H. Floyd Davis

Photodissociation dynamics of OClO

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A 2A2) vibrational states. For all levels studied, corresponding to OClO(A 2A2←X 2B1) excitation wavelengths between 350 and 475 nm, the dominant product (≳96%) was ClO(2Π)+O(3P). We also observed production of Cl+O2 with a quantum yield of up to 3.9±0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A 2A2) excited state vibrational mode. The Cl+O2 yield was enhanced slightly by exciting A 2A2 levels having symmetric stretching+bending, but diminished by as much as a factor of 10 for neighboring peaks associated with symmetric stretching+asymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO(2Π)+O(3P) channel. The photochemical dynamics of OClO possesses two energy regimes with distinctly differe...

Crossed beams study of C-H bond activation: Mo(5S2)+CH4→MoCH2+H2

The reaction of metastable Mo(5S2s1d5) with methane was studied using the method of crossed molecular beams. Angular and velocity distributions of the MoCH2 products were obtained at relative collision energies of 7.2 and 13.0 kcal/mol. The center-of-mass MoCH2 angular distributions exhibited forward–backward symmetry, consistent with a mechanism involving C–H bond insertion forming HMoCH3 followed by H2 elimination. The potential energy barrier for insertion of Mo(5S2s1d5) into CH4 is less than or equal to 7.2 kcal/mol.

Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 produc...

Reactive quenching of OH(AΣ+2) by D2 studied using crossed molecular beams

Reactive quenching of OH(A 2Sigma+,v=0) by D2 forming HOD+D was studied in crossed molecular beams. The D atom products are primarily forward scattered relative to the incident D2. The dominant mechanism involves a direct reaction from relatively large impact parameters with approximately 88% of the available energy appearing in HOD internal excitation.

Dynamics of CO elimination from reactions of yttrium atoms with formaldehyde, acetaldehyde, and acetone

Reactions of neutral, ground-state yttrium atoms with formaldehyde, acetaldehyde, and acetone (Y+RR′CO, where R,R′=H,CH3) were studied in crossed molecular beams. At collision energies greater than 24 kcal/mol, four product channels were observed corresponding to elimination of CO, H2, H, and nonreactive scattering. For the dominant CO elimination channel, a large fraction (34%–41%) of the available energy appeared as kinetic energy of the products. RRKM modeling indicated this was a result of two factors: a large potential energy barrier for R′ migration leading to (R)(R′)YCO and dissociation of this complex prior to complete energy randomization. The CM angular distributions were all forward–backward symmetric, indicating the existence of at least one long-lived reaction intermediate. The angular distributions ranged from being quite forward–backward peaking for the Y+H2CO reaction to isotropic for Y+(CH3)2CO. A simple equation is derived based on statistical complex theory that relates the shape of the...

Reaction geometry from orbital alignment dependence of ion pair production in crossed‐beam Ba(1P1)–Br2 reactions

Strong orbital alignment dependence was observed for Ba+ produced in crossed‐beam reaction of Ba(1P1) with Br2. The peak of this dependence varied strongly with scattering angle for alignment of the p orbital in the scattering plane, with the maximum flux seen for perpendicular alignment with respect to the relative velocity vector. The measured Ba+ was always favored by alignment of the orbital in the scattering plane, regardless of laboratory scattering angle. The experimental results suggest that this charge‐transfer process is dominated by large impact‐parameter collisions which achieve collinear nuclear geometry and Σ orbital alignment at the crossing point. Orbital locking is probably not important owing to the large internuclear distance of the crossing region.

Vibrational vs. translational energy in promoting a prototype metal-hydrocarbon insertion reaction

The reaction Y + CH4 → HYCH3 → YCH2 + H2 is initiated by C–H insertion involving a 20 ± 3 kcal/mol potential energy barrier. The reaction is studied in crossed molecular beams under two different conditions with nearly the same total energy. One experiment is carried out at a collision energy of 15.1 kcal/mol with one quantum of CH4 antisymmetric (ν3) stretching vibrational excitation (8.63 kcal/mol), the other at a collision energy of 23.8 kcal/mol. The reaction cross-section for C–H stretch excited methane (σs) is found to be at least a factor of 2.2 times larger than for ground-state methane (σg) at the same total energy.

Crossed beams study of the reaction 1CH2+C2H2→C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.

C-C versus C-H Bond Activation of Alkynes by Early Second-Row Transition Metal Atoms

The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.

Crossed beams studies of Mo(a 7S3) and Mo*(a 5S2) collisions with CH4 and C2H6

The interactions of Mo(a 7S3) and Mo*(a 5S2) with methane, CH4, and ethane, C2H6, were studied under single collision conditions using the crossed molecular beams technique. Ground state Mo(a 7S3) atoms were found to be unreactive at all collision energies studied up to 〈Ecoll〉=35.4 kcal/mol. Nonreactive scattering of Mo(a 7S3) with methane and ethane was studied and compared to collisions with Ne and Ar. A forward peaking center-of-mass angular distribution, T(Θ), was necessary to simulate the elastic collisions with inert gases as well as inelastic collisions with the alkanes. At a collision energy of 14.4 kcal/mol with CH4 and 21.0 kcal/mol with C2H6, inelastic collisions were found to transfer ∼10% and ∼19% of the initial kinetic energy into alkane internal energy, respectively. For collisions of Mo*(a 5S2)+CH4, the dehydrogenation product, MoCH2, was observed at all collision energies studied down to 2.1 kcal/mol. The reaction Mo*(a 5S2)+C2H6→MoC2H4+H2 was observed down to 〈Ecoll〉=4.5 kcal/mol. For a...