Molecular beam techniques provide a powerful tool for the study of molecular dynamics. Fig. 0.1 shows schematically the apparatus we have used to measure the relative differential cross sections of chemi-ionization of the systems K + Br2 at initial relative kinetic energies of 6.9 and 10.35 eV , Li + Br2 at 6.25 eV and K + I2 at 11.25 eV .
|Figure A05-m3a-F1: Scheme of the apparatus. An explanation is given in the text.|
The sputtering source consists of an ion source (1): . By bombardment of the alkali target (2) by 15 keV ions, alkali atoms are formed with a kinetic energy in the eV range. .
The beam is collimated by two slits of 0.6 x mm2 and velocity-selected by a slotted-disk selector (3) with an energy spread of about 20 percent fwhm. .
The relative beam intensity is measured continuously by a surface-ionization detector (4) target: , the detection efficiency of which is expected to be slightly energy-dependent .
The beam is lead into a big collision chamber (5) filled with a static halogen gas at room temperature at a pressure of about 5 x 10-5 torr; this pressure is low enough to neglect double collisions .
Positive alkali ions formed in the collision region may enter a rotating detector chamber (6-9), which has an angular resolution of 0.5 for scattering angles less than 30 and 2.5 for large scattering angles .
A grid (10) screens the electric field of the lenses and channeltron to avoid as much as possible distortion of the ion paths between collision region and the second detector diaphragm. Other sources of distortion are contact potentials and electrostatic charged spots; therefore all relevant instruments have been made of the same nonmagnetic stainless steel, coated with a thin layer of graphite. .
We measured differential cross-section curve in runs on either side of the axis (symmetry test) . The measuring time for one run is about four hours.