In the following we shall be concerned with a part of the optically active behaviour of the chromophore. With N being the nitrogen atom of ammonia this chromophore is, of course, optically inactive. Its absorption spectrum consists of two bands in the visible which e. g. for
M
= Co(III) are attributed to transitions from a ground state,
1
A
1
g
, to excited states,
1
T
1
g
, and
1
T
2
g
.
ϵ
amounts to between 50 and 100 corresponding to oscillator strengths of
ca
. 10
-3
formal electron in spite of the fact that the transitions are electronically forbidden. This intensity is, I believe, not too well understood in detail but is generally ascribed to the simultaneous excitation of suitable molecular vibration (Moffitt 1956). The transition at lower energy (
1
A
1
g
→
1
T
1
g
is magnetically allowed, whereas the
1
A
1
g
→
1
T
2
g
transition at higher energy is magnetically forbidden (Moffitt 1956). The typical appearance of the absorption spectrum is shown in figure 1. (Similar selection rules apply to Cr(III) and Rh(III).) If the nitrogen atoms are connected in pairs as in a tris-(diamine) chelate the symmetry elements of the second order disappear and the chromophore becomes optically active. The upper level of each of the transitions mentioned splits under the trigonal field into a doubly degenerate
E
and a non-degenerate
A
level, but the splitting is quite small, and I think it is only fair to say that in general we know neither its magnitude nor its sign, although the beautiful crystal circular dichroism measurements by Professor Mason and his co-workers (Ballard, McCaffery & Mason 1962; McCaffery & Mason 1963) seem to show that for the tris-(ethylene-diamine) cobalt (III) ion the sign of the splitting is as indicated in figure 1. In accordance with the smallness of the splitting its spectral effects are quite small, and the selection rules remain almost intact as shown by the fact that the intensity of the absorption increases by less than 100% when ammonia is replaced by a chelating diamine.
Electron Impact Excitation Cross Sections for B III
