Measurements of circular polarization correlations following excitation of Ar and Kr by electron impact

We report measurements of the circular polarization correlation parameterP ₃ for the excitation of the 4s′[1/2] _j=1 state in Ar by 40 eV electrons and for the excitation of the Kr 5s[3/2] _J=1 state by 30 eV electrons at electron scattering angles up to 50°. The measured Ar data are compared to predictions of a distorted wave Born approximation (DWBA). The agreement is in general not very good with theory predictingP ₃ values considerably larger than the measured values in the regime of small scattering angles. In Kr, where the measured data are compared to theoretical predictions from a DWBA, a first-order many-body theory (FOMBT) and a relativistic distorted wave theory (RDW), the level of agreement between experiment and theory is somewhat better. The measuredP ₃ parameters in conjunction with the previously measured linear coherence parametersP ₁ undP ₂ under the same scattering conditions yield the total polarizationP _tot ⁺ which is a measure for the level of coherence in the excitation process. In both cases studied here, we found values of eliminateP _tot ⁺ that were consistent with a fully coherent excitation process.     

Parity dependence in rotationally inelastic collisions of CaF(A2II, υ′ = 0) with He and Ar

CaF, produced from CaF₂ heated in a modified Broida-type oven, is excited by a cw dye into a selected (υ′= 0, low J value) rovibrational level of the A²II electronic state. The fluorescence signal is analysed through a high-resolution spectrometer. The ratio of the population of satellite levels. populated by collisions with Ar or He, to the population of the initially pumped level is derived from intensity measurements at varying buffer gas pressures (0.25–2 Torr). Cross sections are then extracted. The experimental results confirm the theoretically predicted propensity toward conservation of the e/f parity label. In the case of CaF+Ar collisions, the initial and final state dependence of the experimental cross sections agree well with the appropriate sudden limit scaling relation. Less satisfactory agreement is found when He is the scattering partner.     

Test of the asymptotic method as applied to atom-diatom interaction potentials

The accuracy of the asymptotic method in calculating long-range atom-diatom exchange interactions is tested for two sample systems. In both cases high-quality ab initio results are available for comparison. As a first example, the energy splitting between theA′ andA″ surfaces arising from OH(X ²Π)+Ar were expressed analytically in terms of asymptotic parameters of the O and Ar atoms. The agreement with the calculated ab initio quantity is very good for a range of interatomic distances and different angles of approach. Secondly, for the system CN+He, the diabatic off-diagonal matrix elementV _ΣA′ between two diabaticA′ potential energy surfaces arising from CN(X ²Σ⁺)+He and CN(A ²Π)+He is considered. Using the asymptotic approach, this element can be expressed in terms of the three diagonal matrix elements for CN(X andA)+He which are known ab initio. The resulting values ofV _ΣA′ are found to be in very good agreement with the directly ab initio calculated coupling matrix element over a wide range of CN-He geometries.     

Fine structure transition cross sections for several alkali + rare gas systems

The energy dependence, E_c.m. </ 0.2 eV, of the inelastic total cross sections for the ²P_{$\text{1}\text{2}$} → ²P_{$\text{3}\text{2}$} fine structure transition of the lowest excited states of the alkali atoms are calculated for the following systems: Na, K, Rb + He, Ne, Ar and Cs + He. Encouraging agreement between theory and experiment is obtained.     

Elastic differential cross sections and intermolecular potentials for Ar + CH4 and Ar + NH3

Differential elastic cross sections are reported for CH₄ + Ar (E = μg²/2 = 8.43 kJ/mole) and NH₃ + Ar (E = 8.31 kJ/mole) in the region of the rainbow angles. Quantum interference undulations are apparently observed as well for CH₄ + Ar and, possibly, NH₃ + Ar. The measurements are fit to spherically symmetric intermolecular potentials yielding well depths and equilibrium intermolecular separations of 1.32 kJ/mole and 3.82 Å for CH₄ + Ar and 1.32 kJ/mole and 3.92 Å for NH₃ + Ar.     

Simultaneous vibrational and rotational transitions in hydrogen + argon at high collision velocities: Collisions at nonzero impact parameters

The problem of vibrationally and rotationally inelastic scattering processes in H₂ + Ar for nonzero impact parameter b has been investigated in the collision velocity range of 10⁶–10⁷ cm/sec by use of the sudden approximation. The simultaneous vibrational (0 → 1) and rotational (00 → 00, 20, or 40) transitions were studied. For υ > 3 x 10⁶ cm/sec, the probabilities for b/l = 1.0 are found to be very large compared with those for b = 0, where l is the hard-sphere collision diameter; for b/l > 1.0, the probabilities decrease very rapidly with increasing b. The results show that nonzero-b collisions must be included in the calculation of simultaneous transition processes in H₂ + Ar at higher collision velocities.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

Symmetric resonance charge transfer of Ar2+ IN Ar

Merging beams are used to measure cross sections for the reaction Ar²⁺ + Ar → Ar + Ar²⁺ at relative energies from 2 to 1000 eV. The results are in fair agreement with the theory of Fetisov and Firsov.     

Monte Carlo simulations of Na atoms in dynamically disordered Ar systems: Solid,liquid, and critical-point fluid Ar

We present the results of simulations of the structures and optical absorption spectra of Na atoms in solid and liquid Ar at its triple point, and in critical-point Ar fluid. The spectral simulations combine a classical Monte Carlo scheme for generating thermally accessible ground state configurations, along with a first-order perturbation theory treatment of the interactions between the excited Na^*(3p ²P) atom and the surrounding Ar perturbers [Boatz and Fajardo, J. Chem. Phys., 101 , 3472 (1994)]. These simulations predict a “triplet” (i.e., three peaks) absorption lineshape for Na atoms in solid and liquid Ar at its triple point, and an asymmetrical, blue degraded absorption band for Na atoms in critical Ar fluid. We also note and discuss the similarities between the simulated Na/Ar₍₁₎ lineshape and an experimental Li/Ar/Xe mixed host matrix spectrum, and the similarities between the simulated spectrum of Na atoms in critical point Ar fluid, and an experimental Li/H₂ matrix absorption spectrum. © 1997 by John Wiley & Sons, Inc.     

Calculation of transport properties and intermolecular potential energy function of the binary mixtures of H2 with Ne,Ar, Kr and Xe by a semi-empirical inversion method

The present paper contains inspection of the improved corresponding states principle for transport properties of hydrogen and the binary mixtures of hydrogen with Ne, Ar, Kr and Xe. The set of corresponding states parameters has been defined by a complex numerical analysis of a carefully selected body of experimental data. The obtained correlations for the reduced orientation-averaged diffusion and viscosity collision integrals are restricted to low densities in a temperature range from T = ?/k to the onset of ionization. These equations have been inverted directly to give the isotropic and effective intermolecular potential energy curve for binary mixtures of H₂ with Ne, Ar, Kr and Xe corresponding to the viscosity collision integrals. The results are then used to obtain the best Morse-Spline-Van der Waals (MSV) potential parameters. Our inverted potential energies have been compared with experimental intermolecular potentials that were obtained by molecular beam scattering and infrared spectroscopic measurements. In this research, the Chapman–Enskog and Wang Chang-Uhlenbeck-de Boer (WCUB) version of kinetic theory have been used in conjunction with our inverted potential energies to reproduce viscosity, diffusion, thermal conductivity and thermal diffusion factor of binary mixtures of H₂ with Ne, Ar, Kr and Xe in a wide temperature range for equimolar composition. As the deviation plots illustrate, our obtained intermolecular potential energies (on the basis of the algorithm presented in the inversion process) represent the low-density transport properties of binary mixtures of H₂ with Ne, Ar, Kr and Xe within their expected experimental uncertainties. Close agreement between the predicted values and the literature results of transport properties demonstrates the predictive power of the inversion scheme.     

Classical rotational and centrifugal sudden approximations for atom—molecule collisional energy transfer

The application of centrifugal and rotational sudden approximations to classical trajectory studies of rotational energy transfer in atom—molecule collisions to examined. Two different types of approximations are considered: a centrifugal sudden (CS) approximation, in which the orbital angular momentum is assumed to be constant during collisions, and a classical infinite order sudden (CIOS) approximation, in which the CS treatment is combined with an energy sudden approximation to totally decouple translational and rotational equations of motion. The treatment of both atom plus linear and nonlinear molecule collisions is described, including the use of rotational action-angle variables for the rotor equations of motion. Applications of both CS and CIOS approaches to rotational energy transfer in He + I₂ collisions are presented. We find the calculated CS and CIOS rotationally inelastic cross sections are in generally good agreement [errors of (typically) 10–50%] with accurate quasiclassical (QC) ones, with the CS results slightly more accurate than CIOS. Both methods are less accurate for small |Δj| transitions than for large |Δj| transitions. Computational savings for the CS and CIOS applications is about a factor of 3 (per trajectory) compared to QC. We also present applications using the CS method to rotational energy transfer in He, Ar, Xe + O₃ collisions, making comparisons with analogous QC results of Stace and Murrell (SM). The agreement between exact and approximate results in these applications is generally excellent, both for the average energy transfer at fixed impact parameters, and for rotationally inelastic cross sections. Results are better for He + O₃ and Ar + O₃ than for Xe + O₃, and better at low temperatures than at high. Since SM's quasiclassical treatment considered only total internal energy transfer without attempting a partitioning between vibration and rotation, while our CS calculation considers only rotational energy transfer, the observed good agreement between our and SM's cross sections indicates that most internal energy transfer in He, Ar, Xe + O₃ is rotational. The relation of this result to models of the activation process in thermal unimolecular rate constant determination is discussed.     

Study of beta-ray backscattering saturation thickness with two different approximations to the linear transport theory

The behaviour of beta rays backscattered from foils of high atomic number is studied with a view to determine the saturation thickness. Two different approximations (first-order anisotropic flux, called P₁ approximation, and first-order anisotropic scattering) to the linear transport theory are adopted to this purpose. It is observed that, though both approximations predict the saturation of the backscattered flux of beta particles from foils, it is the P₁ approximation which gives values of saturation thickness that agree better with experimental data.     

Investigations of the optical spectra and EPR g factors for the tetragonal Cu2+ centers in trigonal ZnCO3 crystal

Two distinctive theoretical methods, the complete diagonalization (of energy matrix) method (CDM) and the perturbation theory method (PTM), are employed to calculate the optical band positions and EPR g factors g_//, g_⊥ for the tetragonal Cu²⁺ centers in trigonal ZnCO₃ crystal. The results from the two methods coincide and are also in good agreement with the experimental values. So both the CDM and PTM are adequate for the investigations of optical and EPR data for d⁹ ions in crystals. The tetragonal distortion due to the static Jahn–Teller effect for the tetragonal Cu²⁺ centers in trigonal Zn²⁺ site of ZnCO₃ is also acquired from the calculations. The results are discussed.     

BNN-1,3-dipoles: isolation and intramolecular cycloaddition with unactivated arenes

The mono-base-stabilized 1,2-diboranylidenehydrazine derivatives featuring a 1,3-dipolar BNN skeleton are obtained by dehydrobromination of [ArB(Br)NH]₂ (Ar = 2,6-diphenylphenyl (Dpp), Ar = 2,6-bis(2,4,6-trimethylphenyl)phenyl (Dmp) or Ar = 2,4,6-tri-tert-butylphenyl (Mes*)) with N-heterocyclic carbenes (NHCs). Depending on the Ar substituents, such species can be isolated as a crystalline solid (Ar = Mes*) or generated as reactive intermediates undergoing spontaneous intramolecular aminoboration of the proximal arene rings via [3 + 2] cycloaddition (Ar = Dpp or Dmp). The latter reactions showcase the 1,3-dipolar reactivity toward unactivated arenes at ambient temperature. In addition, double cycloaddition of the isolable BNN species with two CO₂ molecules affords a bicyclic species consisting of two fused five-membered BN₂CO rings. The electronic structures of these BNN species and the mechanisms of these cascade reactions are interrogated through density functional theory (DFT) calculations.

The mono-base-coordinated 1,2-diboranylidenehydrazine derivatives exhibiting the BNN-1,3-dipolar reactivity toward unactivated arenes and CO₂ are reported.     

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.     

Multiple scattering method applied to the electronic structure and spectra of the impurity ions Cu2+, Ni2+, Co2+, Fe2+ and their tetrahedral environments in ZnO,ZnS and CdS

The multiple scattering method with local exchange and muffin-tin potentials is applied to tetrahedral clusters with Cu²⁺, Ni²⁺, Co²⁺ and Fe²⁺ as central ions and oxygen and sulphur as ligands. The calculated orbital energy differences ?(t₂^*) - ?(e^*) are compared to Δ_t values deduced from optical spectra. The agreement is fairly good.     

Rotational excitation in the Ar+N2 system.: A computational study of opacity functions and differential cross sections by the P-helicity decoupling approximation

Rotational excitation in the Ar+N₂ system is investigated using the P-helicity decoupling approximation. The accuracy of this approximation is demonstrated. Opacity functions, elastic and inelastic differential cross sections are computed. The results agree reasonably well with those available from molecular beams scattering experiments. The energy dependence and oscillatory structure of the differential cross sections is discussed. It is explained by an interference mechanism very similar in nature to that of elastic scattering. The relative role of the short- and long-range anisotropies, kinematics factors and type of transition in determining the shape of the inelastic differential cross sections is analyzed in some detail.     

Fermi-shuttle processes in the electron emission by ion impact: Contribution to radiation damages

Secondary electrons, formed in biological tissues by high-energy particle impact, significantly contribute to the fragmentation of small molecules and to single- and double-strand brakes in DNA. Differential spectra of electrons emitted in the collisions of decelerating swift ions are of vital importance for estimating ion impact radiation damages. We demonstrate that the so-called Fermi-shuttle-type acceleration mechanism can produce a significant enhancement in the emission of high-energy secondary electrons. Double differential cross-sections for electron emission, measured in the collisions of N⁺ and N₂⁺ ions with Ar targets at 750 keV/u impact energy, clearly show this effect. The measured cross-sections are in good agreement with the theoretical results of CTMC calculations. Multiple scattering contribution to the Ar spectra above 300 eV is proved to be significant.