The total energy splitting of ionic eigenstates in the axial crystal fields

The relationship between the energy total splitting $\Delta E$ of the free-ion electron states in the axial crystal-fields and the second moment of that splitting $\sigma^{2}$ is thoroughly investigated. The non-Kramers and Kramers states with the quantum number $1\leq J \leq 8$ in the axial crystal-fields of any multipolar composition but fixed $\sigma^{2}$ are considered. Since the crystal-field Hamiltonian ${\cal H}_{\rm CF}$ is a superposition of the three effective multipoles various $\Delta E$ can correspond to a fixed $\sigma^{2}$ according to the resultant combination of the independent contributions. This $\Delta E$ variation range is the subject of the study. For the states under examination $\Delta E$ can take the values from $2.00\sigma$ to $3.75\sigma$, whereas the difference $\Delta E_{max}- \Delta E_{min}$, except the states with $J\leq 5/2$, amounts roughly to $\sigma$. For comparison, the one-multipolar ${\cal H}_{\rm CF}$s yield accurately defined $\Delta E$ ranging from $2.50\sigma$ to $3.00\sigma$. The limitations of the allowed $\Delta E$ values exclude rigorously a number of virtually possible splitting diagrams. The documentary evidence for this restriction has been supplied in the paper collating the nominally admissible total energy splittings $\Delta {\cal E}$ (i.e. those preserving the $\sigma^{2}$) with the $(\Delta E_{min}, \Delta E_{max})$ ranges occurring in the actual axial crystal-fields. Although the $\Delta E$ unlike the $\sigma^{2}$ is not an essential characteristic and depends on the reference frame orientation, it is useful to know its dispersion range, particularly attempting to assign or verify complex electron spectra.