Time spectroscopy in theA 1∑ u + state of Li2

We have determined effective cross-sections for resonant transfer of excitation energy from state-selected Li₂ molecules to ground state Li atoms $$Li_2 A\left( {\upsilon ',J'} \right) + Li2S \to Li_2 X\left( {\upsilon '',J''} \right) + Li2P$$ with two different experimental techniques. Absolute cross-sections were derived from lifetime measurements of 114 rotational levels of theA-state subject to resonant collisions which belong to vibrational statesv′=4, 5, 6, 7 and 11 of⁶Li₂ and⁷Li₂. Cross-sections amount to several thousand Ų dropping off sharply with a resonance half-width of about 16 cm^?1. The resonance behaviour of the collision cross-section inferred from lifetime data could be confirmed by a double resonance technique probing the number of excited atoms with a second laser while the first laser was scanned over the Li₂ X?A absorption band. Due to their high reliability our data prove a good basis for the test of theoretical models. In contrast to earlier investigations significant deviations were found to exist between the measured cross-sections and those predicted by the first-order dipole-dipole scattering theory.     

On the determination of HCl+ (A,v′) vibrational populations from HCl+ (A→X) fluorescence spectra: direct comparison between photoionization and penning ionization

Using a beam apparatus, we have measured the HCl⁺ (A,v′→X,v″) fluorescence spectra of HCl⁺ (A,v′) ions formed in HeI (58.4 nm), and NeI (73.6 nm) photoionization and, for the first time, in He (2³ S) Penning ionization under single collision conditions with a wavelength bandwidth around 1 nm. In addition, we have studied Ne (3s ³ P _{2, 0}) Penning ionization of HCl at three different collision energies. The procedure and the problems in extracting HCl⁺ (A,v′) vibrational populations from the data are discussed in some detail. Thedirect comparison of photoionization and Penning ionization data allows definitive conclusions to be drawn on the question whether final state interactions in the Penning reaction change the “nascent” vibrational population (determined by electron spectrometry); for He (2³ S)+HCl, such changes are shown to be absent within the experimental uncertainty (<±10%). For Ne (3s ³ P _{2, 0})+HCl, the HCl⁺ (A,v′=0, 1) populations are also found to be close to those measured by electron spectrometry and essentially independent of collision energy in the range 34–96 meV. From measurements of the fluorescence intensity as a function of HCl density, we have evidence for a fast loss of HCl⁺ (A,v′) ions in collisions with HCl (rate constant around 5·10^?9 cm³s^?1).     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

Crossed Beam Study on the F+D2→DF+D Reaction at Hyperthermal Collision Energy of 23.84 kJ/mol

We presented an experimental apparatus combining the H-atom Rydberg tagging time-of-flight technique and the laser detonation source for studying crossed beam reactions athyperthermal collision energies. The preliminary study of the F+D₂→DF+D reaction at hyperthermal collision energy of 23.84 kJ/mol was performed. Two beam sources were used in this study: one is the hyperthermal F beam source produced by a laser detonation process, and the other is D₂ beam source generated by liquid-N₂ cooled pulsed valve. Vibrational state-resolved di erential cross sections (DCSs) of product for the title reaction were determined. From the product vibrational state-resolved DCS, it can be concluded that products DF(v'=0, 1, 2, 3) are predominantly distributed in the sideway and backward scattering directions at this collision energy. However, the highest vibrational excited product DF(v'=4), is clearly peaked in the forward direction. The probable dynamical origins for these forward scattering products were analyzed and discussed.     

Optical-optical double-resonance spectroscopy of high vibrational levels of the Na2 A 1Σu+ state in a molecular beam

High vibrational bands of the Na₂ A-X system are investigated in a molecular beam by two-step excitation at separate interaction regions. With a first laser A-state levels are optically pumped; they decay radiatively and populate high vibrational levels of the ground state (e.g. υ′' = 31). A second laser excitation starting from these prepared levels allows the investigation of high vibrational levels of the A state, for example υ′ = 66, 67, 68, 69 and υ′ = 70. From analysis of the υ′ = 66, …, 70-υ′' = 31 bands we obtain B(υ) and G(υ) and, including the data of others, we determined a new RKR potential curve for the A ¹Σ_u⁺ state, extending the experimentally determined potential from υ′ = 44 to υ′ = 70 (78% of the well depth).     

Complexation equilibria and spectrophotometric determination of zinc (II) with 2-(4′-methyl-2′-thiazolylazo)-4,6-dimethylphenol

2-(4′-methyl-2′-thiazolylazo)-4,6-dimethylphenol has been synthesized and its acid—base behaviour (pK_a1=0.03 ± 0.01, pK_a2=9.70±0.09) and complexation equilibria with zinc (logβ₁₀₁=6.70±0.04, logβ₁₀₂ = 13.70±0.02) studied in a 40% (v / v) ethanol—water medium at I=0.25 M NaClO₄. A spectrophotometric method for the determination of 0.2–1.5 ppm of zinc has been developed (ε=1.83 × 10⁴ 1. mol⁻¹ cm⁻¹ at 590 nm) and applied to its determination in lubricating oils.     

Collision energy dependence of angular distributions for vibrational excitation of H2 by He

Angular distributions for the vibrational excitation of H₂ by He have been obtained assuming that H₂ is a rotationless anharmonic oscillator. A set of two coupled-channel scattering equations has been solved for all orbital angular momenta, for the n = 0 to n = 1 transition. Increasing the collision energies from twice to six times the first vibrational excitation energy changes intensities in the center of mass frame from backward peaked into forward peaked.     

Semiclassical calculation of energy transfer in polyatomic molecules. V. Differential cross sections for excitation of CO2 and N2O colliding with Li+ at E ≈ 4.72 eV

A semiclassical model has been used to calculate differential cross sections for vibrational excitation of CO₂ and N₂O at the center of mass collision energy E≈ 4.72 eV. Also the average rotational excitation as a function of the scattering angle is reported. Comparison is made with experimental data and previous more approximate theoretical calculations.     

Internal energy disposal in the chemiluminescent reaction CD + NO → ND(A) + CO and the kinetic isotope effect

The vibrational and rotational energy disposal for ND(A) from the CD + NO reaction was measured in a flowing afterglow. The initial vibrational and rotational distributions of ND(A) were obtained from a spectral simulation. The initial vibrational distribution was (0.51 ± 0.05)_{ν′ = 0}: (0.26 ± 0.05)_{ν′ = 1}: (0.16 ± 0.05)_{ν′ = 2}: (0.07 ± 0.05)_{ν′ = 3}. The rotational temperatures in ν′ = 0, 1, 2 and 3 levels were 4500 ± 500, 4000 ± 500, 4000 ± 500 and 4000 ± 500 K, respectively. The fractions of the available energy deposited into the vibration, 〈fv〉 and rotation, 〈f_R〉, were 0.21 and 0.36, respectively. The results for ND(A) were compared with those for NH(A) from the CH + NO reaction reported previously and the reaction dynamics was discussed on the basis of the observed isotope effect on the energy state distributions. The kinetic isotope effect k_H/k_D in the CH, CD + NO reactions was measured to be 1.84 ± 0.23. The experimental result was compared with a theoretical calculation using transition-state theory.     

CARS spectroscopy of the NaH2 collision complex: the nature of the Na(32 P)H2 exciplex — ab initio calculations and experimental results

CARS (Coherent Anti-Stokes Raman Scattering) has been used to analyze the rovibronic state distribution of H₂ after collision with Na(3² P). New lines, which do not correspond to H₂ lines are observed in the CARS spectrum. The experiments point to the formation of a complex of Na(3² P)H₂ inA ² B ₂ symmetry. Ab initio calculations of theA ² B ₂ potential were performed. On this surface the vibrational spectra of the exciplex are evaluated. The observed lines can be attributed to vibrational transitions in the complex, in which combinational modes are involved. The connection of experimental and theoretical results indicates that a collisionally stabilized exciplex molecule is formed during the quenching process.     

Dissociation of hydrogen molecules in collisions with light alkali ions. II. Li+-H2

The dissociation of a ground state H₂ molecule in single collisions with a Li⁺ ion has been studied using a time of flight technique over a large range of center of mass scattering angles (30° ? υ ? 180°) and collision energies (16 eV < E_cm < 55.5 ev).The results have been transformed into the center of mass system to obtain inelastic differential cross sections (contour maps). In contrast to most other scattering experiments on collision induced dissociation, the results at high energies (E_cm > 40 eV) cannot be explained by a two-step mechanism. Instead dissociation appears to occur in a time comparable to the collision time. The results are consistent with several collision models. Of these the spectator model in which only one of the atoms of the molecule is struck by the incident ion is favored since it is in good agreement with the differential cross sections for backward scattering.     

Improved determination of overall rotational and vibronic relaxation rates of BaO(A 1Σ, ν′= 8, J′= 49) colliding with Ar

Using the linear dependence of the ratio of direct and indirect integrated rotational line fluorescence on the inverse. Ar pressure, we obtain more accurate rate constants for rotational relaxation. k = (91.3 ± 1.9) × 10³ Pa^?1 s^?1 and for vibrational plus electronic relaxation: k = (21.0 ± 0.9) × 10³ Pa^?1 s^?1 of cw-laser-excited BaO(A ¹Σ, ν′ = 8, J′= 49) in collision with Ar. The experiments were performed on BaO produced in a gas-flow system using oxidants N₂O, O₂ and CO₂ at variable argon pressures.     

Neutral scattering and vibrational excitation in K+Br2 Collisions

Differential cross sections are presented for neutral scattering of K atoms in collisions with Br₂ molecules in the energy range from 20 to 150 eV. In addition energy-loss spectra for the scattered K atoms are shown. The differential cross sections show a large peak near the forward direction. The energy-loss spectra point to considerable vibrational excitation at small angles. The results are attributed to reneutralization from an ion-pair state formed during the collision. In some cases this process can involve three potential surface crossings. The experimental results can be reproduced in simple trajectory calculations on diabatic potential surfaces. The calculations show that the forward scattering is rainbow scattering, caused by the internal motion of the Br₂^? molecular ion during the collision. There is no analog to this rainbow in atom-atom scattering. The internal moti is also responsible for the observed vibrational excitation.     

Energy transfer as a function of collision energy. I Collision partners: Hydrogen halides + inert gases,hydrogen halides,H2S and propane

Infrared emission has been recorded from a heated seeded supersonic primary beam of HCl or HF (1) prior to collision with a target beam, and (2) subsequent to that collision. Mean collision energy and collision partner were varied systematically. After correction for elastic scattering, the net population change due to inelastic scattering in a translation—rotation (T ? R) energy-transfer encounter was obtained for specific J states ranging from J = 0–16 of vibrational level υ = 1 of the primary-beam molecule. The broad picture is that a net transfer into low-J states out of higher-J states takes place at low collision energies, and the converse at high collision energies. These observations are interpreted in terms of the “exponential model” for the relative cross sections of T ? R inelastic collisions, S^R (J_i → J_f), proposed earlier [J.C. Polanyi and K.B. Woodall, J. Chem. Phys. 56 (1972) 1563], modified here to satisfy microscopic reversibility. The constant C in the model, which governs the exponential decrease in S^R with increasing energy difference ΔE_J between J_f and J_i, can be derived, as a function of collision energy T, from the present experimental data; C decreases as T increases, i.e. larger ΔJ become more probable. In order to check the validity of the model, it was compared with 3D trajectory results; according to this criterion it was found to give a very good representation of S^R(J_i → J_f) with a single value for C, within a limited range of J_i. The collision partners HCl + HF exhibit anomalously efficient rotational deactivation; evidence is presented which indicates that at low collision energies this is due to resonant R → R transfer. Very efficient deactivation of HCl by HCl, at low collision energy, is likely to be due to V — V transfer.     

The reaction Ne+H 2 + (v=0, 1, 2, 3, 4)→NeH++H: 3D potential energy surface and quasiclassical trajectory calculations

A three-dimensional potential energy surface for the endoergic reaction Ne+H ₂ ⁺ →NeH⁺+H in the² A′ ground state of the system NeH ₂ ⁺ has been calculated by quantum chemical ab initio methods (CEPA approximation). The calculated points on this surface were fitted to an analytic ansatz in terms of an extended LEPS functional form augmented by a correction function. The latter was expanded in polynomials in inverse powers of the internuclear distances. This analytic form was used for quasiclassical trajectory calculations of reactive cross sections. In agreement with experimental investigations a strong vibrational enhancement is observed, i.e. the reaction is markedly favored if the necessary reaction energy is supplied as vibrational energy of H ₂ ⁺ rather than as relative translational energy. Other properties of the reaction dynamics such as the backward to forward scattering ratio, the lifetime of the collision complex NeH ₂ ⁺ , and final rotational and vibrational state distributions are also discussed on the basis of the quasiclassical trajectory calculations.     

Electron capture by protons from theK-shell of H and Ar

Whenever a collision takes place between charged particles, the first Born approximation for electron capture from hydrogenlike ions (Z _T ,e) by a bare nucleusZ _P , must be modified in order to account for the long-range Coulomb effects. One of the simplest ways to fulfill this requirement is provided by theT-matrix of the following form: $$T_{if}^{(1)} = \left\langle {\Phi _f exp\left\{ { - i\frac{{Z_T (Z_p - 1)}}{\upsilon } ln (\upsilon R + v \cdot R)} \right\}\left| {\frac{{Z_P }}{R} - \frac{{Z_P }}{{r_P }}} \right| exp\left\{ {i\frac{{Z_P (Z_T - 1)}}{\upsilon } ln (\upsilon R + v \cdot R)} \right\}\Phi _i } \right\rangle $$ where Φ's are the usual unperturbed channel states andZ's are the nuclear charges. In this transition amplitude, both initial and final scattering states satisfy the correct asymptotic boundary conditions in their respective channels. In the present paper, detailed computation of theK-shell cross sections is carried out for charge exchange in H⁺-H and H⁺-Ar collisions. The results are in good agreement with experimental data.     

Hydrogen bonded complexes and the HF vibrational energy distributions from the reaction of F atoms with NH2 and NH3

The gas phase reactions of fluorine atoms with amino radicals and ammonia molecules: F(²P)+NH₂(²B₁) → HF(¹Σ⁺)+NH(³Σ⁻) and F(²P)+NH₃(¹A₁) → HF(¹Σ⁺)+NH₂(²B₁) produce hydrogen fluoride with very different primary vibrational energy distributions as determined by low-pressure chemiluminescence studies. The reaction with NH₂ yields HF with an inverted primary vibrational energy distribution, P(v′=1:2:3:4)=0.23:0.68:0.08:0.01. The HF from the reaction with ammonia is cold (non-inverted), P(v′=1:2)=0.60:0.40. Recent experimental work on these reactions is critically assessed and some discrepancies between low-pressure chemiluminescence results and fast-flowing afterglow studies are resolved. The results of high-level ab initio calculations (up to 6–311G^** CISD) on reactants, products, and the hydrogen bonded complexes FH … NH and FH … NH₂ in the exit channels are reported. The most reliable of the computations predict that FH … NH₂ is significantly more bound than FH … NH (8.1 versus 4.1 kcal mol⁻¹ in comparison with products at the 6-311G^** MP2 level).Also, the calculated vibrational frequencies for the two hydrogen bonded complexes indicate that the FH stretch and NH₂ asymmetric stretch are much closer in frequency in FH … NH₂ than are the FH and NH stretches in FH … NH. The strong interaction and the close match of vibrational frequencies in the FH … NH₂ case both will lead to fast internal vibrational relaxation (IVR) of the reaction exoergicity from the FHN bonds, where it is released, to the NH₂ fragment in the F/NH₃ reaction. Thus, the HF produced in this reaction is expected to have less vibrational excitation than that created in the F/NH₂ reaction, for which these IVR mechanisms are not as important, and simple direct abstraction dynamics are expected.     

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.     

Shear induced deformation of polystyrene coils in dilute solution from small angle neutron scattering

The deformation of the overall conformation of polystyrene,M _w =280000 g/ mole in dilute solution in a constant shear gradient has been investigated by small angle neutron scattering (SANS). The experiments were performed with a new Couette-type apparatus, consisting of two concentric quartz cylinders in which the outer one rotates and the solution is confined in the gap between both cylinders. The observation of the elastic coherent small angle scatterring intensity at a shear gradient ofG=400 s^?1 has revealed a distinct anisotropy of the scattering pattern on the two dimensional detector, as estimated from equilibrium chain dynamics. After extrapolation to zero angle and zero concentration two different values for the overall size of the single macromolecule parallel and perpendicular to the flow-direction are deduced.