S. Wexler

The A←X transition in Cr2: Predissociation, isotope effects, and the 1–1 sequence band

Two‐color resonance‐enhanced ionization spectroscopy with mass analysis is performed on beams of Cr2 produced by laser vaporization and isentropic expansion cooling. For the A←X 0–0 band near 459.6 nm an extensive rotational spectrum is observed and its intensity profile is found to have a systematic modulation with J. This modulation, and its surprisingly strong dependence on isotopic composition, are interpreted in terms of a two‐step predissociation of the A state and suggest a potentially important new procedure for isotope enrichment in metal systems. An additional resonant feature is found ∼1 nm to the red of the origin band and is assigned to the 1–1 sequence band.

Metal‐deficient iron oxide clusters formed in the gas phase

Molecular beams of oxidized iron clusters are produced by pulsed laser evaporation of iron followed by reaction of the resulting iron clusters with O2 in a continuous flow reactor. Nonstoichiometry of the iron oxide clusters is found, the composition rapidly approaching Fe0.9O as cluster size increases. The relation of these results to the similar nonstoichiometries found in bulk FeO is discussed.

Collision‐induced ion pair formation of thallium halides: Threshold behavior

The threshold laws for direct polar disociation A+BC → A+B++C− and for ion‐complex formation A+BC → AB++C− are determined for the collision of Xe and Kr atoms with TlCl, TlBr, and TlI by analysis of the experimental excitation functions presented in the preceding paper. The cross section for direct polar dissociation is parameterized by the expression A(Etot−E0)n/Erel, where Etot and Erel are the total energy and the relative translational energy, respectively, of the reactants, and E0 is the threshold energy. The cross section is averaged over the experimental translational and internal energy distributions, and, for each pair of reactants, E0, n, and A are determined from the best fit to the data. For ion‐complex formation, the data are analyzed in terms of a step behavior at the reaction threshold. For the system Xe + TlCl, experimental measurements of the cross section as a function of internal energy are presented, and the relative importance of translational and internal energy in promoting polar di...

THE DISSOCIATION OF ETHYL BROMIDE AND IODIDE BY NEUTRON CAPTURE

A study has been made of recoil halogen species ejected from gaseous ethyl bromide and iodide after neutron capture. The results demonstrate that a fraction of the radio‐halide fragments carry a positive charge. The fraction charged is 12 percent for the 4.4‐hr Br80m, 25 percent for the 36‐hr Br82, and 50 percent for the 25‐min I128. The corresponding estimate for the 18‐min Br80 is very approximately 18 percent. The positive charges are attributed to internal conversion processes in the stabilization of the compound nucleus formed by neutron capture. The fraction of positive recoil ions is interpreted as the minimum fraction of neutron capture processes which are followed by an internal conversion process. A comment is made upon the possible significance of the ions in the study of chemical reactions induced by radiative neutron capture.A study of the 18‐min Rb88 from β‐decay of the 2.8‐hr Kr88 was made using the procedures and equipment employed in the halogen experiments. The results show that all Rb88...

Dissociation of Methyl Bromide by Nuclear Isomeric Transition of 4.4‐hr Br80m

The pattern of positively charged fragments from methyl bromide as a result of isomeric transition of 4.4‐hr Br80m has been measured by mass spectrometric techniques. A peaked distribution of charge states of atomic bromine ranging from Br+1 to Br+13 was observed, the most probable being Br+7. The finding of singly, doubly, and triply charged carbon‐containing fragments gives evidence that the CH3Br80 daughter ion breaks apart by Coulombic repulsion of redistributed charges following internal conversion and subsequent Auger electron transitions. Multiply charged polyatomic species produced by the nuclear transition are unstable.

Dynamics of electron transfer collisions of fast Li atoms with several molecules

The angular distributions of Li+ ions produced in electron transfer collisions of neutral lithium atoms with the molecules Br2, Cl2, HBr, HCl, O2, and SF6 have been measured. For each system, studied by the crossed molecular beam method, data were obtained at several collision energies, ranging from near threshold [ΔE = 5.390‐target electron affinity (eV)] to approximately 14 eV in the center‐of‐mass (c.m.) system. The fast Li atoms were produced by surface sputtering and then velocity selected. An in‐plane experimental geometry was used, with fixed beam sources and a rotatable quadrupole mass filter detection system. The laboratory angular distributions of Li+ ions fall into two groups. For molecules possessing high electron affinities, i.e., Br2 and Cl2, the distributions exhibit undulations, whereas for targets with low electron affinities, the angular distributions decrease monotonically. All show a strong peaking in the forward direction. Differential cross sections obtained by inversion to the c.m. ...

Collision‐induced ion pair formation of thallium halides: Absolute cross sections

Collision‐induced dissociative ion pair formation (polar dissociation) of thallium chloride, bromide, and iodide by aerodynamically accelerated Xe and Kr atoms has been studied by the crossed‐beam method. Absolute cross sections for total positive ion formation from both the monomer and the dimer were determined from the threshold to several electron volts relative energy above threshold. The cross sections for the monomer varied over three orders of magnitude, ranging from 0–1.0 × 10−16 cm2 for TlCl, to 0–8 × 10−18 cm2 for TlBr, to 0–1.6 × 10−19 cm2 for TlI when Xe was the collidant. Similar ranges were measured for Kr on TlCl and TlI, but the cross sections were approximately a factor of 10 smaller for TlBr. The observed cross sections for positive ion production from the dimer molecules were in the range of 1 to 20 × 10−16 cm2. Time‐of‐flight mass spectrometry permitted the determination of partial cross sections for each positive and negative ion channel in the collision. The excitation functions for ...

Isomeric Effects in Fragmentation by Beta Decay: Monotritiated Propanes and Toluenes

The spectra of positively charged fragments from beta decay of tritium in primary and secondary monotritiated propane and in o‐, m‐, p‐, and α‐monotritiated toluene have been determined by mass spectrometric techniques. Although the number of products from decomposition of each molecule is quite large, the mass patterns from the two propanes are fairly similar while the four mass spectra from the isomeric tritiated toluenes are very nearly the same. These results suggest that the influence of the location of the radioactive atom in the molecule on the fragmentation following beta decay is often obscured by secondary processes, such as radiationless transitions, H‐atom migration, and other rearrangements of the excited molecular ion before breakup. However, there are significant differences within the two groups of mass spectra. Some of the differences in the propane patterns are consistent with the greater stability of the secondary propyl structure as compared to that of the primary isomer. Energetically...

Collision‐induced ion‐pair formation of CsCl and Cs2Cl2

Absolute cross sections have been determined for collision‐induced ion‐pair formation of cesium chloride (monomers and dimers) from collisions with Ar, Kr, and Xe atoms. The rare gas atoms were aerodynamically accelerated in H2, with maximum energies of 6.8, 12.1, and 18.3 eV (in the laboratory frame), respectively. Time‐of‐flight mass spectrometry permitted the determination of partial cross sections for each ion channel resulting from the collision. For the monomer, the cross secions for the channel leading to Cs++Cl− were similar in magnitude for the three rare gas atoms and were approximately 3×10−16 cm2 at 3 eV above threshold. The rearrangement ionization channel leading to CsM+ (M=Ar, Kr, and Xe) exhibited a step behavior at threshold followed by a very rapid fall off at higher collision energies. Collisions with the dimers produced both Cs2Cl++Cl− and Cs++CsCl−2 but no detectable ions containing the rare gas atoms. The magnitude of the cross sections for the Cs2Cl+ channel varied significantly wit...

Survey of chemi-ionization reactions in accelerated atom--O2 crossed-molecular beams

The chemi‐ionization reactions, associative ionization, and rearrangement (reactive) ionization, and the electron transfer reaction have been investigated in the crossed‐molecular beam collisions of more than 20 accelerated (by sputtering) metal and nonmetal atoms with O2 molecules. The observations support a generalization on the scope of these chemi‐ionization reactions and their relation to electronic structure, namely that associative and rearrangement ionization reactions do not occur for collision partners for which the thermodynamic threshold energy for electron transfer is lower than the thresholds for the chemi‐ionizations. On the other hand, rearrangement and associative ionization may be prominent reaction channels when their thermodynamic thresholds are less than that for electron transfer. Relative cross sections for Ce+, CeO+, and CeO2+ formation for collision energies of Ce+O2 up to 50 eV (cm) are presented. A substantial threshold for associative ionization is observed for this system.