Strong field substates

A six- or four-line ESR spectrum that can be fitted to a triplet spin Hamiltonian is strong evidence that the species in the sample embodies two unpaired electron spins. Support for the presence of a triplet spin system often can be found in the weak Ams = 2 line, which appears at one-half the field strength of the center of gravity of the Ams = 1 six-line pattern. This nominally forbidden Amj = 2 resonance results when the ESR spectrometer field and frequency produce a micro-wave quantum of energy just sufficient to jump the gap between the uppermost and lowermost triplet substates, that is, a transition over two quantum levels. 

Other significant spectral changes are also observed. The total splitting increases from 8.3 cm-1 at B=0 T to 24 cm-1 at 5= 12 T. Moreover, due to the field induced mixings of the wave functions, the radiative allowedness of the transitions from the T substates to the ground state is strongly redistributed. The emission from the lowest B-field disturbed substate 1(B) becomes dominant, while the transitions 11(B) —> 0 and III(B) — 0 lose intensity. This is also displayed in the emission decay time of substate I at 1.5 K, which becomes as short as 12 ps at 12 T, while it amounts to 85 ps at zero-field (see next section). Due to this B-field induced increase of radiative allowedness, it also becomes possible to tune magnetically other important properties like the mechanisms of vibrational deactivation [78-82]. 

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