[Legenda]
[Contents of the thesis]
[Comments on this module]
[Legenda]
[Contents of the thesis]
[Comments on this module]

Experimental methods=A08-m3a

Beam techniques provide a powerful tool for the study of molecular dynamics. The general shape of the setup for beam experiments is given in. A scheme of the apparatus we have used to measure relative differential cross sections for the atom-atom collision process at kinetic energies from 13 to 85 eV is given in Fig. A08-m3a-F1

Fig.A08-m3a-F1: Schematic drawing on scale of the apparatus showing the charge-exchange source, the dissociation oven and the rotating detector

The charge-exchange source of the sodium beam (1) is of the same type as constructed by Helbing and Rothe and has been described elsewhere .

For selection of the state of the Na atoms of the beam a pulsed voltage over the directly heated ionization cathode and a chopped-detection technique have been used to avoid energy spread of the beam caused by the potential drop over the cathode. The total energy spread is about 0.5 eV fwhm. The neutral beam is collimated by the exit slit of the source and a second slit (2) placed at a distance of 35 cm. The slits have equal dimensions of 0.5 x 3 mm².

This primary sodium beam interacts with iodine gas, which is generated in an oven (4) described in more detail in .

At working conditions the iodine pressure in the collision region is estimated to be about 2 x 10^-4, while the temperature of the oven was about 1200ºC to ensure complete dissociation. Because the oven rotates simultaneously with the detector, the slots in the tantalum cylinders and heat shields to the detector are only 7 x 2.5 mm². The entrance slots of the sodium beam are 7 x 8 mm², which enables the measurements of scattered particles up to a laboratory angle of 22 degrees.

The reaction products are analyzed in the differentially pumped detection chamber (5) , which they enter via a pumping resistance (6) with identical discs on both ends and with eleven rectangular diaphragms with widths of 0.25, 0.50, 1 and 2 mm to fix the angular resolution of the detector at 0.3º, 0.6º, 1.2º and 2.4º fwhm, respectively. The discs are fixed in identical positions on the axis of a small step motor (7).

The detector is coupled to a multi-channel analyzer, each channel corresponding to a defined angular position of the detector. The detector is rotated by the stepmotor, the smallest step of the detector being 0.07º. A special-purpose computer drives the detector over the desired range with the required angular resolution, rotation step and integration time. An electrostatic 127º energy analyzer (8) has been used at the present measurements only to separate the scattered sodium ions with an energy in the eV range from the large number of thermal ions emitted by the oven.

The ions produced in the chemi-ionization process are detected using a channel electron multiplier (12). The lens system (11) focuses the ions into the detector (12). The ion signal does not contain any background and is measured by pulse counting.

The iridium band (13) and a second channeltron (14) give the possibility to detect scattered neutral alkali atoms passing the energy analyzer by a slit. The relative intensity of the primary sodium beam is measured by a surface-ionization detector (3) .

With respect to the reliability of the analysis and detection, we remark the following. Angular-dependent detection of eV-ions requires much attention in order to avoid electro-magnetic deflection. As much as possible the whole region of the collision center up to the shielding plate (9) has been coated by a thin layer of graphite to avoid contact potentials and stray charges. Distortion of the oven current has been avoided by heating the oven with a current which is pulsed synchronously with the already chopped detection. However, there still exists a drift in time resulting in a smoothing of the fine structure of the differential cross section if scans are added over too long a time. It is suspected that evaporation from the oven causes contamination of the graphite layer and again gives rise to electric charges, and therefore the measurements have been done with a total integration time as short as possible with respect to the statistical noise.

Generally the measurements consist of four scans added in the multi-channel analyzer, each scan consisting of two corresponding angular ranges on both sides of the zero axis to check for symmetry, while each measuring point has an integration time of four seconds per scan.