Superelastic scattering

We have thus far considered the probability of superelastic scattering on a single orbit. To obtain the scattering rate, or autoionization rate, we simply multiply this probability by the orbital frequency, 1/n3.4 Once again we find that T oc 1/n3 and that T decreases with increasing Z. The scattering description we have just given is a two channel description. This picture, when many channels are present, forms the basis of multichannel quantum defect theory.5... 

Superelastic scattering from laser-excited targets... 

An interesting and alternative technique to electron—photon coincidence measurement of coherence effects in excitation processes is superelastic scattering from laser-excited targets. This technique, first developed by Hertel and Stoll (1974, 1978), can be thought of as the time inverse of the (e,e y) coincidence experiment. 

McClelland, Kelley and Celotta (1986) were the first to measure superelastic scattering in the configuration where the spins of both the incoming electron and target atom were polarised, ensuring that the transitions studied are transitions between well-characterised pure quantum states. In particular they studied the superelastic scattering of spin polarised electrons from the mp = 3 and mp = —3 states of Na 3 P3/2 atoms (or the m/ = +1 and m/ = — 1 states on making the conventional assumption... 

Such measurements were first applied with considerable success to elastic scattering. Indeed one was able to discuss experiments which would determine all the theoretically calculable amplitudes (Bederson, 1970). For inelastic processes, such measurements necessitate the simultaneous application of spin selection techniques and the alignment and orientation measurements discussed in the previous chapter. The experiments have become feasible with the advancement of experimental techniques. The first successful differential electron impact excitation study with spin-polarised electrons and alignment and orientation measurements was performed by Goeke et al. (1983) for the e—Hg case. McClelland, Kelley and Celotta (1985, 1986) carried out a systematic study for superelastic scattering of polarised electrons from polarised laser-excited Na (3 P) atoms. This system is essentially a two-electron collision system in which spin exchange is the dominant spin-dependent interaction. It thus allows one to obtain... 

The existence of the fine-structure effect has been demonstrated for sodium (Hanne, Szmytkowski and van der Wiel, 1982 McClelland et ai, 1985 Nickich et al, 1990) using the time-reversed arrangement. A polarised electron beam is superelastically scattered from sodium atoms excited to 3 P /2 or 3 3/2 states by a single-frequency laser. McClelland et al. (1985) measured the spin asymmetry of polarised electrons that de-excite unpolarised atoms from the 3> P3/2 fine-structure state over the angular range —35° < 6 < 35°. As expected from reflection symmetry, the... 

The investigation of sodium as a critical test of the theoretical treatment of scattering is given a new dimension by the spin-dependent measurements of Kelley et al. (1992) in elastic and superelastic scattering experiments with polarised electrons on the polarised 3s and laser-excited 3p states. Not only have asymmetries been measured for these states, but spin-dependent observations of the magnetic substate parameter L have been made for the 3p state. 

The third category of transitions observed is due to the electronic energy levels and it will be recalled that superelastic scattering has been observed in the electronic and vibrational rotation transitions. 

Since an oscillating dipole moment is a source of new waves generated at each molecule, (3.5) shows that an elastically scattered wave at the frequency co of the incident wave is produced (Rayleigh scattering) as are inelastically scattered components with the frequencies co — cOn Stokes waves) and superelastically scattered... 

Those molecules that are initially in excited vibrational levels can give rise to superelastic scattering of anti-Stokes radiation, which has gained energy (/kDs p) = Ei — Ef) from the deactivation of vibrational energy. 

Since an oscillating dipole moment is a source of new waves generated at each molecule, (8.5) shows that an elastically scattered wave at the frequency w is produced (Rayleigh scattering) but also inelastically scattered components with the frequencies (w-w ) (Stokes waves) and superelastically scattered waves with the frequencies (w+w ) (anti-Stokes components). The microscopic contributions from each molecule add up to macroscopic waves with intensities which depend on the population N(E ) of the molecules in the initial level Ej, on the intensity of the incident radiation and on the expression (9Qij/9q )q which describes the dependence of the polarizability components on the nuclear displacements. 

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