Angular distribution of product ions

Since the velocities and angular distributions of products from collisional dissociations at low incident-ion energies have generally not been determined, the precise mechanism by which the products are formed is unknown. Thus in the collisional dissociation of H2+ with helium as the target gas, H+ may result from dissociation of H2+ that has been directly excited to the vibrational continuum... 

Reaction product imaging. In this technique, product ions are accelerated by an electric field toward a phosphorescent screen and the light emitted from the screen is imaged by a charge-coupled device. The significance of this experiment to the study of chemical reactions is that it allows for a detailed analysis of the angular distribution of products. 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

After these two examples, it should not be thought that reactions described by the harpoon model are necessarily direct. This was the case with reactions 4 and 5 because they are very exoergic and the molecular negative ion is unstable. In contrast, in the almost thermoneutral reaction of Cs with NO2, NOj is a stable ion, and the charge-transfer complex Cs - NOj corresponds to a deep well along the reaction coordinate. Hence the reaction proceeds from a persistent complex, with a forward-backward symmetry in the angular distribution of the reaction product CsO [72]. 

A pioneering work on the simultaneous measurement of the angular and velocity distributions was carried out by Champion et al. [98—101] following the velocity work of Vance and Bailey [94] described above. Their apparatus, a tandem mass spectrometer system, consists essentially of three sections a primary ion gun, a collision chamber, and a product-ion analyser and detector. A mass-analysed, velocity-selected ion beam is directed into the collision chamber containing target gas at low pressure. The product ions are velocity-analysed with a 127° electrostatic velocity selector and mass-analysed in a quadrupole mass filter. The angular distributions of the product ions are obtained by rotating the analyser-detector system about the centre of the collision chamber. 

Figure 5.25 Ion TOP distributions for isotropic angular distribution of dissociation products. The mass of the fragment ion is 30 amu, the electric fields in the first two acceleration regions are 10 and 100 V/cm and the acceleration distances are 2 and 1 cm, respectively. The drift distances are indicated in the figure. The d = 55 cm corresponds to the Wiley-McLaren space focusing condition. For each drift distance, the ion TOF was calculated for three product translational energies of 0, 0.5, and 1.0 eV.

The above tests on the primary ion beam are routine. The major uncertainty in the longitudinal tandem technique is the detection efficiency of product ions. This detection efficiency can be calculated if the complementary crossed-beam experiment has been performed to yield the contour map, i.e., both the angular distribution and the velocity distribution. As has been emphasized both earlier in this chapter and in Chapter 12, this is the proper approach to the problem, but the necessary data are rarely available. The alternative is to measure the detection efficiency and, in the absence of the angular distribution data, this must be done before an accurate cross section can be obtained. In practice, it rarely is done because the measurements are not straightforward. This absence constitutes a major source of possible error in most determinations of excitation functions by the longitudinal tandem technique. 

The second and third examples have been selected because these reactions are in principle among the simplest reaction rates to measure. Problems of product ion detection efficiency are eliminated by the collision dynamics, whereby both the reactant and the product ions exhibit very similar velocity and angular distributions, irrespective of the collision energy. However, the reactant ion is formed in two states by electron impact and the cross sections for these states are significantly different. Rate parameters will therefore depend on the relative populations of these two states. In this case, both the chemists techniques (mass-spectrometer ion source and ion cyclotron resonance) and some of the physicists ... 

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