Photodissociation of heptane isomers and relative ionization efficiencies of butyl and propyl radicals at 157 nm

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C3H7X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel

The photodissociation dynamics of astrophysically relevant propyl derivatives (C₃H₇X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photodissociation of six (iso)propyl systems─n/i-propyl cyanide (C₃H₇CN), n/i-propyl alcohol (C₃H₇OH), and (iso)butanal (C₃H₇CHO)─to explore the C₃H₇ loss channel. The distinct center-of-mass translational energy distributions for the i-C₃H₇X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C-H bending modes of the formyl moiety compared to the higher frequency stretching of the cyano and hydroxy moieties. Although the ionization energy of the n-C₃H₇ radical exceeds the energy of a 157 nm photon, C₃H₇⁺ was observed in the n-C₃H₇X (X = CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n-C₃H₇ fragments, photoionization of i-C₃H₇ after a hydrogen shift in vibrationally "hot" n-C₃H₇ radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C₃H₇) and possibly the normal propyl radical (n-C₃H₇) should be present in the interstellar medium and hence searched for by radio telescopes.

Crossed-beam DC slice imaging of fluorine atom reactions with linear alkanes

We report the reaction dynamics of F atom with selected alkanes studied by crossed beam scattering with DC slice ion imaging. The target alkanes are propane, n-butane, and n-pentane. The product alkyl radicals are probed by 157 nm single photon ionization following reaction at a collision energy of ∼10 kcal mol(-1). The analyzed data are compared with the corresponding theoretical studies. Reduced translational energy distributions for each system show similar trends with little of the reaction exoergicity appearing in translation. However, the pentane reaction shows a somewhat smaller fraction of available energy in translation than the other two, suggesting greater energy channeled into pentyl internal degrees of freedom. The center-of-mass angular distributions all show backscattering as well as sharp forward scattering that decreases in relative intensity with the size of the molecule. Possible reasons for these trends are discussed.

Dynamics of chlorine atom reactions with hydrocarbons: insights from imaging the radical product in crossed beams

We present a comprehensive overview of our ongoing studies applying dc slice imaging in crossed molecular beams to probe the dynamics of chlorine atom reactions with polyatomic hydrocarbons. Our approach consists in measuring the full velocity-flux contour maps of the radical products using vacuum ultraviolet "soft" photoionization at 157 nm. Our overall goal is to extend the range of chemical dynamics investigations from simple triatomic or tetraatomic molecules to systematic investigations of a sequence of isomers or a homologous series of reactants of intermediate size. These experimental investigations are augmented by high-level ab initio calculations which, taken together, reveal trends in product energy and angular momentum partitioning and offer deep insight into the reaction mechanisms as a function of structure, bonding patterns, and kinematics. We explore these issues in alkanes, for which only direct reactive encounters are found, and in unsaturated hydrocarbons, for which an addition-elimination mechanism competes with direct abstraction. The results for alkene addition-elimination in particular suggest a new view of these reactions: The only pathway to HCl elimination is accessed by means of roaming excursions of the Cl atom from the strongly bound adduct.