Photodissociation of 2-Bromobutane by Ion-velocity Map Imaging Technique

Energy partitioning and spin-orbit effects in the photodissociation of higher chloroalkanes

We investigate the photodissociation dynamics of the C-Cl bond in chloroalkanes CH3Cl, n-C3H7Cl, i-C3H7Cl, n-C5H11Cl, combining velocity map imaging (VMI) experiment and direct ab initio dynamical simulations. The Cl fragment kinetic energy distributions (KEDs) from the VMI experiment exhibit a single peak with maximum close to 0.8 eV, irrespective of the alkyl chain length and C-Cl bond position. In contrary to CH3Cl, where less than 10% of the available energy is deposited into the internal excitation of the CH3 fragment, for all higher chloroalkanes around 40% to 60% of the available energy goes into the alkyl fragment excitation. We apply the classical hard spheres and spectator model to explain the energy partitioning, and compare the classical approach with direct ab initio dynamics simulations. The alkyl chain appears to be a soft, energy absorbing unit. We further investigate the role of the spin-orbit effects on the excitation and dynamics. Combining our experimental data with theory allows us to derive the probability of the direct absorption into the triplet electronic state as well as the probabilities for intersystem crossing. The results indicate an increasing direct absorption into the triplet state with increasing alkyl chain length.

Hexapole-Oriented Asymmetric-Top Molecules and Their Stereodirectional Photodissociation Dynamics

Molecular orientation is a fundamental requisite in the study of stereodirected dynamics of collisional and photoinitiated processes. In this past decade, variable hexapolar electric filters have been developed and employed for the rotational-state selection and the alignment of molecules of increasing complexity, for which the main difficulties are their mass, their low symmetry, and the very dense rotational manifold. In this work, for the first time, a complex molecule such as 2-bromobutane, an asymmetric top containing a heavy atom (the bromine), was successfully oriented by a weak homogeneous field placed downstream from the hexapolar filter. Efficiency of the orientation was characterized experimentally, by combining time-of-flight measurements and a slice-ion-imaging detection technique. The application is described to the photodissociation dynamics of the oriented 2-bromobutane, which was carried out at a laser wavelength of 234 nm, corresponding to the breaking of the C-Br bond. The Br photofragment is produced in both the ground Br ((2)P3/2) and the excited Br ((2)P1/2) electronic states, and both channels are studied by the slice imaging technique, revealing new features in the velocity and angular distributions with respect to previous investigations on nonoriented molecules.