Differential cross sections (DCS) for the elastic scattering of electrons have been measured by the crossed-beam technique for CO, CO 2 , CH 4 , C 2 H 4 and C 2 H 6 in the angular range 30 • -120 • and at impact energies of 300, 500, 700, 900, 1100 and 1300 eV. The measurements were made with a view to test the validity of the independent atom model (IAM) for polyatomic molecules in this energy region. A comparison of experimental results with the IAM calculations indicates that the model is adequate for the linear molecules CO and CO 2 in the energy region 900-1300 eV. The Born approximation for the evaluation of the atomic scattering amplitudes was found to be valid only at 1300 eV for all the molecules except for C 2 H 6 . A survey of the previous experimental and theoretical data based on different models shows that proper molecular wavefunctions have to be used for the evaluation of short-range potentials and that valence-bond distortion, absorption and non-adiabatic effects in the polarization potential have to be considered.

An innovative technique for three-dimensional imaging is presented, which uses the ratio of intensities of a pair of two-dimensional images to extract timing information. The principal advantage of this method is the ability to measure position and time for an almost unlimited number of particles hitting the detector simultaneously. The detector is capable of subnanosecond time resolution and position resolution of about 50 μm. The photodissociation of H2+ is used to demonstrate the capability of the detector.

The lifetimes of the metastable 1s2s2 p 4 P 5/2 level of He Ϫ , as well as the lifetime of the average of the 4 P 3/2 and 4 P 1/2 levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the 4 P 5/2 state, after correction for decay induced by blackbody radiation, is 343Ϯ10 s. This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the 4 P 3/2 and 4 P 1/2 is 8.9Ϯ0.2 s, which is about 20% lower than the weighted theoretical value. ͓S1050-2947͑99͒09501-3͔

We describe a system combining an ion beam trap and a low energy electron target in which the interaction between electrons and vibrationally cold molecular ions and clusters can be studied. The entire system uses only electrostatic fields for both trapping and focusing, thus being able to store particles without a mass limit. Preliminary results for the electron impact neutralization of C 2 − ions and aluminum clusters are presented.

We describe a bent electrostatic ion beam trap in which cluster ions of several keV kinetic energy can be stored on a V-shaped trajectory by means of an electrostatic deflector placed between two electrostatic mirrors. While maintaining all the advantages of its linear counterpart [Zajfman et al., Phys. Rev. A 55, R1577 (1997); Dahan et al., Rev. Sci. Instrum. 69, 76 (1998)], such as long storage times, straight segments, and a field-free region for merged or crossed beam experiments, the bent trap allows for simultaneous measurement of charged and neutral fragments and determination of the average kinetic energy released in the fragmentation. These unique properties of the bent trap are illustrated by first results concerning the competition between delayed fragmentation and ionization of Al(n) (-) clusters after irradiation by a short laser pulse.

We report on the application of an electrostatic ion beam trap as a mass spectrometer. The instrument is analogous to an optical resonator; ions are trapped between focusing mirrors. The storage time is limited by the residual gas pressure and reaches up to several seconds, resulting in long ion flight paths. The oscillation of ion bunches between the mirrors is monitored by nondestructive image charge detection in a field-free region and mass spectra are obtained via Fourier transform. The principle of operation is demonstrated by measuring the mass spectrum of trapped Ar+ and Xe+ particles, produced by a standard electron impact ion source. Also, mass spectra of heavier PEGnNa+ and bradykinin ions from a pulsed MALDI ion source were obtained. The long ion flight path, combined with mass-independent charge detection, makes this system particularly interesting for the investigation of large molecules.

Differential cross sections for elastic scattering of electrons from dimethylsulfide and dimethylsulfoxide in the energy range from 30 to 500 eV are presented. The cross sections have been measured using a crossedbeam-type high-resolution electron spectrometer. The measured cross sections have been put on an absolute scale using the relative flow technique. The measured differential cross sections show an increase in the midangles at 30 and 50 eV for both the molecules that is characteristic of resonant enhanced d-wave scattering. Total and momentum-transfer cross sections were derived by integrating over all angles after extrapolating the data to forward and backward angles using a least-squares fitting procedure based on the Legendre polynomial expansion. Independent-atom-model-based theoretical calculations incorporating static, exchange, and polarization potentials are also reported and compared with the experimental cross sections. For energies lower than 100 eV, it is seen that the independent-atom-model calculations fail to predict the qualitative behavior of the differential cross sections correctly, while at energies greater than 100 eV it seems to describe the data adequately. The effect of polarization and enhancement of cross sections through d-wave scattering at low energies is also analyzed.

We report on the development of an electrostatic ion trap for high-resolution mass spectrometry. The trap works on purely electrostatic fields and hence trapping and storing of ions is not mass restrictive, unlike other techniques based on Penning, Paul, or radio frequency quadrupole ion traps. It allows simultaneous trapping and studying of multiple mass species over a large mass range. Mass spectra were recorded in "dispersive" and "self-bunching" modes of ions. Storage lifetimes of about 100 ms and mass resolving power of about 20,000 could be achieved from the fifth harmonic Fourier transform spectrum of Xe ions recorded in the self-bunching mode.