Gas beam

A study of interaction of sodium atoms with vibrationaHy excited nitrogen molecules at the intersection of two gas beams shows that conversion of vibrational energy to electronic excitation is substantially more efficient than conversion of translational energy (179). This has also been indicated in other reactions (180). 

Inert gas beams allow the chemistry of a surface to be studied by SIMS without modification by the bombarding species. The achievable values of source brightness allow pA currents into spot diameters of approximately 50 pm for dynamic SIMS, or nA currents into spot diameters < 5 pm for imaging SIMS. For greater spatial resolution a different, higher-brightness source must be used. 

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

For the 71Ga nucleus, the availability of an absolute shielding scale also makes possible a comparison of calculated absolute shieldings with experimental values. The standard used experimentally, [Ga(OH2)6]3+ in solution, is unsuitable as a theoretical reference due to the lack of consideration of water molecules beyond the first solvation shell in the calculations. On the other hand, there exists an absolute shielding scale for 71Ga, based on the NMR measurement in an atomic Ga beam at the same time as in the ion at infinite dilution in a D20 solution (57). 

A values for some gases are plotted in Fig. 10.10, from which an important observation can be drawn since A depends on the gas selected, the association of the gas flow with a specific mode and the properties of the gas beam formed will depend on the gas, i.e., for a given capillary the formation of a gas beam will differ from gas to gas. 

Figure 10.11 Demonstration of the formation of a gas beam by an orifice (left), a transparent tube (middle), and an opaque tube (right), (a) Some particle trajectories in the vicinity of the orifice or within the tube, respectively (f>) resulting angle-dependent intensities /( ). The driving pressure pY is taken to be large compared to the pressure p2 where the beam formed is observed. The conditions for Knudsen flow are always fulfilled in the left-hand and middle diagrams but in the right-hand diagram they are only fulfilled in a restricted region (indicated by /eff, note the different lengths of the arrows which indicate the mean-free-path of some particles). If a particle hits a surface, such as at point A, it is assumed to be repelled with a cosine distribution.

---