Rainbow scattering for Ar + Ar and Xe + Xe

Elastic differential scattering measurements have been performed on Ar⁺ + Ar and Xe⁺ + Xe. The rainbow scattering angle is found at τ = Eθ ≈ 115 eV deg for Ar⁺₂ and τ ≈ 93 eV deg for Xe⁺₂. These data are consistent with a potential well depth of 1.25 eV for Ar⁺₂ and 0.97 eV for Xe⁺₂.     

Inelastic scattering of OH(X2Pi) with Ar and He: a combined polarization spectroscopy and quantum scattering study

One-colour polarization spectroscopy (PS) on the OH A (2)Sigma(+)- X (2)Pi(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X (2)Pi) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X (2)Pi(3/2), J = 1.5-6.5, e states. The PS signal decay rate constant, k(PS), is sensitive to all processes that remove population and destroy polarization. The contribution to k(PS) from pure (elastic) alignment depolarization within the initial level, k(DEP), can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), k(RET), and for Lambda-doublet changing, k(Lambda), collisions from k(PS). Literature values of k(RET) and k(Lambda) are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of k(RET) and k(Lambda) on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for k(DEP) from this subtraction for He are found to be modest, around 0.4 x 10(-10) cm(3) s(-1), whereas for Ar k(DEP) is found to range between 0.6 +/- 0.2 x 10(-10) cm(3) s(-1) and 1.7 +/- 0.3 x 10(-10) cm(3) s(-1), comparable to total population removal rate constants. The differences between k(DEP) for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of k(DEP) may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions.     

Efficiencies of state and velocity-changing collisions of superthermal CN A(2)Pi with He, Ar, N(2) and O(2)

Polarized laser photolysis of ICN is combined with saturated optical pumping to prepare state-selected CN Alpha(2)Pi (nu' = 4, J = 0.5, F(2), f) with a well-defined anisotropic superthermal speed distribution. The collisional evolution of the prepared state is observed by Doppler-resolved Frequency Modulated (FM) spectroscopy via stimulated emission on the CN Alpha(2)Pi-Chi(2)Sigma(+) (4,2) band. The phenomenological rate constants for removal of the prepared state in collisions with He, Ar, N(2) and O(2) are reported. The observed collision cross-sections are consistent with attractive forces contributing significantly for all the colliders with the exception of He. The collisional evolution of the prepared velocity distribution demonstrates that no significant back-transfer into the prepared level occurs, and that any elastic scattering is strongly in the forward hemisphere.     

Kinetics of the CN + CS2 and CN + SO2 reactions

Diode infrared laser absorption spectroscopy was used to measure the rate constant (k(1)) of the CN + CS(2) reaction for the first time. k(1) was determined to be substantially pressure dependent with a value k(1) = (7.1 ± 0.2 to 41.9 ± 2.9) × 10(-12) cm(3) molecule(-1) s(-1) over 2-40 Torr at 298 K. The potential energy surface (PES) of the reaction was calculated using an ab initio method at B3LYP/6-311++G(d, p)//CCSD(T)/6-311++G(d, p) level of theory. Both experimental and computational results suggest that collision stabilization of the adduct NCSCS may dominate the reaction. The rate constant of the CN + SO(2) reaction was measured to be very slow with an upper limit of k(2) ≤ 3.1 × 10(-14) cm(3) molecule(-1) s(-1), in disagreement with an earlier reported measurement. The PES of this reaction reveals an entrance barrier against formation of the low energy adduct NCOSO, in agreement with the experimental result.     

Rate constant for H+H+Ar = H2 + Ar from 1300 to 1700 K

Shock tube experiments on the decay of OH-radical concentration after shock-initiated combustion of H₂:O₂:Ar = 10:1:89 mixtures were analyzed to give the rate constant 1 × 10¹⁵ cm⁶mol^?2s^?1for the reaction H + H + Ar = H₂ + Ar overthe temperature range 1300 to 1700 K.     

Differential energy-loss spectra for CH4 + Ar collisions

In a crossed molecular-beam experiment differential time-of-flight spectra have been measured for CH₄ + Ar collisions at E = 93.1 meV. Ale energy-loss spectra, which show a remarkable transfer of rotational energy, are compared with coupled-states calculations based on a model potential previously derived.     

Ar+ laser-induced fluorescence in NO2

The assignment of the resonant fluorescence induced by the AR⁺ laser lines in ¹⁴N¹⁶O₂ is reported for the first time with the 4727 and 4765 Å laser lines and is further developed with the 4579 and 4965 Å laser lines.     

Scattering experiments with hydrogen atoms. I. Differential collision cross sections for H + Ar,D + Ar and H + CH4

Differential collision cross section measurements for the scattering of hydrogen and deuterium atoms from argon and methane have been carried out with a crossed beams scattering apparatus which uses an oscillating supersonic beam as scattering target and a cryogenic bolometer as beam detector. Diffraction oscillations have been clearly resolved. The data are analyzed with a best fit computing procedure in terms of simple intermolecular energy functions. Well depth parameters for both Ar and CH₄ are 60% larger as compared with those predicted by the geometric mean combination rule while the experimental minimum of the well positions are 10% smaller as given by the arithmetic mean combining rule.     

Luminescence of Ar+-ions emitted after electron capture of Ar2+-ions from K-atoms

The optical attenuation method (OAM) was used to determine the cross-sections for electron capture of Ar²⁺ ions traversing a K vapour. Different values have been found for Ar²⁺ ions in the ground state 3p ^{4 3} P:σ(³ P)=(1.61+/?0.23)·10^?14 cm² and the first metastable state 3p ^{4 1} D:σ(¹ D)=1.88+/?0.21)·10^?14 cm² at the kinetic energy of 4.1 keV applied in the experiment. The values are comparable with Matsumoto's [4] values for the system Ar²⁺+Na, although the energy chosen in the latter experiment was somewhat lower, namely 1.5 keV. At vapour densities 6 times higher than in the above experiment the decay curves cannot be represented any more by a single exponential. Still a superposition of two exponentials fits the decay curve well. We suggest that this behaviour should be attributed to secondary collisions in which excited Ar⁺ ions formed by electron capture are reionized again.     

Renner-Teller bending frequencies of the approximately A 2Pi state of OCS+

There are inconsistencies among previously reported Renner-Teller bending frequencies for the approximately A 2Pi state of OCS+. To resolve these controversies, we have computed vibrational frequencies using high-level excited electronic state ab initio equation-of-motion coupled cluster methods. On the basis of equation-of-motion coupled cluster theory including single, double, and iterative inclusion of partial triple excitations (EOM-CC3) paired with the correlation-consistent polarized valence quadruple-zeta basis set (cc-pVQZ), we predict harmonic bending frequencies of 364 and 401 cm-1 for the A' and A" components of approximately A 2Pi OCS+, respectively. Particularly for the upper Renner-Teller component, these results are lower than the theoretical predictions of 370 and 459 cm-1 reported by Chen, Hochlaf, Rosmus, He, and Ng [J. Chem. Phys. 2002, 116, 5612]. Instead, the presently computed bending frequencies are more consistent with the experimentally derived average value of 357 +/- 5 cm-1 recently reported by Sommavilla and Merkt [J. Phys. Chem. A 2004, 108, 9970], lending credence to the spectral assignments made in this later work. The two components of the Renner-Teller bending frequencies of approximately X 2Pi OCS+ are similarly predicted to be 396 and 453 cm-1. Anharmonicity constants arising from a quartic force field computed at the cc-pVQZ EOM-CC3 level of theory are given, to provide a more complete characterization of the potential energy surface of the approximately A 2Pi state of OCS+.     

Semiclassical theory for atom—surface scattering: Calculations on Ar + W(110)

By using a semiclassical approach we derive an effective potential which governs the motion of the colliding atom/molecule. The effective potential depends on the surface temperature and the phonon excitation during the collision. Numerical results on Ar + W(110) are discussed and compared with experimental data.     

Direct dynamic study on the hydrogen abstraction reaction CH3CN + OH-->CH2CN + H2O

The reaction of acetonitrile with hydroxyl has been studied using the direct ab initio dynamics methods. The geometries, vibrational frequencies of the stationary points, as well as the minimum energy paths were computed at the BHandHLYP and MP2 levels of theory with the 6-311G(d, p) basis set. The energies were further refined at the PMP4/6-311+G(2df, 2pd) and QCISD(T)/6-311+G(2df, 2pd) levels of theory based on the structures optimized at BHandHLYP/6-311G(d, p) and MP2/6-311G(d, p) levels of theory. The Polyrate 8.2 program was employed to predict the thermal rate constants using the canonical variational transition state theory incorporating a small-curvature tunneling correction. The computed rate constants are in good agreement with the available experimental data.     

Ar2H+ 分子振动光谱的理论计算

基于近期由本组提供的Ar2H+分子的基态势能面,应用含时波包演化方法,计算了总角动量J=0时的振动光谱,并对其中的一些谱峰进行了指认.与现有的ab initio结果进行比较,这个新势能面包含了关于Ar2H+基态的比较正确的信息.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Inelastic scattering of vibrationally excited KBr BY Ar and CO2

Scattering of highly vibrationally excited potassium bromide by argon and carbon dioxide has been investigated. Velocity resolved KBr scattering intensities indicate that highly translationally inelastic collisions occur in both systems and, in the CO₂ system, show effects due to VT, VV competition.     

Polarization in elastic scattering: close-coupling studies on ArN2

We present the results of close-coupling calculations of m_j-dependent differential and integral cross sections forj₁ = 2 → j₂ = 2 rotationally elastic ArN₂ collisions. Two potential surfaces were used with differing long-and short-range anisotropies. If the anisotropy is long-ranged the scattering of an isotropic beam results in a significant angle dependent polarization of the elastically scattered products. To a certain extent this reflects a selective loss of m_j-state population due to rotationally inelastic transitions. For quantization along the initial relative velocity vector or perpendicular to the scattering plane, the depolarization of an initially m_j-state selected beam vanishes in the forward direction and is significantly less than the statistical limit at all angles, which indicates a dynamical conservation of the direction of the molecular rotational angular momentum. By contrast, in the helicity frame depolarization is much more pronounced. The oscillatory structure present in the rotationally inelastic differential cross section does not appear to be quenched by the interference between various m → m′ transitions.     

Direct deperturbation analysis of the A 2Pi approximately B 2Sigma+ complex of 7,6LiAr isotopomers

Direct deperturbation analysis of the highly accurate experimental rovibronic term values of the A (2)Pi approximately B (2)Sigma(+) complex of LiAr [R. Bruhl and D. Zimmermann, J. Chem. Phys. 114, 3035 (2001)] has been performed in the framework of inverted close-coupling approach implicitly adjusted to the unified treatment of the overall A approximately B coupling effect without reducing the rovibrational dimensionality. The nonlinear fitting procedure was supported by the ab initio calculations on the spin-orbit and angular coupling matrix elements between the lowest X (2)Sigma(+), A (2)Pi, and B (2)Sigma(+) states. The analytical grid mapping based on the reduced variable representation of the radial coordinate r was used to improve the efficiency of the solution of the close-coupling radial equations near the dissociation limit. The mutual A approximately X perturbation effect on the A (2)Pi term values and spin-rotation splitting of the ground state were evaluated for both (7,6)LiAr isotopomers. The resulting empirical potential-energy curves for the adiabatic A (2)Pi and B (2)Sigma(+) states, along with the refined r-dependent nonadiabatic matrix elements, reproduce the total rovibronic structure of the (7)LiAr complex with the standard deviation of 0.003 cm(-1). The mass invariance of the deperturbed electronic parameters was confirmed by the calculation of the rovibronic term values of the (6)LiAr isotopomer which coincided with their experimental counterparts within 0.004 cm(-1).