Chemiluminescence mapping. I. Experimental method and initial measurments on the D+F2 and F+D2 reactions

Measurements by a new experimental chemiluminescence method of the nascent DF product vibrational distribution confirm earlier findings for the F + D₂ → DF(ν?4) + D reaction. The distribution for D + F₂ → DF(ν?15) + F shows a larger fraction (≈ 78%) of the reaction exothermicity channeled into product vibration than is observed by conventional chemiluminescence measurements on the parallel H + F₂ system (58%). The new method, termed chemiluminescence mapping for its simultaneous recording of spectrally and temporally resolved chemiluminescence, differs from the earlier arrested relaxation and measured relaxation techniques by the introduction of a short duty cycle pulsed molecular reagent source, a modified deuterium dissociation source, and signal averaged (time resolved), detection of the DF infrared emission. The chemiluminescence mapping technique results for D + F₂ and F + D₂, are presented; apparent deviation from the energy distribution in the H + F₂ system is discussed.     

Infrared chemiluminescence from the reactions F + HBr,F + H2Se,and Cl + H2Se

Infrared chemiluminescence from HF and HCl has been observed and yielded vibrational and rotational population distributions for the reactions F + HBr, F + H₂Se, and Cl + H₂Se. Evaluation of the spectra recorded by a commercial Fourier-transform spectrometer under low-flow conditions gave the following relative vibrational populations: for F ? HBr. N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} = 0.45 : 0.31 : 0.13 : 0.11: for F + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} : N_{υ = 5} = 0.29 : 0.35 : 0.24 : 0.09 : 0.03: for Cl + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} = 0.40 : 0.51 : 0.09. All three vibrational surprisal plots show a significant deviation from linearity. Neglecting the contributions from N_{υ = 0}, the total energy is partitioned into vibration and rotation as follows: 〈f_V〉 = 0.49 and 〈f_R〉 = 0.09 for F + HBr, 〈f_V〉 = 0.41 and 〈f_R〉 = 0.07 for F + H₂Se, 〈f_V〉 = 0.53 and 〈f_R〉 = 0.10 for Cl + H₂Se. Inclusion of estimates for N_{υ = 0} gives the more realistic values 〈f_V〉 = 0.24, 0.34, and 0.49 respectively. Whereas 9 ± 3% of the collisions between F + HBr yield Br in the excited ²P_{$\text{1}\text{2}$} state, no rovibrationally excited HSe fragments were detected in the two other systems. Consistent values for the bond dissociation energy D₀⁰(HSeH) = 329 ± 5 kJ/mol and the enthalpy of formation ΔH₁₀⁰ (HSe) = 137 ± 5 kJ/mol are derived from the highest observed HCl and HF levels.     

Vibrational effects in proton and charge transfer in the H+2 + Ar system

Reaction and charge transfer of H⁺₂ + Ar to give ArH⁺ and Ar⁺ have been investigated as a function of H⁺₂ vibrational quantum state and kinetic energy (Ec.m.), using photoionization and guided beam ion optics. Resonance effects are important in charge transfer; proton and charge transfer are closely coupled for Ec.m. 3 eV.     

Triatom-triatom collisions: Fixed angle close-coupling study of vibrational excitation in the 12CO2+13CO2 system

A close-coupling approach to the calculation of quantal vibrational transition probabilities for the fixed angle scattering of a linear triatomic molecule with another linear triatomic molecule is described. The method is applied to the ¹²CO₂+¹³C0₂ collisional system. For a calculated inelastic transition probability to have an appreciable magnitude, it is found that the amount of energy transferred in a transition must be very small and just one quantum of energy must be exchanged between either the symmetric stretch or the asymmetric stretch vibrational modes of ¹²C0₂ and ¹³CO₂. For collisional energies away from threshold, the probabilities for transitions involving the symmetric stretch ¹²CO₂ and ¹³CO₂ modes are insensitive to long range multipole terms in the potential energy surface, while the probabilities for energy exchange between the asymmetric stretch modes are considerably diminished when the long range terms are removed from the potential energy surface. A brief discussion is presented on the possibilities of extending the technique to the calculation of vibrational excitation cross sections for three-dimensional triato—triatom collisions.     

Quantum mechanical scattering calculations on a spline-fitted ab initio surface: the He + H+2 (ν = 0, 1, 2) → HeH+ + H reaction

All-channel time-dependent quantum mechanical reaction probabilities are reported for the collinear He + H⁺₂(ν = 0, 1, 2) → HeH⁺ + H reaction at a total energy of 1.2 eV on previously reported diatomics-in-molecule (DIM) and spline fitted ab initio (SAI) surfaces. These results are in agreement with the previous quasiclassical trajectory results in that there is vibrational enhancement of the reaction probability on the SAI surface but not on the DIM surface. This agreement lends support to our previously drawn conclusion that small differences in the potential-energy surface can lead to substantially different dynamic results.     

A State-selected study of the symmetric charge transfer reaction H2+ + H2

We have measured the relative total charge transfer cross sections of H₂⁺ + H₂ as a function of the vibrational state of H₂⁺, υ′_o = 0–4. using the crossed ion-neutral beam and high-resolution photoionization methods. The experimental results obtained at a center-of-mass collisional energy of 22.5 eV are found to be in excellent agreement with a recent theoretical study.     

Infrared absorption and magnetic circular dichroism of Cs2ZrCl6:Ir4+

The E″_g(²T_2g) å U′_g(²T_2g) transition of Ir⁴⁺ in Cs₂ZrCl₆ is observed in the infrared energy range 5000–6000 cm^?1 by absorption and magnetic circular dichroism spectroscopy. The resolved vibrational structure provides detailed information about the change in vibrational frequencies on excitation and the transition intensity mechanism. The energy of the transition requires the Ir⁴⁺ spin-orbit coupling parameter ζ to be ≈ 2800 cm^?1.     

The microscopic reaction dynamics and branching ratio in the F + HCOOH and F + H2CO reactions

Infrared chemiluminescence under conditions of arrested relaxation has been applied to the study of the hydrogen and deuterium abstraction reactions of HCOOH, DCOOH and H₂CO with F atoms. Two distinctly different modes of product excitation are observed, depending upon whether the reaction proceeds via the formyl or carboxyl hydrogen. Reaction at the formyl hydrogen (or deuterium) causes substantial inversion in the diatomic product internal energy distributions. The F + H₂CO and F + DCOOH reactions respectively channel 56% and 54% of the available energy into vibration in the product diatomic when they occur at the formyl site. In both cases the product energy distributions are qualitatively similar to those observed in direct reactions of triatomic systems on repulsive energy surfaces. In contrast to these, reaction at the carboxyl hydrogen of DCOOH gives an HF2 product vibrational distribution having a Boltzmann equilibrium shape at a temperature of 4300 K. The ratio of HF to DF product from the F + DCOOH study shows that reaction occurs at the carboxyl hydrogen approximately twice as often as at the formyl site. Comparison with triatomic reactions involving the same mass-combinations implies that abstraction of the formyl hydrogen occurs via single-collision, direct encounters, whereas reaction at the carboxyl site involves a long-lived complex in which extensive randomisation of the reaction exoergicity among all the product vibrational modes can occur.     

Quantal and semiclassical studies of the (CO2) laser-induced reaction: F(2P1/2) + H2 → [F(2P3/2) + H2; HF + H]

In this work we studied the collinear F(² P _1/2) + H₂ system in a strong CO₂ laser field. It was found that the trajectory. surface hopping method (TSHM) yields entirely different results than the exact quantum-mechanical treatment. A curvecrossing model was devised to explain the discrepancy.     

Vibrational state analysis of unrelaxed BaI from the reactions Ba + CH3I and Ba + CH2I2

The reactions Ba + CH₃I → BaI + CH₃ and Ba + CH₂I₂ → BaI + CH₂I have been investigated by the method of laser-induced fluorescence. Excitation spectra are reported for BaI products formed under single-collision conditions in a “beam-gas” arrangement. The production of BaI for Ba + CH₂I₂ is found to be a major reaction pathway with a cross section about twice that for Ba + CH₃I. The relative vibrational populations show for both reactions bell-shaped distributions peaking close to υ = 21 for Ba + CH₃I and υ = 39 for Ba + CH₂I₂. The corresponding average fraction of the total reaction exoergicity that appears as BaI vibration is $\text{f}$_υ = 0.18 for Ba + CH₃I and $\text{f}$_υ = 0.29 for Ba + CH₂I₂. In the case of Ba + CH₃I, an estimate for the average relative translational energy of the products, obtained from the primitive angular distribution measurements of Lin, Mims and Herm, can be combined with the average vibrational excitation of BaI to provide evidence that the internal excitation of the methyl radical exceeds that of BaI. A model is discussed which postulates an electron jump in the exit valley of the Ba + CH₃I reaction to account for this feature of the reaction dynamics.     

Predissociation of the c3Πu state of H2, populated after charge exchange of H2+ with several targets at keV energies

A detailed study of the predissocitation of the c³Π_u state of H₂ has been made with a new, very sensitive, experimental technique. A resolution better than 1% is obtained in the measurement of the released kinetic energy of HH pairs after charge exchange of H₂⁺ with Ar, H₂, Mg, Na and Cs by detecting both fragments with a time- and position-sensitive microchannel-plate detector. Eighteen vibrational levels of the c³Π_u state can be clearly distinguished in the range of 7.2–10.2 eV. Detailed information is extracted from the spectra with the aid of a convolution procedure. The vibrational energy levels of the c³Π_u state as calculated by Ko?os and Rychlewski are found to be correct within the experimental accuracy of 5 meV. Predissociative lifetimes are measured, yielding 6.2±0.5 ns for the lowest rovibrational level (υ = 0, N = 1), which are in good agreement with theoretical calculations of Comtet and de Bruijn. The cross section for charge exchange is observed to increase gradually with the vibrational level and seems to follow the geometrical cross section of the molecule. Rotational excitation during the charge exchange is also found to increase considerably with the vibrational quantum number. The final rotational temperature further depends strongly on the target gas used and increases with the resonance energy defect ΔI in the charge exchange collision.     

Transformation of spin-orbit excited states of halogen atoms in the chemical reactions F + Br2 → BrF + Br,I*+ Br2 → IBr + Br,and Br + IBr → Br2 + I

The small laser pulse gain method, based on photochemical Br and I lasers, is used to probe ²P_3/2 and ²P_1/2 states of iodine and bromine atoms in the reactions F + Br₂ → BrF + Br (I), I(²P_1/2) + Br₂ → IBr + Br (II), and Br + IBr → Br₂ + I (III). The results obtained are capable of formulating a conservation rule for the spin-orbit excited state.     

Electronic nonadiabatic transitions in the reactive (Ar+ + H2, Ar + H+2, ArH+ + H) system. Numerical results for the collinear configuration

In this communication are presented exact quantum mechanical nonadiabatic electronic transition probabilities for the collinear reaction Ar⁺ + H₂(v_i = 0) → ArH⁺(v_f) + H. The calculations were performed using a potential surface calculated by the DIM method. It is established that large probabilities (≈ 1.0) can be obtained only if there is enough translational energy to overcome a potential barrier formed due to the crossing between v_i = 0 of the Ar⁺ + H₂ system and v_i = 2 of the Ar + H⁺₂ system. The threshold for the reaction is found to be 0.06 eV.     

Vibrational relaxation in atom exchange reactions: A classical trajectory study of H + H2(ν) and D + D2(ν) collisions using an accurate potential

A quasiclassical trajectory study has been carried out to investigate the dynamics of collisions between H + H₂(1 ? ν ? 4) and D + D₂(1 ? ν ? 4), using the accurate Yates-Lester potential. Reaction is selectively enhanced by the vibrational excitation of the diatomic reagent, as judged by information theory, but the degree of selectivity falls as the vibrational energy is successively increased. The calculated results are in fair agreement with experimental measurements on the removal of H₂(ν = 1) by H but not with those for D₂(ν = 1) by D.     

Muonium chemistry: kinetics of the gas phase reaction Mu + F2 → MuF + F from 300 to 400 K

The MSR (muonium spin rotation) technique was used to measure the chemical reaction rate for Mu + F₂ → MuF + F in N₂ moderator at ≈ 1 atm from 295 to 383 K giving the Arrhenius expression: log₁₀k (?/mole s) = (10.83 ± 0.20) - (200 ± 50)/T, with k = (1.46 ± 0.11) × 10¹⁰ ?/mole s at 300 K. This is at least 6.8 times the room temperature rate constant for the analogous H atom reaction. The measured activation energy and enhancement over the H reaction rate are indicative of significant tunnelling in the Mu reaction, in agreement with the recent collinear quantum mechanical calculations of Connor et al.     

Radiative lifetimes of the B1Π11 states of Li2

Model potential calculations of the B¹Π_uX¹Σ⁺_g transition dipole moment are used in conjunction with the empirical potential energy curves of Vidal and Hessel to calculate the radiative lifetimes of the vibrational levels of the B¹Π_u state. The lifetimes are nearly independent of the vibrational quantum number with a value between 8 and 9 ns.     

Slow energy transfer between vibrational models of CH2Cl2, CD2Cl2, CH2ClBr and CH2Br2

Inefficient vibrational energy exchange between the lowest vibrational mode and the higher lying vibrational modes of CH₂Cl₂, CD₂Cl₂, CH₂ClBr and CH₂Br₂ was investigated by ultrasonic absorption experiments. Breathing sphere theory is used to interpret the data available for VV and VR, T transfer in methylene halides.     

Quantal transition probabilities from classical trajectories: Application to the collinear reactive H + Cl2 system

Moments of the classically computed distribution of products vibrational energy are employed to generate a discrete vibrational state distribution. The built-in convergence criterion of the method is discussed and demonstrated. Comparison is made with the quantal results for the collinear H + Cl₂ reaction and with the experimental results for the H + F₂ reaction.     

Distribution of reaction products (theory). XI. H + F2

The availability for the first time of detailed rate constants k(V′, R′, T′) (where V′, R′ and T′ are product vibrational, rotational and translational excitation) for the highly exothermic reaction H + F₂ → HF(V′, R′) + F has prompted the 3D classical-trajectory study reported here. The potential-energy surface is found to be predominantly repulsive ($\text{A}$_⊥ ≈ 42%, R_⊥ ≈ 58%) corresponding to the rather low fractional conversion of reaction energy into vibration ((f′V) = 0.58 from experiment, and 0.56 from theory). In the homologous series of reactions H + X₂ (X  F, Cl, Br, I) the percentage of repulsive energy-release decreases for X  Cl, Br, I, but increases from X  F to Cl. It is shown that this cannot be due to charge in mass-combination, but can plausibly be explained by the anomolously short range of interaction between the separating X atoms in the case X  F. It is predicted that the more-forward scattered HF will be more rotationally excited. The form of the cross section function S_r(T) (where T is reagent translation) is analysed. In accordance with the expectation for a strongly exothermic reaction, it is found that S_r(T) rises more steeply than S_r(V) (where V is reagent vibrational energy). The effect on the product energy distribution conforms qualitatively to the “adiabatic” behaviour noted in previous work: ΔT → ΔT′ + ΔR′; ΔV → ΔV′. The explanation is to be found in reaction through more-compressed or more-extended intermediate configurations than are characteristic of room temperature reaction. We note the existence of an amplification effect: (ΔT′ + ΔR′)/ΔT ≈ 2, and ΔV′/ΔV ≈ 2.