Velocity relative beam

The two extreme cases cu 1/r and cu -C 1/t are easily understood, but in very different terms, so it is less clear how to think about the case w 1/r. Nonetheless, the low frequency regime suggests that the phase of the rf field relative to when the collision occurs is likely to be important. To control this phase we use transform limited collisions of /C atoms in a velocity selected beam so that we know when the collisions begin and end, and we synchronize the rf field to the laser pulse initiating the collisions [Renn 1991], Specifically, we have studied the process of Eq. (10) in rf fields phase locked to the collisions. 

Velocity selected beams have been used to determine the threshold translational energies and the variation of the reaction cross-section with relative kinetic energy, e.g. K + CH3I [217] and K + HBr [132, 218]. The... 

To measure the reaction cross section as a function of relative kinetic energy requires forming beams of A molecules and BC molecules with specified velocities. The beams are oriented so that they collide collision products are monitored as a function of energy and orientation. A representative apparatus is sketched in Fig. 8.1. 

A unifonn monoenergetic beam of test or projectile particles A with nnmber density and velocity is incident on a single field or target particle B of velocity Vg. The direction of the relative velocity m = v -Vg is along the Z-axis of a Cartesian TTZ frame of reference. The incident current (or intensity) is then = A v, which is tire number of test particles crossing unit area nonnal to the beam in unit time. The differential cross section for scattering of the test particles into unit solid angle dO = d(cos vji) d( ) abont the direction ( )) of the final relative motion is... 

Fig. 4.14 Magnetically perturbed quadrupole spectra simulated for powder distributions of the EFG (Vzz > 0) with an applied field B = 4T which is fixed in the laboratory system perpendicular to the y-beam/ The value of the quadrupole splitting is kept constant at AEq = +4 mm s For negative quadrupole splitting (V z < 0), the spectra would be inverted on the velocity scale. Note the difference in relative intensities for the spectrum for ry = 0 and the single-crystal type spectrum given in Fig. 4.13. Similar patterns are obtained for B y...

More modern systems (diffractometers) follow the same principles but the diffracted X-rays are detected with a solid state detector, as described earlier. Typically, the X-ray source is static and the sample and detector are rotated, with the detector moving at twice the angular velocity of the sample to maintain the equivalent angle. Such instruments typically make use of relatively large samples compressed into the window of a 35 mm sample holder. However, where the sample size is restricted, as is common with archaeological applications, a smaller sample (a few mg) can be attached to a silica wafer. In all cases the sample needs to be hnely ground to ensure a uniform diffracted beam. 

---