Collisionless regimes

Beam Spectroscopy. Both specificity and sensitivity can be gready enhanced by suppressing coUisional and Doppler broadening. This is accomphshed in supersonic atomic and molecular beams (296) by probing the beam transversely to its direction of fiow in a near-collisionless regime. 

The search for a field dependence of the nuclear relaxation rate in the normal phase of (TMTSF)2C104 has been set out by Caretta et al. [74]. Their results obtained at 200 K (Fig. 13) up to 15 Tesla show a square-root field dependence which is indicative of one-dimensional diffusive spin dynamics. The square root dependence is cut off at low enough field due to processes that do not conserve spin along the stacks [66]. The field dependence is found to become essentially undetectable around 150 K and below, which may indicate either a change of dimensionality in the spin dynamics or that the collisionless regime is reached. 

Table 7.2. Approximate pressure limits for collisionless regimes as a function of molecular complexity...

A molecular beam is a collimated ensemble of molecules travelling in a collisionless regime through an evacuated chamber. The beam can be directed towards another beam, a gas cell or a surface. In general, the scattering produced from these interactions provides information about intermolecular forces, energy transfer or the dynamics of a chemical reaction either in the gas phase or gas-surface interaction. 

Figure 1 shows the four gas flow regimes and the applicable models. The Boltzmann equation is valid for the whole range of Am, from 0 to infinity. A simplified Boltzmann equation or the collisionless Boltzmann equation, where the right-hand side reduces to zero, is suitable when Kn is very... 

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