The Crossed Beam Machine

In the primary source, a pulsed beam of open unstable (open shell) species is generated by laser ablation (C, C2, 3), laser ablation coupled with in situ reaction 

FIGURE 11.1 Top view of the crossed molecular beams machine. Shown are the main chamber, the primary (laser ablation configuration) and secondary source chambers, and the rotatable differentially pumped mass spectrometer detector. 

The pulsed primary beam is passed through a skimmer into the main chamber a chopper wheel located after the skimmer and prior to the collision center selects a slice of species with well-defined velocity that reach the interaction region. This section of the beam then intersects a pulsed reactant beam released by a second pulsed valve under well-defined collision energies. It is important to stress that the incorporation of pulsed beams allows that reactions with often expensive (partially) deuterated chemicals be carried out to extract additional information on the reaction dynamics, such as the position of the hydrogen and/or deuterium loss if multiple reaction pathways are involved. In addition, pulsed sources allow that the pumping speed and hence costs can be reduced drastically. 

To detect the product(s), our machine incorporates a triply differentially pumped, universal quadmpole mass spectrometric detector coupled to an electron impact 

FIGURE 11.2 Schematic view of the laser ablation source. 

---

Figure 3. Illustration of the cross-beam machine. N is the nozzle source for the molecular beam, C is the buffer chamber with a beam chopper (not shown), H is the hexapole electric field quantum state selector, U are the homogeneous electric field plates, Q is an on-axis quadrupole mass filter, O is the fast atom beam source, and Q and C,8o are channeltrons.

In a typical CMB experiment, beams of atoms and molecules with narrow angular and velocity spread are crossed in a vacuum chamber and the angular and time-of-flight (TOF) distributions of the products are recorded after well defined collisional events take place. The detector is an electron-impact ionizer followed by a quadrupole mass spectrometer (QMS) filter the whole detector unit can be rotated in the collision plane around the axis passing through the collision center (Figure 14.1). The crossed beam machine used in the present experiments has been described in detail elsewhere [67, 79,80]. Briefly, it consists of two source chambers (10 mbar), a stainless-steel scattering chamber (10 mbar), and a rotatable, differentially pumped quadrupole mass spectrometric detector ( <8 X 10" mbar). 

---