Y. Toker

Electrostatic ion beam trap for electron collision studies

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 C2− ions and aluminum clusters are presented.

A cryogenic electrostatic trap for long-time storage of keV ion beams

We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures <15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.

The decay of ion bunches in the self-bunching mode

The properties of ion bunches stored in an electrostatic ion beam trap (EIBT) have been investigated using the Cryogenic Trap for Fast ion beams (CTF). The extremely high vacuum used rendered the main ion loss mechanism, namely collisions with the rest gas, negligible. Aluminum dimer anions were photo-detached by a pulsed laser to measure the longitudinal ion distribution in the bunch, which for the first time revealed the presence of a dc ion beam component co-existing with the oscillating ion bunch after several hundreds of revolutions. Bunches stabilized by the so-called self-bunching mode of operation have been observed for times as long as 12xa0s (a factor of 100 longer than previous room-temperature experiments) using N+2 and Al−2 bunches at 6–7.1xa0keV beam energies after which the bunch abruptly decayed. The decay of the bunch was observed to be intensity dependent and is well reproduced by a model that includes the expansion of the bunch along the beam axis, intrabeam scattering and collisional losses between the bunch and the dc component. Radio-frequency bunching of the ions resulted in the extension of the bunch observation time to 600xa0s, placing upper limits on all other EIBT ion bunch and trap losses as well as supporting the newly developed decay model and EIBT-adapted bunch dynamics.

A bent electrostatic ion beam trap for simultaneous measurements of fragmentation and ionization of cluster ions.

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.

Radiative cooling of Al4− and Al5− in a cryogenic environment

We have investigated the radiative cooling of initially hot Al−4 and Al−5 cluster anions during storage in a cryogenic ion beam trap at an ambient temperature of <15xa0K. By applying a statistical rate model to the ions measured delayed detachment rate following excitation with a laser, we have obtained their vibrational temperature as a function of cooling time. The temperature curves for Al−4 suggest that the vibrational cooling slows down considerably once the ion reaches approximately room temperature. This suggests the participation of transitions from recently found low-lying electronic states of the anion in the cooling process, prior to reaching approximately room temperature. The experimental results for Al−5 suggest slightly slower radiative cooling than forxa0Al−4.

Radiative cooling of Al−4 and Al−5 in a cryogenic environment

We have investigated the radiative cooling of initially hot Al−4 and Al−5 cluster anions during storage in a cryogenic ion beam trap at an ambient temperature of <15xa0K. By applying a statistical rate model to the ions measured delayed detachment rate following excitation with a laser, we have obtained their vibrational temperature as a function of cooling time. The temperature curves for Al−4 suggest that the vibrational cooling slows down considerably once the ion reaches approximately room temperature. This suggests the participation of transitions from recently found low-lying electronic states of the anion in the cooling process, prior to reaching approximately room temperature. The experimental results for Al−5 suggest slightly slower radiative cooling than forxa0Al−4.

Size effects in the interaction between ionic clusters and low-energy electrons

The interaction between size specific negative clusters (Cn−, 1 < n < 12, Agn−, 1 < n < 11) and low-energy electrons has been studied using a new experimental setup where the ionic clusters are first cooled to room temperature in an electrostatic ion trap. The electron impact detachment cross-sections were measured for electron energies between 5 and 30 eV. The results are analysed in terms of a classical model.

Competition between ionization and fragmentation of small aluminum cluster anions

The competition between delayed fragmentation and delayed ionization of laser excited Al-4 clusters was measured by simultaneous detection of charged and neutral fragments.