Patrick J. Cooney

Apparatus for measurements on the fragmentation of MeV molecular‐ion beams

An apparatus for the study of the fragmentation of MeV molecular ions is described. The system permits high‐resolution measurement of the joint distribution in energy and angle for fragment ions of selected charge and mass. Two movable detectors allow the study of spatial and temporal correlations among up to three fragments resulting from a single dissociation event. All experimental parameters are monitored, controlled, displayed, and recorded by a computer system that uses a linked network of four processors. Real‐time, computer‐generated color graphics are employed to give a visual rendition of the relation between the detector positions and the trajectories of outgoing fragments.

Measurement of the distributions of internuclear separations in 3.0 MeV H2+ and 3.63 MeV HeH+ beams☆

Abstract Angular distributions of charged dissociation fragments are measured for 3.0 MeV H2+ and 3.63 MeV HeH+ ions incident on ∼ 160 A carbon targets. By using the reflection method for a pure Coulomb potential we unfold from these data the distributions of internuclear separations for each molecular-ion species prior to dissociation. These results are insensitive to ion-source conditions. For H2+ this distribution, while ∼2 times wider than a pure ground vibrational state population, is markedly different from the Franck-Condon distribution that has been previously assumed by other authors with similar rf and duo-plasmatron ion sources. For HeH+ the distribution is slightly broader (∼1.5 times) than that expected for a pure ground state population. From the data, we are able to extract the initial vibrational state population in the incident beam.

The transmission of fast molecular ions through thin foils

Abstract We present new results on the transmission of fast molecular ions through thin foils and propose a mechanism for the transmission process. The main feature of the postulated mechanism is that a finite fraction of the incident molecular beam does not undergo a strong Coulomb explosion while traversing the foil. Because the emerging fragments are at large internuclear separations, there is an enhanced probability for the formation of bound, long-range, excited electronic states following electron capture at the rear surface of the target.

Neutral hydrogen from the foil-induced dissociation of 4HeH+, 3HeH+, and H2+

Abstract High-resolution energy spectra and angular distributions of H 0 from the dissociation of 4 HeH + , 3 HeH + , and H 2 + in thin carbon foils are presented for incident ion energies ranging from 170 to 1800 keV/amu. For the range of dwell times inside the target foils studied (about 1–10 fs), the dominant mechanism for such dissociation is the Coulomb explosion of the ions inside the target followed by electron capture near the rear surface of the foil. At the shortest of these dwell times, the yield of H 0 for incident ions oriented perpendicular to the beam relative to that for ions aligned with the beam is a factor of two greater than at longer dwell times. These data indicate that, for ion fragments which emerge from the rear surface of the target within a few A of one another, those in the perpendicular orientation have a greater electron capture probability than those emerging in other orientations.

Determination of molecular-ion structures through studies of the collisionally-induced dissociation of fast (MeV) molecular ions

Abstract High-resolution studies of the angle and energy distributions of fragments detected in coincidence from the foil- and gas- induced dissociation of fast (MeV) polyatomic molecular ions are described. The aim of the measurements is to explore the possibilities that these new techniques offer for determining the stereochemistry of molecular ions.

Molecular structure studies by 3d imaging of fast ion beams

Abstract The use of the Coulomb explosion technique combined with a radically new multiparticle detector, extremely thin film targets, and low-excitation ion source has enabled, for the first time, direct measurements of the complete stereostructure of complex polyatomic molecular ions. We outline the methods used and present results for protonated acetylene (C 2 H 3 + ) and the methane cation (CH 4 + ) as examples. We demonstrate the techniques by which these methods can be generalized to determine directly vibrational motions in polyatomic molecules.

The structure of molecular ions studied with fast (MeV) heavy projectiles

Abstract Fast molecular ions excited in collisions with solid or gaseous targets loose most of their binding electrons and the remaining molecular fragments are then strongly repelled by their mutual Coulomb force. High resolution measurements on the fragments arising from this “Coulomb explosion” show promise as an unique technique to determine the geometrical structure of molecular ions. Similar information may be obtained studying the influence of the “Coulomb explosion” on the Auger decay of electronic states in the fragments of the molecular ion projectiles.