Excitation contour exciting beam

Since the critical configuration is reached when P is far from M, then the reaction will have a large cross section. The velocity of the products will be low, and the products will initially be in excited vibrational levels. The molecular beam contour diagram will show predominantly forward scattering typical of a stripping mechanism. 

The above tests on the primary ion beam are routine. The major uncertainty in the longitudinal tandem technique is the detection efficiency of product ions. This detection efficiency can be calculated if the complementary crossed-beam experiment has been performed to yield the contour map, i.e., both the angular distribution and the velocity distribution. As has been emphasized both earlier in this chapter and in Chapter 12, this is the proper approach to the problem, but the necessary data are rarely available. The alternative is to measure the detection efficiency and, in the absence of the angular distribution data, this must be done before an accurate cross section can be obtained. In practice, it rarely is done because the measurements are not straightforward. This absence constitutes a major source of possible error in most determinations of excitation functions by the longitudinal tandem technique. 

FIGURE 25.6 The 6- and 20-dB beam contours for the beam generated by a 3.0-MHz, 19-mm-aperture, 100-mm-focus transducer undergoing CW excitation. The X and y axes are to scale so that one can get a sense of the beam dimensions with respect to the depth of penetration. 

Laser methods make it possible to obtain particles with both a definite value and direction of the velocity vector and a definite orientation in the space. If a laser producing the radiation with the spectral width much shorter than the width of the absorption line is used for the excitation of a molecule, whose contour of the absorption line is due to the Doppler broadening, only molecules moving with a definite velocity in the direction of the laser beam will undergo excitation. For example, if the generation frequency of the laser corresponds to the frequency of the center of the absorption line of molecule, then only molecules at rest will be excited. 

---