Atomic recoil experiments

Anyone who imdertakes to design a polyvalent atom recoil experiment must keep clearly in mind that the observed reaction products will rqp-... 

Atomic recoil experiments offer an alternative procedure for investigating the chemical effects of translational excitation (IQ.20). Thermal (21.22) and energetic gas (23-2 )... 

The potential content of atomic recoil experiments has recently been shown to include useful dynamical information (15-16.19.25-27.51.52). A central interpretive difficulty, however, follows from the need for tinique and accurate specifications of the reactant non-Boltzmann momentum distribiitions. 

In this experiment a week of target bombardment was required to produce a single fused nucleus. The team confirmed the existence of element 109 by four independent measurements. The newly formed atom recoiled from the target at predicted velocity and was separated from smaller, faster nuclei by a newly developed velocity filter. The time of flight to the detector and the striking energy were measured and found to match predicted values. 

In the past few years there has been a very intense effort to understand the process of single ionization in ion-atom collisions. Experiments in this area have followed largely along three main paths recoil-ion momentum spectroscopy... 

The amount of Es available at this time was very small about N = 10 atoms (a 4- 10 g). At a flux density of a particles = 10 " cm s , a cross section (Ta n = 1 mb and an irradiation time of 10 s a yield N

single atoms, the recoil technique was applied (Fig. 14.6). Es was electrolytically deposited on a thin gold foil. The recoiling atoms of Md were sampled on a catcher foil. After irradiation, the catcher foil was dissolved and Md was separated on a cation-exchange resin. In 8 experiments 17 atoms of Md were detected and identified by their transmutation into the spontaneously fissioning the properties of which were known ... 

An example of the inferences that have been drawn is as follows. Methylene formation in the reactions of recoiling carbon atoms was deduced from comparison of product distributions from recoil experiments with those obtained from singlet state CH2 (16yl7). The hexanes produced from the reactions of recoiling atoms with n-pentane were... 

These examples point to the symbiotic relationship between polyvalent atom recoil chemistry and the study of chemically generated reactive intermediates. Hot atom experiments identify products that suggest the intermediacy and delineate the reactivity of exotic species such as cycloproplyidene, methyne, and silylene. Thus, hot atom experiments have stimulated the search for other ways of generating these species, and in turn these chemical experiments can help interpret the hot atom experiments. 

An interesting application of Husain s kinetic data to the interpretation of the results of a recoil experiment have been made by Kremer and Spicer (10). These workers studied the exchange reactions of sulfur atoms with carbon disulfide. 

Since the necessity for examining reaction sequences initiated by polyvalent recoiling atoms via their end products has been a severe experimental constraint, it may be worthwhile to repeat the prediction that high sensitivity laser absorption spectroscopy will in the near future permit direct detection of reactive intermediates in both chemical and recoil studies of atomic reactions. If this proves to be the case, then an exciting new era will have dawned in which recoil experiments may be able to answer many questions about the dynamics as well as the sys-tematics of polyvalent atomic reactions. 

Statistical models for unimolecular decomposition (16) have figured prominently in these applications. For example, theoretically predicted energy dependencies of branching ratios have often been compared with experimental yields to estimate excitation distributions (3,4,5,13-15). Significantly, one of the first experimental indications of the importance of dynamical influences in unimolecular decomposition was provided by a nuclear recoil experiment (3). In more recent work, hot atom activation combined with statistical rate theory and cascade models for collisional deactivation have been used to investigate energy transfer for highly excited polyatomics (17). 

In well-designed experiments the mean hot atom lifetime is much shorter than the mean thermal reactive lifetime. The MNR technique thus oflFers good utility for precise equilibrium kinetics studies wdth the radioactive atoms and unstable radicals produced using nuclear recoil methods. Small residual nonthermal reaction yields are invariably observed in recoil experiments, but these have no direct bearing on the validity of the MNR equilibrium hypothesis. 

Several factors decide whether or not the bonds attached to a newly formed atom are sufficiently strong to hold it in place. The impulse is imparted to the newly formed atom in a time that is short compared to the time required for bond dissociation, bond dissociation times being approximately the time for a bond vibration (10 — 10 sec). Consequently the other bonds in the molecule probably do not experience any traumatic change in energy state as the atom recoils. In considering the nuclear transformation of bromine in the HBr molecule, Suess (1940) derived a simple expression, given in equation (1), relating... 

The remainder of this article is mainly concerned with the theoretical analysis of nuclear recoil hot atom chemistry experiments. Under typical laboratory conditions the recoil species are generated consecutively through irradiations having much longer duration than the characteristic hot atom mean free lifetime (25-27). It is not unusual for the individual recoil events to be isolated in real time. On this basis the early hot atom kinetic theories utilized stochastic formulations for Independent recoil particle collision cascades occurring in thermally equilibrated molecular reaction systems. 

Later sections describe the modeling procedure and results obtained for nuclear recoil atoms reacting with pure (70) and inert gas moderated (71) H2. These reaction systems have been chosen for study because of the availability of quasl-classical trajectory reaction cross sections (51) together with results from thermal (21.72) and nonthermal ( 7 nuclestr recoil experiments. 

This uniform distribution is achieved by having the exciting electron beam coaxial with the atomic beam. Early experiments with transverse electron beam excitation led to atom recoil and serious systematic effects due to non-uniform velocity distributions over the beam cross-section. The mean radiative lifetime is determined by taking the ratio of the number of metastable atoms in the same velocity interval at the two spatially separated detectors since... 

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