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.

The photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-atom distribution. Under appropriate molecular beam conditions, the entire H-atom loss signal from the photolysis of pyrrole at both 240 and 250 nm (including the sharp features) disappear when using Xe as opposed to He as the carrier gas. We attribute this phenomenon to cluster formation between Xe and pyrrole, and this assumption is supported by the observation of resonance enhanced multiphoton ionization spectra for the (Xe...pyrrole) cluster followed by photofragmentation of the nascent cation cluster. Ab initio calculations are presented for the ground states of the neutral and cationic (Xe...pyrrole) clusters as a means of understanding their structural and energetic properties.

The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S(1), which, in turn, undergoes intersystem crossing to the first excited triplet state T(1). On the triplet surface, the molecule dissociates into CH(3) and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T(1). However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S(0) after internal conversion from S(1). We have characterized the fragments' KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T(1) is presented.

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study.

Thermal HCl and HBr molecules were photodissociated using circularly polarized 193 nm light, and the speed-dependent spin polarization of the H-atom photofragments was measured using polarized fluorescence at 121.6 nm. Both polarization components, described by the a(0)(1)(perpendicular) and Re[a(1)(1)(parallel, perpendicular)] parameters which arise from incoherent and coherent dissociation mechanisms, are measured. The values of the a(0)(1)(perpendicular) parameter, for both HCl and HBr photodissociation, are within experimental error of the predictions of both ab initio calculations and of previous measurements of the polarization of the halide cofragments. The experimental and ab initio theoretical values of the Re[a(1)(1)(parallel, perpendicular)] parameter show some disagreement, suggesting that further theoretical investigations are required. Overall, good agreement occurs despite the fact that the current experiments photodissociate molecules at 295 K, whereas previous measurements were conducted at rotational temperatures of about 15 K.

The photodissociation dynamics of the methyl iodide cation has been studied using the velocity map imaging technique. A first laser pulse is used to ionize methyl iodide via a (2 + 1) REMPI scheme through the 5pπ → 6p Rydberg state two-photon transition. The produced CHI(X[combining tilde]E) ions are subsequently excited at several wavelengths between 242 and 260 nm. The reported translational energy distributions for the methyl and iodine ions present a Boltzmann-type unstructured distribution at low excitation energies as well as a recoiled narrow structure at higher excitation energies highlighting two different dissociation processes. High level ab initio calculations have been performed in order to obtain a deeper understanding of the photodissociation dynamics of the CHI ion. Direct dissociation on a repulsive state from the manifold of states representing the B[combining tilde] excited state leads to CH(X[combining tilde]A') + I*(P), while the CH + I(P) channel is populated through an avoided crossing outside the Franck-Condon region. In contrast, an indirect process involving the transfer of energy from highly excited electronic states to the ground state of the ion is responsible for the observed Boltzmann-type distributions.