Multiple-collision expansion

Quantitative results for high-energy processes may be obtained from a multiple-collision expansion. Indicating with k the free total state in channel k (a product of relative, internal and atomic spin states), the transition matrix element M31 for A + BC -> AB + C is given by... 

Fig. 12. Spectator-stripping cross sections for T + H2(0,0) -+ TH(h3. 73) + H, calculated from the first term of the Faddeev-Watson multiple collision expansion (from D. A. Micha and P. McGuire, 1972).

We briefly examined vibrational relaxation of l2 X,v > 20) induced by collisions with He at low temperatures. These measurements were performed by focusing state preparation and probe laser beams into the downstream region of a He free-jet expansion that had been seeded with l2-Transfer out of the initial levels u = 23 and 42 was examined at 5 K. For these conditions the probe laser spectra were quite simple and uncongested. A typical probe laser spectrum is shown in Fig. 5. The delay used to record this trace was long enough for multiple collisions to occur, so populations are seen in levels corresponding to Av = — 1 and —2 transitions. 

The reactivity of the clusters can then be studied by various experimental techniques, including fast flow reactor kinetics in the postvaporization expansion region of a laser evaporation source [21, 22], ion flow tube reactor kinetics of ionic clusters [23, 24], ion cyclotron resonance [25, 26], guided-ion-beam [27], and ion-trap experiments [28-30]. Which of these techniques is applied depends on the charge state of reactants (neutral, cationic, anionic), on whether the clusters are size-selected before the reaction zone, on single or multiple collisions of the clusters with the reactants, on the pressure of a buffer gas if present, and on the temperature and collision energy of the reactant molecules. 

Two physical limits of the general formalism have proved very useful in applications. They are the impulsive collision limit, which can be derived from the multiple scattering expansion, and the semiclassical limit which derives from a short-wavelength or eikonal expansion. These are considered in detail in separate sections which include several of our applications to scattering by diatomics and polyatomics. 

The laser-desorbed molecules are injected into the colhsion zone of a pulsed supersonic expansion where they are entrained in the carrier gas pulse and internally cooled by multiple collisions with the carrier gas atoms. When the molecules are desorbed into the gas phase, it is assumed that the different conformations are populated according to a Boltzmann distribution at a temperature of about 600-800 K. An important question is to what extent this initial distribution is retained in the supersonic cooling process. 

---