Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

9 MOLECULAR BEAMS 173jet in that the mean free path of molecules, X,, in the former is very much longerthan the width of the beam. This means that when a beam passes through a lowpressureregion it retains its identity whereas a jet disintegrates within a very shortdistance. The details of a molecular beam apparatus depend to a large extent onthe nature of the experiment, and Fig. 23 shows schematically how a single molecularbeam may be produced. A source of beam material A has a slit throughwhich the molecules enter the oven chamber B. The beam then passes through ashort isolating chamber C and into the observation chamber which contains a collimatingslit S. The position and intensity of the beam are detected at D. Traditionally,the three chambers are connected to independent pumping systems. The requirementthat X, be large compared with the beam width restricts the width ofthe slits and also, because X, is inversely proportional to it, the pressure. This iswhy the intensity of a molecular beam cannot be raised above what is really a verylow level (and one which requires a detector of extreme sensitivity); if it were, thecondition for effusive flow would no longer be satisfied,The critical feature of any apparatus for following chemical reactions in crossedmolecular beams' is the detector. The surface-ionization detector, which providesa very sensitive way of estimating alkali metals in the presence of their salts, comprisesparallel filaments of tungsten and a platinum-tungsten alloy which can beheated in turn and are partly surrounded by a metal sheath held at a negative potentialrelative to the wires. Suppose, for example, that the reaction being studied isMeI+K + Me+=. The ionization potential of potassium in K and KI and thework function of tungsten are such that, if a K atom or a KI molecule strikes theheated tungsten wire, the alkali metal may lose an electron to the wire. (In fact,the ionization potential of the beam molecules must be less than the work functionof the wire material.) This produces a positive ion which is attracted to the sheath,and the positive ion current is a direct measure of the rate at which K and KI arestriking the wire. Now the ionization potential of potassium in its salts is higherthan that of the free metal and the work function of the Pt-W alloy is (under certainconditions) less than that of W. The second wire gives a positive current withfree potassium but not with K18. Thus the concentrations of K and KI can be determinedat any position from the two positive ion currents. The surface-ionizationdetector is restricted to reactions involving the alkali metals and a few others. Inthe original experiments the angle between the beams was kept constant at 90"and the detector moved only in the plane described by the two beams. It is now apparent that additional useful information may be had if the beam angle can bevaried and the density of the products outside the beam-plane can be measured.Other variables are the temperatures of the beams, which need not be the same.The source of the alkali metal is a double-oven; the inner oven contains the moltenmetal and is kept at a constant temperature, while the temperature of the outeroven may be varied. By this method the beam temperature can be varied by about300" without affecting the vapour pressure. It was found that the normal triple-References pp .176-179