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 photodissociation of nitromethane at 193 nm is reviewed in terms of new stereodynamical information provided by the measurement of the first four Dixon's bipolar moments, β0(2)(20), β0(0)(22), β0(2)(02), and β0(2)(22), using slice imaging. The measured speed-dependent β0(2)(20) (directly related with the spatial anisotropy parameter β) indicates that after one-photon absorption to the S3(2 (1)A″) state by an allowed perpendicular transition, two reaction pathways can compete with similar probability, a direct dissociation process yielding ground-state CH3 and NO2(1 (2)A2) radicals and a indirect dissociation through conical intersections in which NO2 radicals are formed in lower-lying electronic states. A particularly important result from our measurements is that the low recoil energy part of the methyl fragment translational energy distribution presents a contribution with parallel character, irrespective of the experimental conditions employed, that we attribute to parent cluster dissociation. Moreover, the positive values found for the β0(0)(22) bipolar moment indicates some propensity for the fragment's recoil velocity and angular momentum vectors to be parallel.

The stereochemistry of methyl iodide photodissociation in the onset of the second absorption B-band has been studied using slice imaging of the CH3(ν = 0) and I*((2)P1/2) photoproducts. The stereodynamical data have been crucial to disentangle the photochemistry of methyl iodide in terms of the competition between direct dissociation and electronic predissociation. The origin of the B-band has been established with high accuracy at 201.11 ± 0.12 nm and a depolarization factor due to parent molecule rotation during predissociation has been found to be 0.29 ± 0.06. Analysis of the semiclassical Dixon's bipolar moments extracted from the CH3(ν = 0) sliced images indicates that direct excitation to the A-band (3)A1 repulsive state in the vicinity of the origin of the B-band is remarkably enhanced by vibrational coupling between the electronic states involved at the conical intersection through in-plane vibrational motion of the molecule.

The photodissociation dynamics of pyrrole-ammonia clusters (PyHÁ(NH 3 ) n , n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH 4 (NH 3 ) nÀ1 products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (B0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.

NH3(Ã) photodissociation dynamics has been studied using a combination of velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) of the H-atom product. H(+) ion images have been recorded after excitation to the first five NH3 (Ã, ν2' = n) ← (X, ν = 0) vibronic transitions (denoted as 0(0)(0) and 2(0)(n) with n = 1-4). The measured high-resolution H-atom kinetic energy distributions (KED) show a dense set of sharp structures related to rovibrational states of the NH2 co-fragment. A careful simulation of the KEDs in terms of the known internal energies of the NH2 fragment has allowed the extraction of the non-adiabatic NH2(X) rovibrational populations for the 0(0)(0), 2(0)(1) and 2(0)(2) transitions, which are in good agreement with previous measurements. For the 2(0)(3) and 2(0)(4) transitions, some features of the KED have been assigned to rovibrational states of NH2(Ã) fragments produced adiabatically. In particular, the sharp feature distinctively observed at very low kinetic energies in the H-atom KED for the 2(0)(4) transition has been undoubtedly assigned to H atoms produced in correlation with rotationally excited NH2 fragments in the à electronic state. For these two transitions, the analysis of the KEDs has allowed the determination of the NH2(X, Ã) rovibrational populations and precise electronic branching ratios, Φ* = [NH2(Ã)]/([NH2(X)] + [NH2(Ã)]). A speed-dependent anisotropy analysis of the H-atom images has been made for all transitions, which provides a picture of the partitioning of the available energy among the NH2 co-product internal modes - including the electronic branching ratios - in terms of a roaming-like mechanism.