Dynamical studies of elementary gas-phase bimolecular reactions have progressed significantly during the last few years owing to advancements in molecular beam and laser techniques as well as in theoretical methodologies In this article we give a brief overview of the recent progress in the field of reaction dynamics and then survey recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method using mass-spectrometric detection Emphasis is on three-atom (A + BC) and four-atom (AB + CD) reactions for which the interplay between experiment and theory is the strongest and the most detailed Reactive differential cross-sections for the three-atom Cl + H2 and four-atom OH + H2 and OH + CO reactions are presented and compared with the results of quasiclassical and quantum-mechanical scattering calculations on ab initio potential-energy surfaces in an effort to assess the status of theory versus experiment The reaction dynamics of electronically excited atoms are discussed too; the effect of electronic excitation on the reaction dynamics of atomic oxygen is examined using the reaction O(3P, 1D)+ H2S as an example

Reactive scattering of atoms and radicals

When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (〈ETr 〉=0.82Eavl ) and angular distribution (β=−0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration.

Dissociation of methanol and ethanol activated by a chemical reaction or by light

The development of a new time‐resolved Fourier transform spectrometer that is capable of 0.25 cm−1 spectral resolution and better than 10−7 s temporal resolution in the visible is reported. The time‐resolved capability of the spectrometer is achieved by coupling a step‐scan interferometer to a transient digitizer/laser system. The operation of the spectrometer is described in detail, and scattered light and laser‐induced fluorescence spectra from an I2 gas cell are presented to demonstrate the temporal and spectral resolution of the spectrometer.

Time‐resolved Fourier transform spectroscopy with 0.25 cm−1 spectral and <10−7 s time resolution in the visible region

The initial vibrational distributions in both energetically accessible channels of the reaction between O(1D) atoms and HCl molecules are reported. The measurements were made using very fast time‐resolved Fourier transform spectroscopy to observe the emission spectra of the products before their vibrational populations could be altered by collisional relaxation. Both the OH from the reaction and the HCl created in the E–V energy transfer process have strongly inverted vibrational distributions. The cross section of the former is found to be about 20 times larger than that of the latter. Although spin forbidden, the E–V process is fast, due to the fact that the 1A″ and 3A″ surfaces of the HOCl intermediate are nearly energetically degenerate over a large region of configuration space. The results suggest that the dynamics of the interaction are dominated by the relatively slow O–Cl collision. The rapid motion of the H atom, in response to changes in the potential created by the motion of the heavier atoms,...

The interaction of O(1D2) with HCl: The initial vibrational distributions in the OH(2Π) produced by chemical reaction, and the HCl(1Σ+) produced by E–V energy transfer

The energy disposal and branching ratios in the reactions of O(1D2) with CHCl3 and CHF3 have been measured using an implementation of time‐resolved Fourier transform spectroscopy in this laboratory. The infrared emission from the products of the reactions is measured as a function of time after the creation of the O(1D2) atom by UV photolysis of ozone. The reaction with CHCl3 produces OH, HCl, and CO as primary products. The OH vibrational excitation indicates simple abstraction dynamics. The HCl has much lower vibrational excitation, characteristic of a longer‐lived insertion‐elimination process, which also produces CO in the decomposition of the internally excited Cl2CO product. Only HF is observed in the reaction with CHF3. In this case the vibrational distribution is nonmonotonic, indicating contributions from two microscopic channels, possibly associated with the formation of F2CO in both the ground (X 1A1) and first excited (A 1A2) states.

The dynamics and microscopic kinetics of the reactions of O(1D2) atoms with CHCl3 and CHF3

Energy partitioning measurements on laser- initiated reactionsby time -resolved Fourier transform spectroscopyJ.J. Sloan, P.M. Aker and B.I. NieferNational Research Council of Canada, 100 Sussex Drive, Ottawa, Canada, K1A OR6 andDepartment of Chemistry, Carleton University, Ottawa, Canada, K1S 5B6AbstractWe report the implementation of a technique for the observation of laser- initiatedchemical processes using time- resolved Fourier transform spectroscopy. This technique com-bines state- specific creation of reagents and the high resolution and broad spectral cover-age of conventional Fourier transform spectroscopy with 10 microsecond time resolution ofthe spectral measurement. It has been applied to the study of energy partitioning in thereactions of O(1D2) atoms with several small molecules in the gas phase at pressures of afew millitorr. Using this technique, collisional deactivation of the reaction products canbe eliminated and the initial energy distribution created by the reaction is obtained.

B. I. Niefer

Papers

The dynamics and microscopic kinetics of the reactions of O(1D2) atoms with CHCl3 and CHF3

Energy Partitioning Measurements On Laser-Initiated Reactions By Time-Resolved Fourier Transform Spectroscopy