Reaction and Photodissociation Dynamics

1. Reaction dynamics: Current experimental and theoretical studies investigate the elementary bimolecular reactions of Cl atoms with organic molecules. They explore the effects of molecular shape and structure on the dynamics of formation of HCl products, and non-adiabatic dynamics involving reaction pathways on more than one potential energy surface.

We study the dynamics of reactions of Cl atoms, using UV laser photodissociation of molecular chlorine to initiate the reaction, and detection of products via multiphoton ionization (MPI) combined with velocity map imaging. This detection method enables us to characterize not only the speeds of the reaction products and their rotational energies, but also the directions of their velocity and angular momentum vectors. We can thus determine the angles at which the products scatter away from the direction of approach of the reagents (the differential cross section), and can probe how the product molecules are rotating in space (the rotational alignment). Such measurements are highly sensitive to the forces acting in the reaction transition state. The laser methods used allow us to probe the angular scatter of reaction products formed in individual vibrational and rotational quantum states. We have carried out extensive studies of reactions of Cl atoms with organic molecules such as methanol, ethanol, methylamine and oxirane, and attribute the enhanced rotation of the HCl product (compared to reactions of Cl atoms with alkanes) to dipolar interactions between the HCl and polar organic radial products. There is a clear linear correlation between the mean rotational energy of the HCl products (as measured by experiments) and the dipole moment of the radical co-product (as computed by electronic structure theory methods) - see the figure below.

Correlation between the mean rotational quantum number of HCl from the reaction Cl + RH -> HCl + R and the calculated dipole moment of the radical R.

The functional groups in the organic molecules thus influence the dynamics after the reaction transition state. Animations of "on-the-fly" ab initio trajectories illustrate the different dynamics of reactions of Cl atoms with alkanes, alcohols and amines. These were calculated in collaboration with Dr Jeremy Harvey.

Potential energies and structures along the reaction coordinate for the reaction of Cl atoms with methanol.

Studies of the dynamics of reactions of Cl atoms with the heterocyclic organic molecules oxirane and thiirane demonstrate two interesting observations: (i) ring-opening dynamics (e.g. to form vinoxy radicals) does not appear to occur despite being a more exothermic channel than the ring-retaining dynamics (e.g. to form oxiranyl radicals); (ii) the HCl formed from the reactions is much more rotationally cold than that from reaction of Cl atoms with dimethylether or methanol. We proposed that the reason for the latter observation stems from different rotational dynamics of the rigid triangular product radicals after break-up of the reaction transition state.

Recent experimental developments include the first reported observation of non-adiabatic scattering dynamics in the Cl + CH₄ reaction system, by measuring the formation of both ground (Cl) and spin-orbit excited (Cl*) atoms from the reverse reaction of HCl molecules with CH₃. A significant branching into Cl* is seen, and is likely to be a consequence of collisions at high collision energies which promote non-adiabatic coupling between the ground and low lying excited state potential energy surfaces. See references [9,14] below for more details. Images of the velocities of the Cl and Cl* products suggest that the non-adiabatic transitions occur late on the reaction pathway, past the transition state for the reaction.

2. Photodissociation dynamics: Experimental studies of the dynamics of photodissociation of small molecules also make use of the velocity map imaging technique. We have recently demonstrated measurement of the polarization of the electronic angular momentum of Cl atoms from the 467-nm photolysis of BrCl, with determination of alignment and orientation parameters up to rank K=3. These are being analysed to derive phase shifts and probabilities for non-adiabatic dissociation dynamics on the various potentials correlating to ground state Br + Cl atoms. This work is carried out in collaboration with Prof Oleg Vasyutinskii and Andrey Smolin of the Ioffe Institute, St Petersburg. We observe an interesting interference effect between dissociating wavefunctions on different potentials that causes the K=1 orientation parameters to be small (through destructive interference) but the K=3 orientation parameters to be large (constructive interference). More details are given in reference [13].

The figure shows difference images (top) and fits (bottom) for right and left circularly polarized REMPI detection of Cl atoms from BrCl photolysis at 467 nm, using a circularly (geometry I) and 45 degree linearly (geometry II) polarized photolysis laser. Red and blue show negative and positive values in the difference images. The weighting of basis functions used in the fits allows orientation and alignment parameters to be extracted, from which non-adiabatic dynamics mechanisms are deduced.

The figure below left shows the arrangement of photolysis and probe lasers and the polarizations used for geometry I. The polarization of the probe laser is switched from linear (vertical) to right circular, linear (horizontal) and left circular on a shot-to-shot basis using a Pockels cell. The right hand figure shows the images accumulated for these four polarizations of the probe laser, and the two geometries of the photolysis laser. Differences between selected pairs of these images give the data shown in the figure above.

References:

[1] The dynamics of formation of HCl products from the reaction of Cl(²P) atoms with methanol, ethanol and dimethyl ether, S. Rudic, C. Murray, D. Ascenzi, H. Anderson, J.N. Harvey and A.J. Orr-Ewing, J. Chem. Phys. 117 5692 (2002).

[2] On-the-fly ab initio trajectory calculations of the dynamics of Cl atom reactions with methane, ethane and methanol, S. Rudic, C. Murray, J.N. Harvey, and A.J. Orr-Ewing, J. Chem. Phys. 120, 186 - 198 (2004).

[3] Imaging the quantum-state specific differential cross sections of HCl formed from reactions of chlorine atoms with methanol and dimethyl ether, C. Murray, A.J. Orr-Ewing, R.L. Toomes and T.N. Kitsopoulos, J. Chem. Phys. 120, 2230 - 2237 (2004).

[4] The dynamics of the H atom abstraction reactions between chlorine atoms and methyl halides, C. Murray, B. Retail and A.J. Orr-Ewing, Chem. Phys. 301, 239 - 249 (2004).

[5] Imaging the dynamics of reactions of chlorine atoms with methyl halides, R.L. Toomes, A.J. van den Brom, T.N. Kitsopoulos, C. Murray and A.J. Orr-Ewing, J. Phys. Chem. A 108, 7909-7914 (2004).

[6] The dynamics of chlorine atom reactions with polyatomic organic molecules, C. Murray and A.J. Orr-Ewing, Int. Rev. Phys. Chem. 23, 435-482 (2004).

[7] H-atom abstraction dynamics of reactions between Cl atom and heterocyclic organic molecules, J.K. Pearce, C. Murray, P.N. Stevens and A.J. Orr-Ewing, Molecular Physics 103, 1785-1796 (2005).

[8] Stereodynamics of chlorine atom reactions with organic molecules, C. Murray, J.K. Pearce, S. Rudic, B. Retail and A.J. Orr-Ewing, feature article for J. Phys. Chem. A 109, 11093 - 11102 (2005).

[9] Non-adiabatic dynamics in the CH₃ + HCl ® CH₄ + Cl(²P_J) reaction, B. Retail, J.K. Pearce, C. Murray and A.J. Orr-Ewing, J. Chem. Phys. 122, 101101 (2005).

[10] How do the structures of polyatomic molecules affect their reaction dynamics? J.K. Pearce, C. Murray and A.J. Orr-Ewing, Physica Scripta 73, 1 - 6 (2006).

[11] Vector correlations and alignment parameters in the photodissociation of HF and DF, G.G. Balint-Kurti, A.J. Orr-Ewing, J.A. Beswick, A. Brown and O.S. Vasyutinskii, J. Chem. Phys.116, 10760 (2002).

[12] Ion imaging studies of Cl(²P_3/2) fragments arising in the visible photolysis of BrCl: measurement of orientation, alignment and alignment free anisotropy parameters, E.R. Wouters, M. Beckert, L.J. Russell, K.N. Rosser, A.J. Orr-Ewing, M.N.R. Ashfold and O.S. Vasyutinskii, J. Chem. Phys. 117, 2087 (2002).

[13] Velocity map imaging study of BrCl photodissociation at 467 nm: determination of odd rank (K=1 and 3) anisotropy parameters for the Cl photofragments, A.G. Smolin, O.S. Vasyutinskii, O.P.J. Vieuxmaire, M.N.R. Ashfold. G.G. Balint-Kurti and A.J. Orr-Ewing, J. Chem. Phys. 124, 094305 (2006).

[14] Imaging the nonadiabatic dynamics of the CH₃ + HCl reaction, B. Retail, S.J. Greaves, J.K. Pearce, R.A. Rose and A.J. Orr-Ewing, PCCP 9, 3261 - 3267 (2007).