Detectors and angular distributions

The conventional way of scanning the angular distribution of the products is by means of using rotatable sources. The two sources are attached to a circular turntable and the point of intersection of the beams may be swept past the detector [6, 8]. 

Herm and Herschbach [104] and Wilson [142] have studied the reactions of alkali metals with NO2, N2O, CH3NO2, several alkyl nitrites, nitrates and nitro compounds and dialkyl peroxides. Differential surface ionization could not be used as the Pt—8% W filament was in the 

The detector of choice for molecular beam experiments is a mass spectrometer. Wilson [142] used a quadrupole mass spectrometer to exmnine alkali exchange reactions [173], e.g. 

The main advantages of quadrupoles are their small size and absence of small focusing slits, i.e. high transmission properties, and also the relative cheapness. It is predicted that in the future quadrupoles will become the most commonly used detector (see, for example, refs. 177 and 227-229). 

Generally in molecular beam studies, both beams have comparable velocities and intersect one another at 90°, and thus the CM velocity vector points at a wide angle intermediate between the two beams. Measurement of the displacement of the laboratory angular distribution of products from the centre-of-mass vector enables an estimate of the velocity of the products to be derived. Reaction products have been velocity analysed (e.g. see refs. 8 and 231) and the results support the view that the product relative translational energy is usually within ca. 1 kcal mole of the reactant relative translational energy. Most of the alkali metal reactions studied to date are exothermic, thus the products must be internally excited. It is believed [8] that, for most reactions, the internal excitation consists mainly of vibrational excitation however, the partition of the vibrational energy between, for example, KI and CH3 is as yet unknown. There are a few exceptions, e.g. the K + HBr reaction where KBr is rotationally excited rather than vibrationally excited [8], and the 

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