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 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.

Uptake of several atmospheric molecules on free ice nanoparticles was investigated. Typical examples were chosen: water, methane, NO x species (NO, NO 2 ), hydrogen halides (HCl, HBr), and volatile organic compounds (CH 3 OH, CH 3 CH 2 OH). The cross sections for pickup of these molecules on ice nanoparticles (H 2 O) N with the mean size ofN ≈ 260 (diameter ∼2.3 nm) were measured in a molecular beam experiment. These cross sections were determined from the cluster beam velocity decrease due to the momentum transfer during the pickup process. For water molecules molecular dynamics simulations were performed to learn the details of the pickup process. The experimental results for water are in good agreement with the simulations. The pickup cross sections of ice particles of several nanometers in diameter can be more than 3 times larger than the geometrical cross sections of these particles. This can have significant consequences in modelling of atmospheric ice nanoparticles, e.g., their growth.