Crossed beam mass spectrometric experiments

A simple mass spectrometric experiment with a well-defined positron beam would give us much useful information. Much more information would be obtained by the application of recoil ion momentum spectroscopy (RIMS) [23, 24] to annihilation from positron-molecule resonances. This would provide the energies and masses of all the ionic fragments. One possible configuration of a RIMS spectrometer involves crossed beams of a supersonic molecular beam of target molecules and a pulsed beam of positrons. This experiment is possible with existing technology [25]. 

In this chapter we have discussed the successful implementation in our laboratory, for the first time, of the soft (i.e. low energy) electron-impact ionization method for product detection in crossed molecular beams reactive scattering experiments with mass spectrometric detection. Analogous to the approach of soft photoionization by tunable VUV synchrotron radiation,... 

In a typical CMB experiment, beams of atoms and molecules with narrow angular and velocity spread are crossed in a vacuum chamber and the angular and time-of-flight (TOF) distributions of the products are recorded after well defined collisional events take place. The detector is an electron-impact ionizer followed by a quadrupole mass spectrometer (QMS) filter the whole detector unit can be rotated in the collision plane around the axis passing through the collision center (Figure 14.1). The crossed beam machine used in the present experiments has been described in detail elsewhere [67, 79,80]. Briefly, it consists of two source chambers (10 mbar), a stainless-steel scattering chamber (10 mbar), and a rotatable, differentially pumped quadrupole mass spectrometric detector ( <8 X 10" mbar). 

We highlight some recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method with mass-spectrometric detection. Emphasis is on three-atom (Cl + H2) and four-atom (OH + H2 and OH + CO) systems for which the interplay between experiment and theory is the strongest and the most detailed. Reactive differential cross sections are presented and compared with the results of quasiclassical and quantum mechanical scattering calculations on ab initio potential energy surfaces in an effort to assess the status of theory versus experiment. 

Photoionization mass spectrometric studies of BHg, produced by the pyrolysis of BgHg, allowed the appearance potential of [BH]+, from the process BH3 + hv [BH]" + H2 + e", to be measured as 308.38 0.39 kcal/mol. This value is deemed by the authors to be close to the true value [7]. Energetics of the B + H2- -[BH]" + H reaction channel were studied at the ab initio level. Of the allowed processes, only the B-" ( P) atoms are considered to be reactive species. The internuclear distances for [BH]" (2I+), [BH]" (211), and H2 ( Ig), 1.22, 1.26, and 0.7415 A, respectively, are in good agreement with experiment. The triplet potential energy surface is obtained and it is deemed useful for interpretation of results from crossed beam experiments [11]. 

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