Metastable beams

Faubert, D. Paul, G. J. C. Giroux, J. Bertrand, M. J. Selective fragmentation and ionization of organic compounds using an energy-tunable rare-gas metastable beam source. Int. I. Mass Spectrom. Ion Proc. 1993,124, 69-77. 

In merging beam experiments, in which the metastable beam is formed by charge exchange with alkali atoms and the target beam by resonant... 

A beam of metastable Xe atoms is excited to a Rydberg state or photoionized and the product Xe+ ions are expelled by a field pulse from 2-5 /is later and impinge upon a position sensitive detector. When the metastable Xe is photoionized the pattern reflects the geometry of the metastable Xe beam, and the same is true when Xe+ is produced by attachment of the electron from a Xe 60f state to SF6. Apparently, there is no deflection of the Xe + by the SF6 . However, at low n,n< 40, the pattern of Xe+ detected is twice as broad as the metastable beam, indicating deflection of the Xe+ ion by the SF6-. As expected, the deflections of the Xe+ become more pronounced at lower n. 

Finally, thermal beams of atoms can be excited using one or more lasers, as described in Chapter 3. An interesting variant is the metastable beam used by Post et al,4 A Ba beam from an effusive oven is used to conduct a current from a heated... 

A numerical calculation of the line profiles due to the combined effect of the natural lifetime, the light-shift and the saturation has been performed taking into account all possible trajectories of atoms inside the metastable beam. Actually, the study of experimental linewidths shows there are some other stray effects responsible for the broadening of the lines. He have considered their contribution by making a convolution of the line profile with a gaussian curve. 

Because of the inelastic collisions with electrons, the metastable atomic beam is tilted with an angle of 20°, with respect to the incident atomic beam. The tilting from the incident beam is used to make the metastable beam colinear with the Laser beams. This colinear geometry... 

The third chamber is used to detect the metastable atoms two electrodes produce an electric field which quenches the 2S metastable states they are mixed to 2P state which radiates to ground state emitting the Lyman a fluorescence. A photomultiplier measures the L3nnan a fluorescence which is proportional to the number of metastable atoms in the beam. The metastable beam intensity can thus be estimated to be about 10 atoms per second. The detection of the two-photon transition is done by monitoring the population of the metastable level. It uses the fact that when the Rydberg atoms fall down in cascade, about 90 % of them come back to the ground state. So the metastable state is depopulated by the two-photon transition. 

It is not surprising that most results in atomic spectroscopy were obtained on singly charged ions which are difficult to prepare for the usual Doppler-free techniques on thermal samples. On the other hand, the fast-beam technique has certain advantages also on neutral atoms, such as the availability of metastable beams, the sensitivity, and the Doppler-tuning. 

In these experiments the corrections amount to only 3 per cent and the final result is given in Table 7.2. The result is in agreement with a less accurate value obtained in experiments made by Kocher et al, (1972) in which the metastable beam intensity was measured as a function of distance from the source using the apparatus shown in Fig.7.4. There... 

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