B. Rosner

EXPERIMENTAL EVIDENCE FOR LONG-LIVED HEAR2+ RARE-GAS DIMERS

Abstract The first observation of long-lived doubly charged HeAr2+ rare-gas dimers is reported. These dications were obtained in charge-stripping collisions of 850 keV HeAr+ in Ar gas. The mean lifetime for spontaneous dissociation of the 3 He 40 Ar2+ isotope was determined to be larger than about 40 μs using a new experimental technique. The cross-section for collision-induced dissociation of these molecular ions in air was found to be σdiss∼2×10−15 cm2. Calculations of the vibrational state population and mean lifetimes for a few low-lying electronic states indicate that all four low-lying states are very long lived and may be populated in fast charge-stripping collisions.

Methods for measuring mean lifetimes of long lived molecular ions formed in fast collisions

Abstract The direct measurement of the mean lifetime of molecular ions in a fast-beam (0.1–1 MeV) which were produced in collisions with a thin target requires methods which are different from the ones used for metastable atomic cases. This is due to the nature of the molecular ions breakup. The kinetic energy released in the dissociation is sufficient to spread the fragments on a relatively large area on the detector plane, thus special care and appropriate detectors have to be used to ensure the detection of all fragments. Several experimental techniques to determine the mean lifetime of long lived molecular ions are described. They are based on evaluating the ratio of dissociated molecular ions to the non-dissociated ones as a function of the distance they traveled. Measurements of the mean lifetimes of the recently discovered long lived rare gas dimers, NeAr2+ and HeNe2+ are used to describe these methods.

Formation and mean lifetime measurements of the long-lived doubly charged HeNe2+ rare-gas dimer

Abstract The first observation of a long-lived doubly charged HeNe 2+ rare-gas dimer is reported. This dication was obtained in charge-stripping collisions of 900 keV HeNe + and Ar. The mean lifetime of HeNe 2+ , τ = 184 ± 32 ns, was determined by measuring the yield of its He fragments, which have dissociated after passing the analyzer, as a function of the distance between the target cell and analyzer exits. Theoretical calculations of the 1 Σ + ground state indicate no bound vibrational states for this shallow potential curve. Further calculations are needed to determine the long-lived electronic state which is most likely an excited state.

Long lived CH2+ and CD2+ dications

Abstract A search for long lived CH 2+ and CD 2+ dications formed in fast charge stripping collisions of CH + and CD + on Ar was conducted. An experimental method based on the detection of the H (or D) fragments of the dication was developed, in order to eliminate possible confusion with 13 C 2+ for the first and 14 N 2+ for the latter. The flight time of these dications through the apparatus is about 70 ns, well below the 3 μs time associated with earlier observations of CH 2+ . Our measurements indicate that no long lived states of either of these dications are formed in fast charge stripping collisions. However, this result does not exclude the possibility that long lived states, like the excited A 2 Σ + metastable state, are populated in slow charge stripping collisions.

Formation of long-lived CDn2+ and CHn2+ dications

A systematic study of the formation of CDn2+ and CHn2+ dications in fast charge-stripping collisions with Ar atoms was conducted. The experimental method was based on the detection of the D (or H) fragments of the molecular ion of interest, and thus reducing the effect of the fraction of molecular ions containing the 13C isotope and other beam impurities. We observed long-lived CD22+, CD42+, and CD52+ dications. In the same process neither long-lived CD2+ nor CD32+ were observed. The mean lifetime of CD22+ was determined to be 4.0±1.11.3 µs, and those of CD42+ and CD52+ were longer than 2.1 and 3.3 µs, respectively. The production cross sections of CDn2+ from different CDm+ beams were measured. Long-lived CD22+ was formed from all CDm+ beams (m2) and also directly from the rf ion source. In contrast, CD42+ and CD52+ were formed only from CD4+ and CD5+, respectively.

A method for measuring mean lifetimes of short-lived (nanosecond region) molecular ions formed in fast collisions

Abstract The HeD2+ molecular ions are bound in their first excited electronic state, and decay to their repulsive ground state with a mean lifetime of a few nanoseconds. These short lived molecular ions are produced by charge stripping collisions of 900 keV HeD+ ions in Ar gas. The ions emerging from a short target cell are analysed by a small powerful permanent magnet. The trajectories of all reaction products are determined within a few nanoseconds. The simultaneous detection of both fragments i.e. a “fragment-fragment” coincidence, peaking at the proper deflection angle, is a clear signature of an HeD2+ molecular ion surviving through the analyzer. The average mean lifetime of HeD2+ was determined from the measured number of the “fragment-fragment” coincidences as a function of the distance between the target cell and the magnet. Similar measurements were performed to determine the mean lifetime of other HeH2+ isotopes.

Comparison between two computer codes for PIXE studies applied to trace element analysis in amniotic fluid

Abstract Two different methods of analysis applicable for PIXE data are introduced and compared. In the first method Gaussian shaped peaks are fitted to the X-ray spectrum, and the complete analysis can be done on a microcomputer. The second is based on the Bayesian deconvolution method for simultaneous peak fitting and has to be carried out on a larger IBM computer. The advantage of the second method becomes evident for regions of poor statistics or where many overlapping peaks occur in the spectrum. The comparisons between the methods made on PIXE measurements obtained from 55 amniotic fluid samples gave satisfactory agreement.

The ''Foil-Mesh'' method for measuring mean lifetimes of long-lived molecular ions

Abstract A method for measuring the mean lifetime of molecular ions undergoing unimolecular dissociation is presented. It can be used to measure molecular ions even if the beam has a large impurity fraction in it, as demonstrated by a measurement of the mean lifetime of 4 He 20 Ne 2+ . Furthermore, this method is not limited to mean lifetimes comparable to the flight time of the ions through the experimental setup as other commonly used methods. It can be used to measure mean lifetimes up to about two orders of magnitude longer than the flight time, as demonstrated using 12 CD 2 2+ and 3 He 40 Ar 2+ as examples.

A fast multiparameter multicoincidence data acquisition device using an IBM-PC microcomputer

Abstract A multiparameter single slot unit for an IBM-PC microcomputer controlling 16 parallel inputs is described. A MACRO assembler subroutine sorts out singles and various coincidence events with processing times less than 50 μs.