Magnetic-dipole emission

Table 6. Ratio of electric to magnetic dipole emission intensity and position of the lowest excitation band for some Eu +-activated ABO4 compounds [after Ref. (33))...

Spin Orbit Energies of the 2P% States of the Halogen Atoms and Mean Radiative Lifetimes for Magnetic Dipole Emission (following Garstang13)... 

However, the actual situation is mote complicated. In the first place Y2O3 offers two sites to the Eu ion, one with C2 and one with Ss symmetry (see Fig. 6.11). There are three times more C2 sites, and Eu is assumed to occupy these two types of sites in a statistical way. The So site has inversion symmetry, so that the Eu ion on this site will only show the Dq - Fi magnetic-dipole emission (Sect. 3.3.2) which is situated around S95 nm. The strongly forbidden character of the Dq - Fj transitions of Eu (S6) becomes clear from the value of the decay time of 8 ms compared with 1.1 ms for Eu " (C2) [15]. 

This is nicely confirmed by a study of some Eu -activated phosphates and vanadates with zircon structure (Blasse and Bril, 1969). The observed ratio of electric to magnetic dipole emission of the Eu " luminescence in these hosts is correlated with the position of the lowest excitation (and absorption) band of these materials and the intensity ratio. This absorption band is a c.t. transition in which either europium or vanadium or both are involved. It has, therefore, been proposed that the parity-forbidden 4f-4f transitions of the Eu " ion borrow intensity from the lowest strong absorption band (either host lattice absorption or charge-transfer absorption within the centre) and not from the 4f-5d absorption band. In conclusion we find that for intense forced electric-dipole emission from Eu two conditions must be fulfilled, viz. absence of inversion symmetry at the Eu " crystallographic site and c.t. transitions at low energies. 

This calculation also shows that spontaneous magnetic dipole emission in the radio or microwave region will be quite unobservable in laboratory experiments. The techniques of radio frequency spectroscopy of atoms and nuclei in solids, liquids, and gases therefore depend on the stimulated emission and absorption processes wliich are discussed in detail in Chapter 9. 

Provided that a transition is forbidden by an electric dipole process, it is still possible to observe absorption or emission bands induced by a magnetic dipole transition. In this case, the transition proceeds because of the interaction of the center with the magnetic field of the incident radiation. The interaction Hamiltonian is now written as // = Um B, where is the magnetic dipole moment and B is the magnetic field of the radiation. 

As shown in Example 5.2, it is easy to obtain that (A)(./(A) 10, where (A)m is the probability of spontaneous emission for a magnetic dipole transition. Thus, using the previous estimation of (A)e, we obtain that, for a magnetic dipole transition,... 

It is also interesting to compare similar results for Eu3+ in trigonal prismatic coordination (JO). The ratio of electric to magnetic dipole Eu + emission is 1.2 for YF3 Eu, 4.0 for GdTiSbOe Eu and 8.0 for YAI3B4O12 Eu. Note the much lower ratio for the fluoride than for the oxides which must be due to the difference in position of the c.t. band (in fluorides much higher than in oxides). 

Suppose one first considers electric-dipole and magnetic-dipole transitions. As is now well recognized, these are the major contributors to rare-earth absorption and emission spectra. We know that the electric-dipole operator transforms as a polar vector, that is, just as the coordinates (23, 24). This means that it has odd parity under an inversion operation. On the other hand, the magnetic-dipole operator transforms as an axial vector or pseudovector and of course must have even parity (23, 24). 

As stated in an earlier paragraph, the sharp emission and absorption lines observed in the trivalent rare earths correspond to/->/transitions, that is, between free ion states of the same parity. Since the electric-dipole operator has odd parity,/->/matrix elements of it are identically zero in the free ion. On the other hand, however, because the magnetic-dipole operator has even parity, its matrix elements may connect states of the same parity. It is also easily shown that electric quadrupole, and other higher multipole transitions are possible. 

For quadrupole radiation, they estimate P 2x 10-9, whereas for magnetic-dipole radiation their result was P 2 x 10-8. The experimental values lie in the range of 10 7 to 10 5. From these estimates, one concludes that the probability of significant electric-quadrupole and higher-order-multipole radiation is very small indeed. The magnetic-dipole radiation is weak but probably is of some importance, particularly in cases where the electric-dipole emission is strictly prohibited. 

Magnetic dipole interactions are possible for the 1AJ -3S ," and 129+ <-3E9 transitions in 02, although naturally they are weak. The transitions are, however, known both in absorption, especially in the atmosphere, and in emission indeed, the two band systems are frequently known as the infrared atmospheric and atmospheric bands of oxygen, and the observation of emission from these systems has often been used to demonstrate the presence of excited singlet oxygen both in the laboratory and in the atmosphere. 

PAC atomic probes (e.g., mIn or mHf) possess a nuclear quadrupole moment and a magnetic dipole. Even if no field acts on the PAC nucleus, the successive emission of the y-photons through an intermediate state exhibits an appreciable angular anisotropy between the emission directions. If the (isolated) nucleus is then brought into a perturbing field (e.g., on a specific lattice site which is next to a vacancy), the angular anisotropy becomes time-dependent due to the precession of the nuclear spin. For example, if the PAC nucleus in the crystal is exposed to a (static) electric... 

Circularly polarized luminescence (CPL) from chiral molecular systems is the emission analog of circular dichroism (CD) and as such reflects the chirality of the excited state in the same maimer as CD probes reflect the chirality of the ground state (Riehl and Muller, 2005). For lanthanide ions, laige CPL (and/or CD) signals are expected for f-f transitions obeying magnetic dipole selection rules, in particular A J = 0, 1 Eu(5Do —> 7Fi), Tb(5D4 7F4, 5D4 7F5), Dy(4F9/2 6Hn/2), Yb(2Fs/2 2F7/2) emissions are typical examples. Recent... 

The ground-state electronic configurations (levels) of neutral and singly ionized berkelium were identified as 5f 7s2 (6H15/2) and Sf s1 (7H8), respectively (82). A nuclear magnetic dipole moment of 1.5 nuclear magnetons (61) and a quadrupole moment of 4.7 barns (83) were determined for 249Bk, based on analysis of the hyperfine structure in the berkelium emission spectrum. 

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