonian gives very accurate simulations and predictions of the ro‐vibrational quantum energies. Then, in the main part of the work, an experimental and theoretical study of vinyl fluoride self‐broadening collisions is carried out for the first time. The broadening coefficients obtained experimentally are compared with those calculated by a semiclassical theory, demonstrating a significant contribution of collisional coupling effects between lines connecting pairs of degenerate (or nearly degenerate) rotational levels. Finally, the experimentally retrieved integrated absorption coefficients are used to calculate the absorption cross‐section of the ν₇ normal mode, from which dipole transition moments are derived. The obtained results provide a deep insight into the spectral behaviour of vinyl fluoride, in a spectral region of primary relevance for atmospheric and environmental determinations. Indeed, the data presented constitute an accurate model for the remote sensing of vinyl fluoride—a molecule of proved industrial importance which can lead to hazardous effects in the atmosphere and affects human′s health.     

Line frequency shift measurements by diode-laser spectroscopy for CH(3)D-Xe

The pressure-induced Xe shifting and broadening coefficients for five lines of 12CH(3)D in the nu(3) band near 7.5 microm have been measured using a tunable diode-laser spectrometer. The frequency shift was determined from the simultaneous record of the Xe-broadened line and the same line of pure CH(3)D at low pressure. Comparisons are made with the results of theoretical calculations based on a semiclassical model involving the atom-atom Lennard-Jones (LJ) potential.     

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.     

The effect of the electron spin on angular momentum transfer in Na*(32 P 3/2)+Na+ collisions — crossed beam experiment and semiclassical calculation

The angular momentum transfer between electronic and heavy particle motion has been studied for inelastic collisions of laser state-prepared Na*(3² P _3/2,M _J ) with Na⁺ leading to Na*(3² D) or Na(3² S) in the energy rangeE _cm=5?47.5 eV. The measurements are compared to semiclassical calculations employing the coupled channel method in the impact parameter approximation but including dynamics of the electron spin coupling to the heavy particle motion.     

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.     

Stark broadening of halogen atom lines from (1 D)n p levels

We report results of a study of the Stark broadening of halogen atom lines from (¹ D)n p levels. Wall stabilized arc is used as a plasma source. Electron densities 2.2–3.2×10²² m^?3 are determined from the width ofH _α line and electron temperature 9300–10000 K from plasma composition data. The agreement with the results of simple semiclassical calculations is within the limits of the estimated errors of both experiment and theory. An explanation for the large discrepancy between theory and experiment detected for three BrI lines is offered.     

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.     

Emission spectroscopic studies of Grimm-type glow discharge plasma with argon-helium gas mixtures

The effect of argon/helium pressure ratios on the emission intensity of various Ar II lines is investigated for a Grimm-type glow discharge radiation source, operated with Ar-He mixtures. The relative intensities of the Ar II lines are altered significantly by mixing helium with argon. It is found that the population of the Ar⁺ excited states can be redistributed through He-Ar collisional energy transfer. The energy level of the He singlet metastable state (¹S₀,20.62 eV) is very important for these processes. If the excitation energy of Ar II lines is higher than that of the He singlet metastable, strong quenching of the Ar II line intensity is observed. However, when the excitation energy is slightly lower, some of the Ar II lines are enhanced by adding helium to the argon plasma. Energy exchanges between the Ar⁺ doublet term states and the He singlet metastable are favoured because the total spin remains unchanged before and after the He-Ar collisions. Furthermore, the helium mixing also exerts a great influence on the emission intensities of the elements sputtered from the cathode of the discharge lamp. The enhancement of Al I and Al II emission intensities at suitable Ar-He mixture ratios is discussed for when aluminum is employed as a cathode material.     

Coherent excitation of H(n=2) in H+, H - He collisions

The coherent excitation of H(n=2) in H⁺, H - He collisions was investigated at incident energies of 5–25 keV. From a polarization analysis of the emitted Lyman-α radiation as a function of an external electric field, the partial cross sections for excitation to the H(2s) and the H(2p _m) magnetic substates and the real part of thes ?p ₀-coherence were extracted. For H⁺-He collisions, the measured partial cross sections are in fair agreement with previous two-electron calculations by Kimura and Lin; the agreement with one-electron calculations of Jain et al. is, particularly at the lower incident energies, less satisfactory. For both collision systems, an energy-dependent forward-backward asymmetry corresponding to a shift of the center-of-charge relative to the center-of-mass (dipole moment) was observed. In H⁺ - He collisions, the measured dipole moment was positive; it thus corresponds to an electron trailing behind the proton. The same analysis applied to the H - He system showed the electron riding in front of the proton.     

Infrared Cavity Ringdown Spectroscopy of Jet‐Cooled Polycyclic Aromatic Hydrocarbons

Infrared absorption spectra of the CH stretching region were observed for naphthalene, anthracene, phenanthrene, pyrene, and perylene using a heated, supersonic, slit‐jet source and cavity ringdown spectroscopy. Band positions and intensities recorded with 0.2‐cm^?1 resolution were compared with previous gas‐phase and argon matrix isolation experiments, as well as theoretical calculations. The largest matrix shift in the absorption maximum (‐7.4 cm^?1) was observed for anthracene, with all others shifted by 3.0 cm^?1 or less. Spectral features in the supersonic jet spectrum were generally narrower than those observed in the Ar matrix, with the largest matrix broadening found for the perylene (80 % increase). Low number densities observed for the larger polycyclic aromatic hydrocarbons (PAHs) suggest that the lower vapor pressure of PAHs with catacondensed four‐membered rings and with five‐membered rings other than perylene will not be detectable using our current configuration.     

Semiclassical Trajectory Studies of Reactive and Nonreactive Scattering of OH(A 2Σ+) by H2 Based on an Improved Full-Dimensional Ab Initio Diabatic Potential Energy Matrix

We present a new full-dimensional diabatic potential energy matrix (DPEM) for electronically nonadiabatic collisions of OH(A ²Σ⁺) with H₂, and we calculate the probabilities of electronically adiabatic inelastic collisions, nonreactive quenching, and reactive quenching to form H₂O+H. The DPEM was fitted using a many-body expansion with permutationally invariant polynomials in bond-order functions to represent the many-body part. The dynamics calculations were carried out with the fewest-switches with time uncertainty and stochastic decoherence (FSTU/SD) semiclassical trajectory method. We present results both for head-on collisions (impact parameter b equal to zero) and for a full range of impact parameters. The results are compared to experiment and to earlier FSTU/SD and quantum dynamics calculations with a previously published DPEM. The various theoretical results all agree that nonreactive quenching dominates reactive quenching, but there are quantitative differences between the two DPEMs and between the b=0 results and the all-b results, especially for the probability of reactive quenching.     

Excitation and charge transfer in He++H collisions

For the conflictive case of He⁺+H collisions, we present a norm-method optimization of the parameters included in the (often used) two-electron translation factor of Errea et al. As surmised in a previous publication, a strong cut-off is needed at short internuclear distances to prevent the translation factor from marring the properities of the molecular expansion there. With a basis of 16 molecular states, we present the first calculations including translation factors, of total and partial charge exchange and excitation cross sections in He⁺+H collisions, as well as the alignment parameter A₂₀ for hydrogen excitation. Good agreement with experiment is reached up to the energy range where ionization and charge exchange cross sections are comparable.     

Reaction pathways of Sc+ (3D, 1D) and Fe+ (6D, 4F) with acetone in the gas phase: metal ion oxidation and acetone deethanization

The reactions of Sc⁺ (³D, ¹D) and Fe⁺ (⁶D, ⁴ F) with acetone have been investigated in both high‐ and low‐spin states using density functional theory. Our calculations have indicated that oxidation of Sc⁺ by acetone can take place by (1) metal‐mediated H migration, (2) direct methyl‐H shift and/or (3) C = O insertion. The most energetically favorable pathway is metal‐mediated H migration followed by intramolecular ScO⁺ rotation and dissociation. For the deethanization of acetone mediated by Fe⁺, the reaction occurs on either the quartet or sextet surfaces through five elementary steps, i.e. encounter complexation, C–C bond activation, methyl migration, C–C coupling and non‐reactive dissociation. The rate‐determining step along the quartet‐state potential‐energy surface (PES) is similar to that in the case of Ni⁺ (² F, 3d⁹), namely the methyl‐migration step. For the sextet‐state PES, however, the energy barrier for methyl migration is lower than that for C–C bond activation, and the rate‐determining step is C–C coupling. In general, the low‐spin‐state pathways are lower in energy than the high‐spin‐state pathways; therefore, the reaction pathways for the oxidation of Sc⁺ and the Fe⁺‐mediated deethanization of acetone mostly involve the low‐spin states. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical state-to-state cross sections for collisions of N2+ (X, υ) or N2+(A, υ) with Ar

State-to-state cross sections have been calculated for collisions of N⁺₂ (X, υ) or N⁺₂ (A, υ) with Ar at relative energies of 8 and 20 eV. The computations utilize potential energy surfaces computed recently by Archirel and Levy. In the calculations the translational motion is treated classically, and the time-dependent Schrödinger equation is solved exactly for the vibronic states of the system. In addition to the charge transfer and vibrational excitation and deexcitation processes, cross sections are also obtained for internal conversion between N⁺₂ (A) + Ar and N⁺₂ (X) + Ar. The results are in good agreement with the available experimental data at these energies.     

Design,Synthesis, and Structural Analysis of Divalent NI Compounds and Identification of a New Electron‐Donating Ligand

The dative‐bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L→N←L′)⁺ arrangement are of special interest because of their therapeutic importance. This work reports the design and synthesis of novel chemical species with the general structural formula (L→N←L′)⁺ carrying the unusual ligand cyclohexa‐2,5‐diene‐4‐(diaminomethynyl)‐1‐ylidene. Four species belonging to the (L→N←L′)⁺ class carrying this unconventional ligand were synthesized. Quantum chemical and X‐ray diffraction analyses showed that the electronic and geometric parameters are consistent with those of already reported divalent N^I compounds. The molecular orbital analysis, geometric parameters, and spectral data clearly support the L→N and N←L′ interactions in these species. The newly identified ligand has the properties of a reactive carbene and high nucleophilicity.