Beam sources excited states

Increased ion-source pressures and/or introduction of paramagnetic molecules, such as NO, cause deactivating collisions that convert excited ions into ground-state species13 61,63,64-66 (see Table I). The fractional abundance of excited states in an ion beam may thus be lowered without a concomitant reduction in total ion intensity, which would occur if the ionizing electron energy were simply lowered. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84). In this technique, a beam of high-energy ( 10 MeV) charged particles (e.g., O4+, Cl7 +) is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mossbauer effect experiment must be performed at the location of the charged particle beam. 

X-band cavity changes the fluorescence polarisation, which can be detected. The main features of the apparatus are illustrated in figure 11.12. One of the advantages of excitation with an electron beam, rather than with conventional monochromatic or white light sources, is that transitions between electronic states of different spin multiplicity are allowed consequently both singlet and triplet excited states can be populated. 

Figure 2 Experimental arrangement for measurements of the Fe nuclear resonance at the Advanced Photon Source (APS). In the standard fill pattern, electron bunches with a duration of 100 ps are separated by 153 ns. X-ray pulses are generated when alternating magnetic fields in the undulator accelerate these electron bunches. The spectral bandwidth of the X-rays is reduced to 1 eV by the heat-load monochromator and to 1 meV by the high-resolution monochromator. At the sample, the flux of the beam is about 10 photons/s. APD indicates the avalanche photodiode used to detect emitted X-rays. The lower right inset illustrates that counting is enabled only for times weU-separated from the X-ray pulse, so that only delayed photon emission resulting from decay of the nuclear excited state contributes to the experimental signal...

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