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Abstract : It was pointed out some years ago by Mizushima 1 that although magnetic dipole radiation is forbidden in LS-coupling for intercombination lines, magnetic quadrupole radiation is allowed for ƊS = ⊥ 1 transitions. This can make a magnetic quadrupole transition of this type of the same order-of-magnitude intensity as many intercombination transitions due to electric dipole radiation and spin-orbit perturbations. Both magnetic quadrupole and electric dipole radiation require the initial and final states to be of opposite parity. For many transitions both multipoles can occur, but there are a few cases where only magnetic quadrupole radiation is possible. One such case is s2 1S0-sp 3P2, which is forbidden for electric dipole radiation in the absence of nuclear spin interactions because ƊJ = 2. A series of transition probability calculations by Garstang 2 showed that for heavy elements such as Hg I 6 s2 1S0-6 s 6 3P2 magnetic quadrupole radiation is negligible in comparison with nuclear-spin-induced electric dipole radiation. In the corresponding transition in Cd I magnetic quadrupole radiation forms about 10 per cent of the total, and there is some evidence from the laboratory intensity ratio 1S0-3P2-1S0-3P0 that magnetic quadrupole radiation is indeed present. In Zn I magnetic quadrupole radiation dominates in the line 4 s2 lS0-4 s 4 p 3P2, and this line has been observed in the laboratory. In Mg I magnetic quadrupole radiation dominates in the line 3 s2 lS0-3 s 3 p 3P2 ; this line has not been seen in the laboratory, but was observed in the planetary nebula NGC 7027 by Bowen. In a later paper Garstang 3 investigated the occurrence of magnetic quadrupole radiation in highly ionized atoms, including among others C V, Ne IX, Ar XVII, Fe IX, Fe XVII and Fe XXV. It was shown that magnetic quadrupole radiation is a significant radiative depopulation mechanism for several levels, including 3 p5 3 d 3P2 in Fe IX, 2 p5 3 s 3P2 in Fe XVII and 1 s 2 p 3P2 in Fe XXV. In Fe XXV the magnetic quadrupole line 1 s 2 p 3P2-1 s21S0 has a higher probability than the electric dipole line 1 s 2 p 3P2-1 s 2 s 3S1 . The latter situation holds in the He I isoelectronic sequence above S XV. Recently R. Marrus and his group unpublished in Berkeley have made a measurement of the lifetime of the 1 s 2 p 3P2 state in Ar XVII, and obtain a result in agreement with that calculated from the sum of the electric dipole and magnetic quadrupole transition probabilities to the 3S1 and 1S0 states, giving an excellent confirmation of Garstang-s transition probability for the magnetic quadrupole line.

Autor: R. Garstang



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