Collinear Geometry Research Articles

Wave Packet Simulation on the Cage Effect in the Full Scattering : CF3H + Ar(3P) → CF3* + H + Ar

Abstract The orientation-dependent wave packet dynamics was investigated for the dissociative energy transfer reaction : CF3H + Ar(3P) → CF3* + H + Ar. Wave packets were propagated numerically by the Chebychev scheme recently developed by Kosloff et al. The calculation was done for the two orientations of CF3H, one is the H-end orientation and another is the CF3-end orientation under the collinear geometry. The results showed clear orientation dependence in the dissociation process. The H-end orientation seems to give a quantum effect, while the CF3-end does not.

Orientational correlation functions and polarization selectivity for nonlinear spectroscopy of isotropic media. I. Third order

The contribution of orientational relaxation to the tensor components of the third-order nonlinear polarization is evaluated for off-resonance Raman and dipole resonant experiments in the perturbative limit. Orientational correlation functions are calculated within the model of orientational diffusion for all third-order tensor components relevant to isotropic media. General expressions for polarization geometries that are selective to particular components of the signal, i.e., magic angles, are derived for collinear and crossed-beam excitation geometries. It is shown that although limited selectivity exists for Raman spectroscopies, no combination of polarizations will give complete selectivity to the isotropic, anisotropic, or nonresonant contributions to the Raman polarizability tensor. For resonant spectroscopies, the four-time correlation function that describes the orientational polarization decay can be written as the product of three two-time correlation functions. While magic angles for orientational relaxation will exist for experiments that probe population dynamics, such as pump–probe and transient grating spectroscopies, orientational relaxation cannot be removed from coherence experiments such as the photon echo.

Ab initio study of the O2(X 3Σ−g)+He(1S) van der Waals cluster

Potential energy surface for the He(1S)+O2(X 3Σ−g) interaction is calculated using the supermolecular unrestricted Mo/ller-Plesset perturbation theory approach and is analyzed via the perturbation theory of intermolecular forces. The latter has been generalized to provide a decomposition of the interaction energies into electrostatic, exchange, induction, and dispersion constituents for monomers described by unrestricted Hartree–Fock determinants. The global minimum occurs for the T-shaped geometry, around 6.0a0. Our UMP4 estimate of the well depth of the global minimum is De=27.7 cm−1. This value is expected to be accurate to within a few percent. The potential energy surface reveals also a local minimum for the collinear geometry at about 7.0a0. The well depth for the secondary minimum is estimated at De=25.5 cm−1(UMP4) and is expected to be accurate within a few percent. The minima are separated by a barrier of 7.5 cm−1. The energy partitioning reveals that the origin of interaction in this complex is typical for van der Waals clusters involving He. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary linear minimum results from the maximum dispersion attraction and enhanced by a slight flattening of the electron density near the ends of the interoxygen axis.

On the Interaction of the S–Ion with H2 at low Collision Energies

Potential energy functions have been calculated for the reactions S- (2P) + H2 → HS + H- and S-(2P) + H2 → HS- + H for low collision energies and collinear geometries. Unlike in the O- + H2 system, only very flat polarisation minima in the structures S- . . . H2 and SH...H- have been found. They are not deep enough to support the formation of a complex in the detachment, in the reactive detachment and in the reactive charge transfer processes of the S- + H2 collision system.

Target structure and in-beam electron spectra

The main features of the interplay between the target structure and the accelerated ion beam characteristics suitable for an in-beam e-γ measurement are discussed in detail. In particular, the different types of “spurious” electron (and photon) spectra overlapped with in-beam internal conversion electrons (and discrete γ-rays) are reviewed as well as the most efficient experimental methods used for their attenuation. Furthermore, the general characteristics of particle beams, targets and electron spectrometers required for prompt and delayed in-beam e, γ and e-γ measurements are also compared. Moreover, the relative magnitude of the electron line Doppler broadening corresponding to the perpendicular and the collinear geometry is briefly discussed and illustrated by a few in-beam e-γ studies of Pm and Eu isotopes produced by accelerated ions ( 2 1H and 7 3Li) bombarding very thin Nd(+C) backed targets. Finally the most recent developments made at Orsay towards multi e-γ detector assemblies lead to a broad discussion concerning the most suitable targets to be inserted within such instrumental arrangements.

Photofragmentation dynamics of Mg2(CO2)+1,2

We have investigated the photofragmentation spectroscopy of Mg2(CO2)+1,2 in a reflectron time-of-flight mass spectrometer. For the linear bimolecular complex Mg2(CO2)+, we have observed three distinct molecular absorption bands, in the red, the green, and the near uv spectral regions. In each band both Mg+ and Mg+2 fragments are observed, although with different action spectra. In the uv band, we also observe reactive fragmentation to form the chemical product Mg2O+. As an initial step in investigating the dissociation mechanism and the dynamical effects which determine the final state branching, we have carried out preliminary calculations of the low lying Mg2(CO2)+ potential energy curves in collinear geometry. Photofragmentation of the complex Mg2(CO2)+2 occurs over a broad spectral range in the visible. We have found that the addition of the second solvent molecule dramatically enhances the solvent cage effect, as evidenced by a large increase in the Mg+2/Mg+ branching ratio throughout this spectral region.

Collinear Quantum Calculation of Transition-State Spectrum for the K + NaCl Reaction

Abstract The laser-induced reaction, K + NaCl + hν → KCl + Na*, has been studied quantum-mechanically, with consideration of only collinear geometries. Laser-dressed equations are numerically solved by the R-matrix propagation method. Quantum reaction probabilities are dominated by many resonances corresponding to the vibrational states of the reaction complex formed in the reaction. The results obtained are compared with surface hopping trajectory calculations. The trajectory surface hopping model fails in predicting the translational energy dependence of the reaction probabilities but a Landau–Zener model is qualitatively correct in the calculation of the laser wavelength dependence of the reaction probabilities. These results suggest that the dynamics of the laser-induced reaction can be divided into two independent processes; the ground state reaction K + NaCl → KCl + Na and the non-adiabatic transition from the laser-dressed ground state to the excited state.

Study of saturated triglycerides in oil based on the c.w. transverse CO 2 laser excited photothermal deflection signals

A photothermal beam deflection spectrometer comprising a novel sample cell, CO 2 laser (pump source) and a diode laser (probe beam) in a transverse (crossed) configuration was constructed and used to study triglycerides in diet oil. Prospects of this method for trace detection are evaluated and compared to the results expected from the collinear geometry with focused pump and probe beams.

Heterodyne nondegenerate pump–probe measurement technique for guided-wave devices

We describe a new heterodyne nondegenerate pump-probe waveguide measurement technique that permits independent control of wavelengths, pulse widths, and polarizations of the pump and probe pulses in a collinear geometry. This technique provides both time- and spectral-domain information and can be applied to transmission and index measurements alike. We demonstrate this technique for the measurement of gain dynamics in a strained-layer single-quantum-well diode laser.

Determination of90Sr in environmental samples with resonance ionization spectroscopy in collinear geometry

A new, fast technique for trace analysis of the radioactive isotopes89Sr and90Sr in environmental samples has been developed. Conventional mass separation is combined with resonance ionization spectroscopy in collinear geometry, which provides high selectivity and sensitivity. In addition, a chemical separation procedure for sample preparation has been developed. The described technique was used to determine the90Sr content in ≈ 870 m3 air samples collected near Munich during and shortly after the Chernobyl reactor accident in April 1986. The content of90Sr was measured to be 1.4 mBq per m3, corresponding to 1.6 × 109 atoms of90Sr per sample. This value is in good agreement with the results of radiochemical measurements.

High-resolution field ionizer for state-selective detection of Rydberg atoms in fast-beam laser spectroscopy

The design of a novel electrode structure for electric-field ionization of Rydberg atoms in a fast atomic beam is described. It allows the resolution of the ion signal from Rydberg atoms with adjacent effective principal quantum numbers n* in the range of 14≤n*≤40 and minimizes the volume where field ionization takes place. These features drastically reduce the number of background events arising from collisions of fast atoms with residual gas molecules and thus are essential for trace analysis applications of the technique of resonance ionization spectroscopy in collinear geometry.

Fast, low-level detection of strontium-90 and strontium-89 in environmental samples by collinear resonance ionization spectroscopy

Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes 89Sr and 90Sr. Useful measurements must be able to detect 10 8 atoms in the presence of about 10 18 atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 10 6. The ions are then neutralized by charge exchange and the resulting fast strontium atoms are selectively excited into high-lying atomic Rydberg states by narrow-band cw laser light in collinear geometry. The Rydberg atoms are then field-ionized and detected. Thus far, a total isotopic selectivity of S > 10 10 and an overall efficiency of ξ = 5 × 10 −6 have been achieved. The desired detection limit of 10 8 atoms 90Sr has been demonstrated with synthetic samples.

Theoretical study of the lowest potential energy surfaces for the reaction O(3P) + HBr(X 1∑+) → OH(X 2Π) + Br(2P)

Potential energy surfaces for the reaction of O(3P) with HBr(X 1∑+) to OH(X 2Π) + Br(2P) have been calculated by means of non-relativistic quantum chemical ab initio methods. Dynamic correlation effects have a rather large influence on the height and the position of the barrier as well as on the exoergicity of the reaction, therefore all calculations have been performed at the SCF or CASSCF levels and with inclusion of electronic correlation using the single-reference or multi-reference CEPA method. For collinear geometries, the (spatially) triply degenerate 3P ground state of the O atom is split into a 3Π and a 3∑− component. The reaction to OH + Br is possible on both potential surfaces; the one of the 3Π state possesses a slightly lower barrier (0.42 eV at ROH = 2.45a0 and RHBr = 2.90a0) than that of the 3∑− state (0.58 eV at ROH = 2.50a0 and RHBr = 3.05a0). Upon bending, the 3∑− state (3A″) is lowered below the two components of 3Π(3A′, 3A″); this gives rise to a true and an avoided crossing. The overall barrier is shifted to a bent conformation with an OHBr angle of ∼150° on the 1 3A″ surface, but the height of the barrier is lowered only by 0.05 eV.

Quantum dynamics of Renner–Teller vibronic coupling: The predissociation of HCO

A Hamiltonian model and parity-adapted wave packet representation are developed to describe a rotating triatomic system with two Renner–Teller coupled potential surfaces, and HCO predissociation is studied. New configuration interaction calculations on HCO are performed to determine its excited Ã(2A″) potential surface, and Bowman, Bittman, and Harding’s X̃(2A′) ground potential surface is employed. The properties of many resonances, correlating with stretch/bend excitations on the A″ surface, are determined. Resonance energies and decay constants are in good agreement with experimental results of Houston and co-workers, but CO rotational product distributions tend to be hotter and narrower than experiment, particularly for pure bend excitations. Wave packet dynamics involves growth of amplitude on the A′ surface near collinear geometries via Renner–Teller coupling, and subsequent adiabatic evolution to determine product distributions. The wave packets probe a previously untested part of the A′ surface, and point to the need for improvements of this surface.

In vivo monitoring of glutamate in the brain by microdialysis and capillary electrophoresis with laser-induced fluorescence detection

Glutamic acid, an excitatory neurotransmitter, was monitored in vivo in the corpus striatum of freely moving rats by brain microdialysis and capillary electrophoresis with laser-induced fluorescence detection. A procedure to derivatize glutamate in complex matrices was developed. Capillary electrophoresis in 12 μm I.D. capillaries was performed to determine glutamate with a migration time of 195 s. Laser-induced fluorescence detection with 488-nm radiation from an argon ion laser and with collinear geometry was used. An injection of haloperidol decreased the concentration of glutamic acid in the dialysates. These experiments support the hypothesis that dopamine receptor blockade decreases glutamate release. The potential of these techniques for the study of chemicals in biomedical experiments is discussed.

Neutron decay of 10Li produced by fragmentation.

The neutron decay of [sup 10]Li, produced in the reaction of 80 MeV/nucleon [sup 18]O + [sup nat]C, was studied by the coincidence detection of neutrons and [sup 9]Li nuclei in a collinear geometry. An analysis of the relative velocity spectrum indicates the presence of a [sup 10]Li state which decays by very low energy neutron emission. This state corresponds to either the ground state of [sup 10]Li, in which case it is probably an [ital s]-wave resonance, or an excited state at [ital E][sub [ital x]][approx]2.5 MeV.

On the Role of High‐Order Multiple Scattering Terms in Br K‐Edge XANES of the Ionic Conductor AgBr

AbstractThe role of the multiple scattering terms in the Br K‐edge X‐ray absorption near edge structure (XANES) spectrum of AgBr is studied. In this system, in spite of the arrangement of atoms in collinear geometry, the first term in the expansion of the absorption coefficient (the single‐scattering term) is shown to account for the main features of the experimental spectrum in the range of 80 eV above the absorption threshold. The role of both the large Br‐Ag distance 0.289 nm and the short core hole lifetime in reducing the amplitude of multiple scattering terms is discussed. The size of the cluster of neighbor atoms probed by the XANES spectrum is studied: the second shell consisting of twelve Br atoms is shown to give the main contribution to the observed experimental features showing the minor role of the first coordination shell.

Quantum functional sensitivity analysis for the 3-D (J = 0) H + H2 reaction

The sensitivity of state-to-state transition probabilities for the 3-dimensional H + H2 hydrogen-exchange reaction (at zero total angular momentum) has been investigated with respect to variations in the interaction potential. Several regions of configuration space where the dynamics is highly sensitive to inaccuracies in the potential have been identified. These regions of importance vary with collision energy, but do not change as dramatically as the previously studied [J. Chem. Phys. 97, 6226 (1992)] collinear case. Near the reaction threshold, the dynamics is most sensitive to the saddle point region as expected. At higher energies (about 1.0 to 1.5 eV), however, the inner corner of the potential, where the dynamics “cuts the corner” in going from reactant to product arrangements, is most important for collinear geometries, and the outer corner, where the H3 conformation is more compact than the transition state conformation, is most important for bent geometries. Surprisingly, the region of the potential traversed by the minimum energy path across the saddle point region has rather insignificant sensitivities at these higher energies. © 1993 John Wiley & Sons, Inc.

Hemiquantal time dependent calculation of the absorption spectrum of a photodissociating triatomic

A half-collision process is described within a hemiquantal time dependent model to calculate the absorption line shape. The photodissociation of ICN in a collinear geometry serves as an illustrative example with the ‘‘light’’ CN motion treated quantum mechanically, whereas the ‘‘heavy’’ I–CN motion is treated classically. The emphasis is put on (i) the way to introduce the Wigner distribution which appears to be the equivalent of the classical density matrix, and (ii) the choice of the initial positions and momenta in the sampling of trajectories. The so-obtained converged results compare, within good accuracy, with the full quantal ones. Both conceptually and computationally, the hemiquantal approach appears as an interesting alternative to the full quantum wave packet calculation.

Comparative study of temperature measurement with folded BOXCARS and collinear CARS

A theoretical model has been developed to simulate temperature profile measurements performed with Coherent Anti-Stokes Raman Spectroscopy (CARS) using collinear and BOXCARS phase matching geometries. Spatial averaging is a more serious problem for collinear geometry in which the observed spectra have CARS signal contributions generated outside the focal volume. The shape of the extracted temperature profile close to the boundary of a gas stream is quite sensitive to the temperature gradient at the boundary for both collinear and BOXCARS measurements. Based on the modeling results, collinear CARS phase-matching optics have been modified to reduce the contribution of the CARS signal from the cold region. The error in the temperature extracted from a spectrum generated in a bimodal temperature distribution medium has been studied. Temperature and temperature profile measurements have been performed with both collinear and folded BOXCARS geometries in a CH 4/air, flat flame burner and the Diagnostic Instrumentation and Analysis Laboratory (DIAL)/MSU test stand to compare the results of these two phasematch geometries. Collinear CARS data taken with the usual and modified collinear CARS in the DIAL test stand have been compared to study the application of collinear CARS in practical combustion. A temperature profile obtained from the measurements with modified collinear CARS and folded BOXCARS phase matching technique is in agreement with the theoretically predicted results. The studies also show that BOXCARS is the best choice for the temperature profile measurement. However, collinear CARS can be used with a modified optics arrangement and an expanded laser beam to improve the spatial resolution whenever folded BOXCARS is not able to provide a sufficient signal.