In addition to the nucleophile and solvent, the leaving group has a significant influence on SN2 nucleophilic substitution reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains unclear. Here, we uncover the influence of the leaving group on the gas-phase dynamics of SN2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F(-) + CH3Cl and compared it to F(-) + CH3I. For the two leaving groups, Cl and I, we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial and the differences are attributed to the relative orientation of the approaching reactants. This effect occurs on short timescales and may also prevail in solution-phase conditions.

Functional group dependence is observed in the dissociative electron attachment (DEA) to various organic molecules in which the DEA features seen in the precursor molecules of the groups are retained in the bigger molecules. This functional group dependence is seen to lead to site-selective fragmentation of these molecules at the hydrogen sites. The results are explained in terms of the formation of core-excited Feshbach resonances. The results point to a simple way of controlling chemical reactions as well as interpreting the DEA data from bigger biological molecules.

We have measured fully differential cross sections for ionization of He by 3 MeV proton impact. A projectile beam with a coherence length larger than typical atomic scales was prepared at the Test Storage Ring taking advantage of electron cooling. Pronounced effects due to the projectile coherence are observed which are absent for an incoherent 100 MeV u−1 C6 + projectile beam and the present data are in much better agreement with theory. Thus, these results represent a major advancement in resolving the so far unexplained discrepancies between experiment and theory for the 100 MeV u−1 C6 + case.

We present a novel experimental tool allowing for kinematically complete studies of break-up processes of laser-cooled atoms. This apparatus, the 'MOTReMi,' is a combination of a magneto-optical trap (MOT) and a reaction microscope (ReMi). Operated in an ion-storage ring, the new setup enables us to study the dynamics in swift ion-atom collisions on an unprecedented level of precision and detail. In the inaugural experiment on collisions with 1.5  MeV/amu O(8+)-Li the pure ionization of the valence electron as well as the ionization-excitation of the lithium target was investigated.

We use the forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H 2 as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H 2 exhibits oscillatory structure due to Young-type interference in contrast to atomic targets such as He. The asymmetry parameter technique provides a selfnormalized method to reveal the interference oscillation independent of theoretical models and complementary measurements on atomic H target. Angular distribution of various types of radiations ͑particles and photons͒ is known to be quite sensitive to various effects associated with different physical processes in atomic, nuclear, plasma physics and other branches of physics. In fast ion-atom ionization, the long range Coulomb interaction of the final state electrons with the target and the projectile ions influences the evolution of the electron wave function and thereby the angular distribution of electron emission. Such two-center effect is known to cause a large forward-backward asymmetry ͓1-4͔ in the electron emission spectrum. The electron emission spectrum from the simplest diatomic molecule H 2 manifests yet another important aspect of interference ͓5͔ in ion-atom ionization besides the wellknown mechanisms such as soft collision, two-center effect and binary encounter ͓1-4,6-8͔. Since the two indistinguishable H atoms in the H 2 molecule may be considered as the coherent emission sources of phase coupled electrons in a large impact parameter collision, their contributions add coherently and an interference effect should be observed. Therefore, the electron emission from H 2 may be viewed as a natural coherent system which is similar to Young's double slit interference phenomenon ͓5͔. We demonstrate here that the additional mechanism of Young-type interference plays a major role in the angular asymmetry of electron double differential cross section ͑DDCS͒ and asymmetry parameter itself would be a sensitive test to study the interference for a diatomic molecular target.Following the initial theoretical studies on the interference effect in electron scattering ͓9͔ and photoionization ͓5͔, very recently the evidence of Young-type interference was found in the fast-ion collisions with H 2 ͓10-12͔. Ideally one would have expected an oscillation in the DDCS spectrum due to interference. But a steep fall of the DDCS by about four or five orders of magnitude ͑see below͒ does not allow one to observe the oscillation directly. The oscillations, thereby, were observed in the DDCS ratios ͑H 2 -to-2H͒ which was explained due to the interference. However, the experiments using H are rare due to the experimental constraint and oscillations in the DDCS ratios were observed ͓4,12͔ in such experiment with H. Theoretical DDCS for atomic, or effective atomic H have also been employed ͓10,11͔ in the absence of an atomic H target. In such cases, the shapes of the oscillations are sensitive to the atomic parame...

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A reaction microscope (ReMi) has been combined with a magneto-optical trap (MOT) for the kinematically complete investigation of atomic break-up processes. With the novel MOTReMi apparatus, the momentum vectors of the fragments of laser-cooled and state-prepared lithium atoms are measured in coincidence and over the full solid angle. The first successful implementation of a MOTReMi could be realized due to an optimized design of the present setup, a nonstandard operation of the MOT, and by employing a switching cycle with alternating measuring and trapping periods. The very low target temperature in the MOT (∼ 2 mK) allow for an excellent momentum resolution. Optical preparation of the target atoms in the excited Li 2 2 P 3/2 state was demonstrated providing an atomic polarization of close to 100 %. While first experimental results were reported earlier, in this work we focus on the technical description of the setup and its performance in commissioning experiments involving target ionization in 266 nm laser pulses and in collisions with projectile ions.

Dissociative electron attachment (DEA) cross sections for simple organic molecules, namely, acetic acid, propanoic acid, methanol, ethanol, and n-propyl amine are measured in a crossed beam experiment. We find that the H(-) ion formation is the dominant channel of DEA for these molecules and takes place at relatively higher energies (>4 eV) through the core excited resonances. Comparison of the cross sections of the H(-) channel from these molecules with those from NH(3), H(2)O, and CH(4) shows the presence of functional group dependence in the DEA process. We analyze this new phenomenon in the context of the results reported on other organic molecules. This discovery of functional group dependence has important implications such as control in electron induced chemistry and understanding radiation induced damage in biological systems.