No abstract

Absolute total positive-ion electron ionization cross-sections from threshold to 220 eV are reported for a range of halogenated methanes and small perÑuorocarbons (2È4 carbon atoms). Correlations between the measured ionization cross-section and related molecular properties, in particular the vertical ionization potential (or vertical appearance energy) and molecular polarizability volume, are noted. Contributions to the total cross-section from individual bonds are also determined. Cross-sections predicted using these " bond contributions Ï are in agreement with experiment for a wide range of molecules to better than ^10% accuracy, and in most cases to better than ^5%. The experimental data are also compared with ionization efficiency curves calculated using the (DM) and binary encounter Bethe (BEB) models.

The "pixel imaging mass spectrometry" camera is used to perform femtosecond laser-induced Coulomb explosion imaging of 3,5-dibromo-3 ,5-difluoro-4-cyanobiphenyl molecules prealigned in space. The experiment allows the concurrent detection of the correlated two-dimensional momentum images of all the ionic fragments resulting from fragmentation of multiple molecules in each acquisition cycle. The Coulomb explosion studies provide rich information about the parent molecular structure and fragmentation dynamics, and open new opportunities for real-time imaging of intramolecular processes.

The experimental determination of absolute total electron impact ionization cross-sections for polyatomic molecules has traditionally been a difficult task and restricted to a small range of species. This article reviews the performance of three models to estimate the maximum ionization cross-sections of some 65 polyatomic organic and halocarbon species. Cross-sections for all of the species studied have been measured experimentally using the same instrument, providing a complete data set for comparison with the model predictions. The three models studied are the empirical correlation between maximum ionization cross-section and molecular polarizability, the well-known binary encounter Bethe (BEB) model, and the functional group additivity model. The excellent agreement with experiment found for all three models, provided that calculated electronic structure parameters of suitably high quality are used for the first two, allows the prediction of total electron-impact ionization cross-sections to at least 7% precision for similar molecules that have not been experimentally characterized.

Femtosecond laser-induced Coulomb-explosion imaging of 3,5-dibromo-3 ,5-difluoro-4-cyanobiphenyl molecules prealigned in space is explored using a pixel-imaging mass-spectrometry (PImMS) camera. The fast-event-triggered camera allows the concurrent detection of the correlated two-dimensional momentum images, or covariance maps, of all the ionic fragments resulting from fragmentation of multiple molecules in each acquisition cycle. Detailed simulation of the covariance maps reveals that they provide rich information about the parent molecular structure and fragmentation dynamics. Future opportunities for imaging the real-time dynamics of intramolecular processes are considered.

Measurements of electron impact ionization cross sections have been made for methane and the series methyl fluoride to methyl iodide. The results for methane and methyl fluoride to methyl bromide have been compared with ionization efficiency curves calculated using Deutsch-Märk (DM) and binary-encounter-Bethe (BEB) methods, and also with the results of an ab initio model which gives the maximum cross section as a function of molecular orientation. In addition, the ab initio and DM methods have been used to calculate the steric ratios for the electron impact ionization of methyl chloride which have been compared with experimental measurements made previously.

State-to-state dynamics of the Cl + CH 3 OH → HCl + CH 2 OH reaction Differential cross section polarization moments: Location of the D-atom transfer in the transition-state region for the reactions Cl+C 2 D 6 →DCl (v ′ =0,J ′ =1)+ C 2 D 5 and Cl+CD 4 →DCl (v ′ =0,J ′ =1)+ CD 3The hydrogen atom abstraction reaction between Cl( 2 P 3/2 ) and ethane has been studied at a mean collision energy of 0.24 eV. The experiments were performed in a coexpansion of molecular chlorine and ethane, with the atomic Cl reactants generated by laser photodissociation of Cl 2 at 355 nm. HCl(vЈ, jЈ) products were detected quantum state selectively using (2ϩ1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. The ion images were used to determine center-of-mass angular and kinetic energy release distributions. Several analysis methods were employed and have been carefully assessed. It is shown that the single beam experiments can be used with confidence to determine both center-of-mass angular and energy release distributions. For the title reaction the angular distribution is found to be forward peaking, with on average 22% of the available energy channeled into internal excitation of the ethyl coproducts. Possible sources of this internal excitation are discussed.

Rigorous quantum dynamical calculations have been performed on the ground 1 1A' and first excited 1 1A" electronic states of the title reaction, employing the most accurate potential energy surfaces available. Product rovibrational quantum state populations and rotational angular momentum alignment parameters are reported, and are compared with new experimental, and quasiclassical trajectory calculated results. The quantum calculations agree quantitatively with experiment, and reveal unequivocally that the 1 1A" excited state participates in the reaction.