Polarisation spin-orbit mechanism

The use of polarised beams in collision studies has enabled experimentalists to perform very detailed tests of theoretical models, particularly with regard to the role of electron exchange and the spin—orbit interaction in spin-dependent scattering. We will now briefly discuss the role of these interactions before using the general density matrix method to describe the more general case where more than one mechanism may contribute to the spin-dependent effects. 

It can be seen that electron—photon coincidence experiments with polarised electrons permit the investigation of spin effects in electron impact excitation of atoms at the most fundamental level. It can lead to direct information on both exchange effects and spin—orbit effects in the excitation mechanism. The information on the population of the magnetic sublevels can be visualised by charge-cloud distributions. These can tilt significantly out of the scattering plane for incident electrons transversely polarised in the scattering plane. 

The differences in the population and depopulation rate constants and the phosphorescence probabilities of the three components of the triplet states form the basis of all the methods for Optical Detection of Magnetic Resonance in triplet states of jr-electron systems. These methods were developed after the discovery of optical spin polarisation and extended to inorganic solids. The essential physical difference from the optical double resonance in atoms developed by Alfred Kastler is to be found in the selection mechanism in optical double resonance, the polarisation of the resonant UV light, i.e. the symmetry of an applied field, is responsible for the selection. In optical spin polarisation, the selection is due to the spin-orbit coupling, and thus to an internal field. 

The net effect of the live unpaired electrons in the 3d orbitals is to produce a spherically symmetric charge distribution centred on the chromium atom. Using the intemu-clear distance Ro calculated from Bo above, the dipolar constant c is calculated to be 51.2 MHz which compares quite well with the measured value of 41.81 MHz. The Fermi contact constant bp is found to be negative and small, consistent with a spin polarisation mechanism The determined value of gf is in remarkably good agreement with the value calculated from a relationship due to Curl [72], gf = -y/25 = —0.414 x 10 . ... 

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