The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band ͑286-333 nm͒ has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state ͑ =0͒. The kinetic energy distributions ͑KED͒ of the produced CH 3 ͑ =0͒ fragments show a vibrational structure, both in the I͑ 2 P 3/2 ͒ and I ‫ء‬ ͑ 2 P 1/2 ͒ channels, due to the contribution to the overall process of initial vibrational excitation in the 3 ͑C-I͒ mode of the parent CH 3 I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH 3 I when the photolysis wavelength is increased. The I͑ 2 P 3/2 ͒ / I ‫ء‬ ͑ 2 P 1/2 ͒ branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH 3 I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, 3 Q 0 , 1 Q 1 , and 3 Q 1 , as well as the probability of nonadiabatic curve crossing 1 Q 1 ← 3 Q 0 . The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality ͑pseudotriatomic͒ model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I͑ 2 P 3/2 ͒ / ͓I͑ 2 P 3/2 ͒ +I ‫ء‬ ͑ 2 P 1/2 ͔͒ branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies.

The photodissociation of acetaldehyde in the molecular channel yielding CO and CH(4) at 248 nm has been studied, probing different rotational states of the CO(nu = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained. One of the mechanisms is consistent with the well-studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism. This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding to a high internal energy in the CH(4) counter-fragment.

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ∼308 nm. Towards the blue edge, at 230 nm, an upper limit of ∼50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.