Domenico Stranges

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center‐of‐mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty‐two percent of the available energy is released as product translational energy.

Reactive scattering of atoms and radicals

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.

UV Photodissociation Dynamics of allyl radical by photofragment translational spectroscopy

Photodissociation of the allyl radical, CH2CHCH2, has been studied using the method of molecular beam photofragment translational spectroscopy following excitation to the C(2 2B1) and A(1 2B1) states by 248 and 351 nm photons. Two different primary channels have been detected following 248 nm excitation: H-atom loss (84%) and CH3 elimination (16%). From the product translational energy distribution and polarization dependence studies, dissociation processes from the ground-state C3H5 potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronically excited state. Rice–Ramsperger–Kassel–Marcus (RRKM) calculations show that the formation of cyclopropene is not important, while formation of allene and methylacetylene from dissociation of 1- and 2-propenyl radicals are important reaction pathways at both wavelengths. Translational energy distributions peaking well away from zero provide evidence fo...

Photodissociation of ozone at 193 nm by high‐resolution photofragment translational spectroscopy

The photodissociation of ozone has been studied at 193 nm using high resolution photofragment translational spectroscopy. The results show six distinct peaks in the time‐of‐flight spectra for the O2 product and its momentum‐matched O atom counterpart. The translational energy distributions determined from the time‐of‐flight spectra reveal the production of a range of electronic states of the photofragments. The product electronic states were identified based on the translational energy distributions, with the aid of state‐resolved imaging experiments by Houston and co‐workers. The results reveal the production of a substantial yield of highly excited triplet states of O2, recently suggested to play an important role in the stratospheric ozone balance. In addition, peaks corresponding to O2(a 1Δg) and O2(b 1Σg+) were observed, the latter confirming a previous report [A. A. Turnipseed et al., J. Chem. Phys. 95, 3244 (1991)]. Evidence was seen for a small contribution from the triple dissociation O3→3O(3P), ...

The effect of reagent electronic energy on the dynamics of chemical reactions: A high‐resolution crossed beam study of O(3P,1D)+H2S

Crossed beam reactive scattering studies of the H displacement reaction of both ground 3P and excited 1D oxygen atoms with H2S show that the reaction dynamics changes dramatically upon electronic excitation: while the reaction of O(3P) is direct, that of O(1D) proceeds via a long‐lived complex.

NEUTRAL-NEUTRAL REACTIONS IN THE INTERSTELLAR MEDIUM. I. FORMATION OF CARBON HYDRIDE RADICALS VIA REACTION OF CARBON ATOMS WITH UNSATURATED HYDROCARBONS

The reactions of ground-state atomic carbon with acetylene, C2H2 (1), methylacetylene, CH3CCH (2), ethylene, C2H4 (3), and propylene, C3H6 (4), are investigated at relative collision energies between 8.8 and 45 kJ mol-1 in crossed-beam experiments to elucidate the reaction products and chemical dynamics of atom-neutral encounters relevant to the formation of carbon-bearing molecules in the interstellar medium (ISM). Reactive scattering signal is found for C3H (1), as well as the hitherto unobserved interstellar radicals C4H3 (2), C3H3 (3), and C4H5 (4). All reactions proceed on the triplet surface via addition of the carbon atom to the molecular π-bond. The initial collision complexes undergo hydrogen migration (1/2) or ring opening (3/4) and decompose via C-H-bond rupture to l/c-C3H (1), n-C4H3 (2), propargyl (3), and methylpropargyl (4). The explicit identification of the carbon-hydrogen exchange channel under single collision conditions identifies this class of reaction as a potential pathway to carbon-bearing species in the ISM. Especially, the formation of l/c-C3H correlates with actual astronomical observations and explains a higher [c-C3H]/[l-C3H] ratio in the dark cloud TMC-1 as compared to the carbon star IRC +10216. Our findings strongly demand the incorporation of distinct structural isomers in prospective chemical models of interstellar clouds, hot cores, and circumstellar envelopes around carbon stars.

Combined crossed molecular beams and abinitio investigation of the formation of carbon-bearing molecules in the interstellar medium via neutral–neutral reactions

The crossed molecular beams reaction of atomic carbon C(3Pj) with hydrogen sulfide, H2S, allene, H2CCCH2, the vinyl radical, C2H3, and deuteroacetylene, C2HD, have been studied at different collision energies up to 42.2 kJ mol-1 and combined with high level abinitio calculations. All reactions are barrier-less and are dominated by a carbon–hydrogen exchange to form thioformyl (HCS), butatrienyl (HCCCCH2), C3H2 isomer(s), and deuteriated tricarbon hydride(s). This carbon–hydrogen replacement channel represents a one-step alternative reaction pathway to competing ion–molecule reactions to form complex, carbon-bearing molecules in the interstellar medium as well as in the outflow of carbon stars.

Chemical reactivity of ionic clusters formed by laser ablation of solid oxides utilized in superconducting materials

Abstract Time-of-flight mass spectra of positive and negative cluster ions produced by laser evaporation of several solid oxides (pure as well as mixtures) are reported. The spectra are characterized by the formation of complex oxide cluster ions. The results are interpreted on the basis of the chemical reactivity of the species desorbed from the solid surface.

The enthalpy of formation of the HSO radical

Abstract The enthalpy of formation, Δ H 0 f,0 , of the HSO radical has been determined to be −0.9±0.7 kcal/mol from the analysis of high-resolution crossed beam reactive scattering experiments on the reaction O( 3 P)+H 2 S→HSO+H at different collision energies. The results are compared with previous experimental and theoretical estimates.

Abundance distribution of cluster ions in the laser mass spectra of the pure elements C, Si, Ge, Sn and of binary cluster ions from their mixtures

Abstract Homonuclear cluster ions formed from C, Si, Ge and Sn powders as well as mixtures of these elements have been investigated by laser mass spectrometry. The mechanism of formation of silicon and germanium carbide cluster ions is discussed in relation to the cluster ions produced from the pure elements.