William A. Lester

Theoretical study of inelastic scattering of H2 by Li+ on SCF and CI potential energy surfaces

Integral and differential cross sections for pure rotational and simultaneous rotational−vibrational excitation of H2 by Li+ impact have been computed following the coupled−channel formalism using two different SCF potential energy hypersurfaces and a CI hypersurface at 0.6 and 1.2 eV. Sensitivity of integral cross sections to (a) choice of ab initio potential energy surface and (b) expansion length of a Legendre polynomial representation of one of the energy surfaces is examined. It is seen that preparation of H2 in the v = 0, j = 2 state leads to four− and fivefold increases in excitation cross sections to the v′ = 1, j′ = i, i = 0,2,4 states relative to excitation of ground state (v = 0, j = 0) H2. Differential cross sections are reported at 1.2 eV for up to five quantum rotational and for vibrational transitions on one of the energy hypersurfaces. All angular distributions required for determining ratios (inelastic : elastic) of differential cross sections needed for comparison with recent time−of−fli...

Trajectory studies of O+H2 reactions on fitted abinitio surfaces. II. Singlet case

Classical trajectory calculations of cross sections for the reaction 0(3P)+H2(1Σ+g; ν, j) →OH(2Π; ν′, j′)+H(2S) have been performed for collision energies 1 kcal/mol⩽E⩽40 kcal/mol using an analytical fit to a recent ab inito potential energy surface. Three initial vibrational states of H2, ν=0, 1, and 2, are considered in order to study the influence of vibrational reactant energy on the OH production. With increasing vibrational quantum number, (a) the threshold shifts to lower energies, and (b) the cross sections rise more steeply with collision energy. Rotational excitation of H2 enhances the total reaction cross section for each vibrational state over the range of H2(ν,j) states studied. The cross sections have been used to calculate reaction rate constants for temperatures 300°K⩽T⩽1000°K and the three lowest vibrational states. The ratio k (T,ν=0):k (T,ν=1):k (T,ν=2) is found to be 1:1.13×104:1.42×106 at 300°K and 1:2.12×101:1.24×102 at 1000°K, demonstrating that vibrational energy strongly enhances ...

Nonadiabatic effects in the collision of F(2P) with H2(1Σg+). III. Scattering theory and coupled‐channel computations

The theory of nonreactive atom–diatom collisions in the presence of multiple electronic surfaces is developed in both space‐fixed and body‐fixed coordinate frames. The formalism is applied to the scattering of fluorine atoms by para‐ and ortho‐hydrogen molecules. Coupled‐channel computations of integral cross sections for fine structure and rotational transitions are carried out in the rigid rotor approximation using ab initio self‐consistent‐field potential energy surfaces and are facilitated by the use of a diabatic representation of the molecular channel states. The magnitudes of the cross sections at a specific translational energy are found in general to decrease with increasing energy defect. For F(2P1/2)+p‐H2 the cross section for a near resonant electronic‐to‐rotational energy transfer process dominates other inelastic transitions by at least an order of magnitude.

Coupled channel study of rotational excitation of H2 by Li+ collisions

Integral cross sections for rotational excitation of H2 by Li+ impact have been computed in a coupled channel formalism for the energy range 0.05–0.5 eV. Computational results are reported for the rigid rotator, ``energy corrected rigid rotator, and vibrating rotator models (the latter using accurate numerical vibrational functions of the Kolos‐Wolniewicz potential including adiabatic correction). The ``energy corrected model, which adjusts the energy levels of the rigid rotator to agree closely with the experimental levels of the diatomic molecule by inclusion of the centrifugal distortion term, affords a noticeable improvement over the usual rigid rotator approximation for H2 with no increase in computational effort. Comparison with the classical rigid rotator results of LaBudde and Bernstein [J. Chem. Phys. 59, 3687 (1973)] show reasonable agreement with the present rigid rotator cross sections retaining open channels only. Peaks are obtained in the partial integral cross section in the threshold r...

Nonadiabatic effects in the collision of F(2P) with H2(1Σ+g). I. SCF interaction potentials for the 1 2A′, 2 2A′, and 2A″ states in the reactant region

Interaction potentials for the 1u20092A′, 2u20092A′, and 2A″ states of the F+H2 system are computed in the self‐consistent field (SCF) approximation for a range of geometries suitable for the study of nonadiabatic interactions at thermal energies. Notable features found for the energy surfaces are (a) dominance at long range of the quadrupole–quadrupole interaction, (b) strong repulsive forces at small F–H2 separations for the excited‐state surfaces, and (c) a pseudocrossing region with a conical intersection for the collinear nuclear arrangement at R (F–H2) =5.2 a.u.

Nonadiabatic effects in the collision of F(2P) with H2(1Σ+g). II. Born–Oppenheimer and angular momentum coupling in adiabatic and diabatic representations

The matrix elements necessary for the description of electronic interactions between the 1u20092A′, 2u20092A′, and 2A″ surfaces of FH2 are evaluated from Born–Oppenheimer states. It is shown that these couplings are handled most appropiately in a diabatic basis. The transformation between adiabatic and diabatic bases is explicitly obtained.

Effect of rotation on vibrational excitation of H2 by Li+ impact☆

Coupled channel calculations of integral cross sections for rotational and vibrational excitation of H2(X1Σ+g by collision with Li+ are reported for 1.2 eV in the c.m. system employing an ab initio potential energy surface and numerical vibration—rotation functions of the Koos—Wolniewicz potential function including adiabatic correction. Pure rotational excitation is found to strongly dominate the inelastic scattering occurring at this energy. Preparation of H2 in various allowed non-zero rotational states is seen to enhance the 0 → 1 vibrational cross section by approximately an order of magnitude.

Coupled‐channel study of rotational excitation of a rigid asymmetric top by atom impact: (H2CO,He) at interstellar temperatures

A quantum mechanical scattering study is carried out to test a collisional pumping model for cooling the 6 and 2 cm doublets of interstellar formaldehyde. The Arthurs and Dalgarno formalism is extended to the collision of an s‐state atom with a rigid asymmetric top molecule and applied to rotational excitation of ortho formaldehyde by helium impact. Using a previously determined configuration interaction potential energy surface, the coupled‐channel (CC) equations are integrated at 12 scattering energies between 20 and 95°K. Up to 16 ortho formaldehyde states, yielding a maximum of 62 CC equations, are retained to test convergence of computed cross sections. Resonance structure is obtained at ∼20.2, 32.7, and 47.7°K. The computed inelastic cross sections are averaged over a Maxwell–Boltzmann distribution and the resultant rates used to solve the equations of statistical equilibrium for the relative populations. The 6 and 2 cm doublets are found to be cooled only upon inclusion of the j=3 doublet.

MOLECULAR PROPERTIES OF EXCITED ELECTRONIC STATES: THE a 3a"" AND A 1A"" STATES OF FORMALDEHYDE

Ab initio self‐consistent‐field wavefunctions and molecular properties have been calculated for the three lowest electronic states of H2CO. For the ground state, a variety of basis sets were used, the largest being an uncontracted Gaussian basis: C(11s 7p 2d), O(11s 7p 2d), H(6s 1p). For the excited states, the above basis was contracted to C(7s 5p 2d), O(7s 5p 2d), H(4s 1p). Ground state molecular properties agree well with the earlier theoretical study of Neumann and Moskowitz, and with available experimental data. The z components (along the CO bond axis) of the excited state dipole moments have been measured, and the present a priori predictions reproduce experiment rather closely. Other properties reported include quadrupole moments, octupole moments, and electric field gradients.

Extension of a HeH2 potential energy surface

The CI surface of Tsapline and Kutzelnigg is extended to smaller H2ue5f8He separations. Defining R as the H2ue5f8He distance, r as the H2 separation, and γ as the agnel between them, the ab initio values are fit to a Legendre series in cosγ retaining the first three (even) terms with the coefficients given as analytic functions of R and r to facilitate semiclassical scattering computations. The fit is quantitative for 1.0 <r < 1.8 au, 1.8 au <R < ∞ and is expected to extrapolate well in the region 0.5 <r 2.5 au, R ⪆ r/2 + 1.