Many perturbations in diatomic molecules can be characterized as either homogeneous or heterogeneous [l] (pp. 284-286). Homogeneous perturbations take place between electronic states satisfying the selection rule ΔΛ = 0 or ΔΩ = 0. Heterogeneous perturbations take place between electronic states satisfying the selection rule ΔΛ = ±1 or ΔΩ = ±1. Ambiguities can clearly arise in this classification scheme. Is, for example, the perturbation between a case (a) 2Π1/2 state and a case (a) 2Π3/2 state homogeneous (ΔΛ = 0) or heterogeneous (ΔΩ = ±1)? Some measure of consistency can be achieved by requiring the nomenclature to reflect the selection rules on Ω when, as in cases (a) and (c), the rotational energy levels are given approximately by BJ(J + 1), and to reflect the selection rules on Λ when, as in case (b), the rotational energy levels are given approximately by BN(N + 1). It is interesting to note that the transition from Hund's case (a) to Hund's case (b) as J increases in a 2Π state might thus be described as resulting from a heterogeneous perturbation of the rotational levels of the 2Π1/2 state by those of the 2Π3/2 state (or vice versa).
These two types of perturbations can be described in another way. Homogeneous
perturbations are those which can occur in the nonrotating molecule,
i.e., perturbations caused by . The rigorous selection rule is thus
ΔΩ = 0 for nonvanishing homogeneous-perturbation matrix
elements in the basis sets used in this monograph, with the approximate
selection rules ΔS = 0, ΔΛ = 0,
ΔΣ = 0, when S, Λ, Σ are good quantum
numbers in the basis set. Heterogeneous perturbations are those which can only
occur in the rotating molecule, i.e., perturbations caused by
. The rigorous
selection rules are thus ΔJ = 0 and
ΔΩ = ±1 for nonvanishing heterogeneous-perturbation
matrix elements in the basis sets used in this monograph, with the approximate
selection rules ΔS = 0, and
ΔΛ = ±1, ΔΣ = 0 or
ΔΣ = ±1, ΔΛ = 0, when S,
&Lambda, Σ are good quantum numbers in the basis set, as well as the
approximate rule ΔL = 0 when L is a good quantum
number in the basis set. Matrix elements for homogeneous perturbations do not
involve the rotational quantum number J; matrix elements for
heterogeneous perturbations do involve J. Many of the selection rules
stated in this paragraph can be derived by considering the operators
and
together with
appropriate angular momentum commutation relations [7]
(pp. 59-64).
Homogeneous perturbations arise most frequently in practice because of spin-orbit interaction (see sect. 4.4), but they may also occur because of configuration interaction. In the latter case, the perturbations are often very large and difficult to treat accurately [22-24]. Heterogeneous perturbations occur because of uncoupling phenomena, i.e., uncoupling of the spin angular momentum (see sect. 1.9) or uncoupling of the orbital angular momentum (see sect. 4.3) from the internuclear axis because of rotation. These uncoupling phenomena can be attributed to Coriolis interactions in the rotating molecule.
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