Rotational-mode specific effects on the stereo-requirement in the reaction of prealigned-CHD3(v1 = 1; |JK = |10 or |1 ± 1) with the chlorine atom.

Several aspects of the stereo-specific requirement in the title reaction are systematically investigated in a crossed-beam experiment using a time-sliced, velocity-mapped imaging technique. Specifically, we explored (1) the differential steric effect from pre-aligning two different reagent rotational states and (2) the effect from probing different product rotational states. In the reaction with an aligned JK=10 reagent at Ec = 3.2 kcal mol-1, the head-on geometry yields a predominantly backward-scattered CD3(00) + HCl(v = 0) product pair, whereas the side-on approach results in a pronounced sideway-scattered distribution. The alternative CD3(00) + HCl(v = 1) channel exhibits a sharply forward-scattering feature for both the collisional geometries. The branching of the two product channels shows sensitive dependency on the collisional geometries. Probing different rotational states of CD3(00) reveals little variation in pair-correlated angular distributions, yet yields notable effect on the correlated vibrational branching of the HCl(v = 0, 1) coproducts. Similar steric propensities hold at lower collisional energy of 1.3 kcal mol-1. In stark contrast, diminishing steric effects were observed in the reaction with an aligned 1±1 reagent. Such huge differential, K-dependent stereo-requirements are largely attributed to the distinct "shapes" of the two rotational states of the aligned CHD3(v1 = 1) reagents.

Rotationally inelastic scattering of methyl radicals with Ar and N2.

The rotationally inelastic scattering of methyl radical with Ar and N2 is examined at collision energies of 330 ± 25 cm(-1) and 425 ± 50 cm(-1), respectively. Differential cross sections (DCSs) were measured for different final n' rotational levels (up to n' = 5) of the methyl radicals, averaged over k' sub-levels, using a crossed molecular beam machine with velocity map imaging. For Ar as a collision partner, we present a newly constructed ab initio potential energy surface and quantum mechanical scattering calculations of state-resolved DCSs. These computed DCSs agree well with the measurements. The DCSs for both Ar and N2 collision partners are strongly forward peaked for all spectroscopic lines measured. For scattering angles below 60°, the theoretical CD3-Ar DCSs show diffraction oscillations that become less pronounced as n' increases, but these oscillations are not resolved experimentally. Comparisons are drawn with our recently reported DCSs for scattering of methyl radicals with He atoms.

Rotationally Inelastic Scattering of Quantum-State-Selected ND3 with Ar.

Rotationally inelastic scattering of ND3 with Ar is studied at mean collision energies of 410 and 310 cm(­1). In the experimental component of the study, ND3 molecules are prepared by supersonic expansion and subsequent hexapole state selection in the ground electronic and vibrational levels and in the jk(±) = 1(1) rotational level. A beam of state-selected ND3 molecules is crossed with a beam of Ar, and scattered ND3 molecules are detected in single final j'k'(±) quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state differential cross sections for rotational-level changing collisions are obtained by velocity map imaging. The experimental measurements are compared with close-coupling quantum-mechanical scattering calculations performed using an ab initio potential energy surface. The computed DCSs agree well with the experimental measurements, confirming the high quality of the potential energy surface. The angular distributions are dominated by forward scattering for all measured final rotational and vibrational inversion symmetry states. This outcome is in contrast to our recent results for inelastic scattering of ND3 with He, where we observed significant amount of sideways and backward scattering for some final rotational levels of ND3. The differences between He and Ar collision partners are explained by differences in the potential energy surfaces that govern the scattering dynamics.

Differential and integral cross sections for the rotationally inelastic scattering of methyl radicals with H2 and D2.

Comparisons are presented of experimental and theoretical studies of the rotationally inelastic scattering of CD3 radicals with H2 and D2 collision partners at respective collision energies of 680 ± 75 and 640 ± 60 cm(-1). Close-coupling quantum-mechanical calculations performed using a newly constructed ab initio potential energy surface (PES) provide initial-to-final CD3 rotational level (n, k → n', k') integral and differential cross sections (ICSs and DCSs). The DCSs are compared with crossed molecular beam and velocity map imaging measurements of angular scattering distributions, which serve as a critical test of the accuracy of the new PES. In general, there is very good agreement between the experimental measurements and the calculations. The DCSs for CD3 scattering from both H2 and D2 peak in the forward hemisphere for n' = 2-4 and shift more to sideways and backward scattering for n' = 5. For n' = 6-8, the DCSs are dominated by backward scattering. DCSs for a particular CD3 n → n' transition have a similar angular dependence with either D2 or H2 as collision partner. Any differences between DCSs or ICSs can be attributed to mass effects because the PES is unchanged for CD3-H2 and CD3-D2 collisions. Further comparisons are drawn between the CD3-D2 scattering and results for CD3-He presented in our recent paper [O. Tkác, A. G. Sage, S. J. Greaves, A. J. Orr-Ewing, P. J. Dagdigian, Q. Ma, and M. H. Alexander, Chem. Sci. 4, 4199 (2013)]. These systems have the same reduced mass, but are governed by different PESs.

Collision dynamics of symmetric top molecules: a comparison of the rotationally inelastic scattering of CD3 and ND3 with He.

We compare rotationally inelastic scattering of deuterated methyl radicals (CD3) and ammonia (ND3) in collisions with helium using close-coupling quantum-mechanical scattering calculations performed with ab initio potential energy surfaces (PESs). The theoretical methods have been rigorously tested against angle-resolved experimental measurements obtained using crossed molecular beam apparatuses in combination with velocity map imaging [O. Tkác, A. G. Sage, S. J. Greaves, A. J. Orr-Ewing, P. J. Dagdigian, Q. Ma, and M. H. Alexander, Chem. Sci. 4, 4199 (2013); O. Tkác, A. K. Saha, J. Onvlee, C.-H. Yang, G. Sarma, C. K. Bishwakarma, S. Y. T. van de Meerakker, A. van der Avoird, D. H. Parker, and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. 16, 477 (2014)]. Common features of the scattering dynamics of these two symmetric top molecules, one closed-shell and the other an open-shell radical, are identified and discussed. Two types of anisotropies in the PES influence the interaction of an atom with a nonlinear polyatomic molecule. The effects of these anisotropies can be clearly seen in the state-to-state integral cross sections out of the lowest CD3 rotational levels of each nuclear spin symmetry at a collision energy of 440 cm(-1). Similarities and differences in the differential cross sections for the ND3-He and CD3-He systems can be linked to the coupling terms derived from the PESs which govern particular initial to final rotational level transitions.

State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3 with He.

State-to-state differential cross sections are reported for rotationally inelastic scattering of fully state-selected ND3 (j(k)(±) = 1(1)(-)) with He. Experimental measurements are compared with full close-coupling quantum-mechanical scattering calculations that used an ab initio potential energy surface. Results are presented for final states up to j'(k')(±) = 7(7)(-) at a mean collision energy of 430 cm(-1). For selected final quantum states, the effect of collision energy on the differential cross sections is also explored in the range 230-720 cm(-1). For the experimental studies, a hexapole electrostatic lens was used for the j(k)(±) state-selection of ND3 molecules in their electronic and vibrational ground states in a molecular beam. This state-selected molecular beam was then crossed with a beam of He atoms. The velocities of inelastically scattered ND3 molecules in single j'(k')(±) states were obtained by velocity map imaging, and converted to differential cross sections in the centre-of-mass frame by density-to-flux transformation. The close-coupling calculations reproduce well the measured angular distributions. For small changes in the rotational angular momentum quantum number (j), the ND3 is predominantly forward scattered, but the scattering shifts to the sideways and backward directions as Δj increases. For scattering into a given j'(k')(±) state, cross-sections for collisions that conserve the ± symmetry associated with the ND3 inversion vibration are larger and generally more forward scattered than the corresponding symmetry-changing processes.

Photochemistry of HI on argon and water nanoparticles: hydronium radical generation in HI·(H2O)n.

Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n ∼ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.