Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

9 MOLECULAR BEAMS 175in which E is the initial relative translational kinetic energy, Wand W' are the initialand final internal excitation energies (rotational, vibrational or electronic), respectively,and ADo is the difference in dissociation energies of the new and old bonds,measured from the zero-point vibrational levels. (Note that the translational kineticenergy of the centre of mass remains constant during the collision. E and E'refer to the relative translational kinetic energies.) The direction of the final relativevelocity vector is determined by the conservation of angular momentum. Thusif L, L' are the orbital angular momenta associated with the relative motion of thecollision partners and J, J' (= J, +J2 ; J , +J4, respectively) the sums of the momentaof the individual reactant and product molecules, then L'+J' = L+J. TheNewton diagram shows the recoil spectrum of products as a set of spheres centredon the tip of the centre of mass velocity veotor u, and with radii depending on thevalues of E'. The next steps in the comparison of the observed product densitypattern with a theoretical model are to project these distributions on to the laboratorycoordinate system and to average over the initial velocity distributions in theincident beams. It is possible to obtain additional information about the reactionthrough a velocity analysis of the reaction products.In a recent review1l2 reference is made to all of the reactions of the typeRX + M + R+ MX (where X is a halogen and M an alkali metal) which have beenstudied in crossed molecular beams. The relationship between the rate coefficient kand the parameters measured in molecular beam experiments has been considered" '.The ratio of cross-sections of reactive (ar) and elastic (a,) scattering was shownto be proportional to the ratio of the rate coefficients for reactive and elastic scattering,kr/ke (the collision yield). Several very interesting generalizations have beenmadeti2. It seems that most of the chemical energy released by the reaction goesinto internal degrees of freedom of the products rather than appearing as increasedtranslational kinetic energy. Also, the angular distribution of the products is usuallyquite anisotropic, and the preferred recoil direction is strongly correlated with themagnitude of the reaction cross-section, 0,. For example, in the reaction betweenmethyl iodide and potassium the KI produced recoils backwards with respect tothe potassium beam while the CH, is scattered forwards. Although it is much tooearly to speak of complete agreement between experimental observations and theo-retical predictions, some recent theoretical work on the MeI-K reaction in whichthe methyl group was treated as a point has yielded very encouragingresults.9.1 GAS-SOL~D REACTIONSA conceptually rather simple application of molecular beams to kinetics has beenused to measure the life-time of adsorption of hydrogen molecules on a nickelsurface. The apparatus is shown in Fig. 24"'; the molecular beam strikes a ro-References pp. 176-1 79