We describe the design of a novel type of storage device currently under construction at Stockholm University, Sweden, using purely electrostatic focussing and deflection elements, in which ion beams of opposite charges are confined under extreme high vacuum cryogenic conditions in separate "rings" and merged over a common straight section. The construction of this double electrostatic ion ring experiment uniquely allows for studies of interactions between cations and anions at low and well-defined internal temperatures and centre-of-mass collision energies down to about 10 K and 10 meV, respectively. Position sensitive multi-hit detector systems have been extensively tested and proven to work in cryogenic environments and these will be used to measure correlations between reaction products in, for example, electron-transfer processes. The technical advantages of using purely electrostatic ion storage devices over magnetic ones are many, but the most relevant are: electrostatic elements which are more compact and easier to construct; remanent fields, hysteresis, and eddy-currents, which are of concern in magnetic devices, are no longer relevant; and electrical fields required to control the orbit of the ions are not only much easier to create and control than the corresponding magnetic fields, they also set no upper mass limit on the ions that can be stored. These technical differences are a boon to new areas of fundamental experimental research, not only in atomic and molecular physics but also in the boundaries of these fields with chemistry and biology. For examples, studies of interactions with internally cold molecular ions will be particular useful for applications in astrophysics, while studies of solvated ionic clusters will be of relevance to aeronomy and biology.

In a beam experiment the processes of elastic scattering, pure rotational excitation and deexcitation, pure vibrational excitation, and simultaneous rotational-vibrational excitation have been investigated for e-H

proved to be very useful. The discrepancy in °v+rv($) between theory and experiment in the earlier investigations is found to be mainly due to oy(Aj = 0), while orv(#) agrees quite well.b) The energy range around 12 eV is characterized by the superposition of direct scattering and Feshbach-resonances. It has been discussed that in the case of H2 Feshbach-resonances may be expected to have 2 J?g + -configuration in most cases. For the decay into the electronic ground state of H2, which is of interest here, the dominating partial wave is the s-wave. This type of resonance is effective for vibrational excitation, but less important for rotational excitation. Obviously, this conclusion depends on the configuration of the resonant state and of the final state of the molecule.c) The direct scattering process is present as a background process in the whole energy range. The direct scattering amplitude normally contains several partial waves, so that one finds direct rotational as well as direct vibrational excitation. Usually, the angular distributions vary with energy much more rapidly than in the case of the resonance scattering mechanism. The relative importance of the direct scattering amplitude is decreasing with increasing quantum number of the exit channel {v = 0 for elastic scattering; v = 1, 2, 3,... for vibrational excitation). The contributions to elastic scattering are relatively large, the cross sections for rotational and vibrational excitation due to direct scattering are very small. -These findings are expected to be valid also for other molecules. AcknowledgementsWe would like to thank Prof. Dr. H. EHRHARDT for encouragement and stimulating discussions. The e -He data of D. ANDRICK and A. BITSCH were of great use for the measurements of this work. The help of M. EYB in performing angular distribution measurements is also greatfully acknowledged. Table 1. The half width of the resonance can be estimated to TV» 0.5 meV, which corresponds to a lifetime tof 10" 12 sec for the 02~ states. The angular dependence of pure resonance scattering is rather flat and not in accordance with the simplest theoretical model. An analysis of the angular dependence and of the rotational structure of the resonance in a somewhat extended model have been performed. -No electronically excited 02~ states could be detected in the energy range up to 3 eV. Experimental Study of Low Energy e-Oa Collision Processes IntroductionLow energy e -02 collision processes are of special interest in high atmosphere physics. In a more general context, the e -02 system represents an interesting example for the study of reaction mechanisms of electron-molecule scattering. It is well known that higher vibrational states of 02~, which is a stable negative ion in its lower vibrational levels, play an important role in electron scattering processes at very low energies. The formation and decay of these resonant states can be explained within the frame of the Born-Oppenheimer approximation. They are interpreted as very low-lying shape resonances la ...

We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations (C 16 H + 10+m , m = 0, 6, or 16) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions (C 16 H + 10 ) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C 16 H + 16 ) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C 16 H + 26 ) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H 2 -formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H 2 -formation at least for small hydrogenated PAHs like pyrene.

The intrinsic absorption profile and radiative cooling rate of coronene cations are reported.