Magnetic field dependence of spin-forbidden electronic excitations reflects the Haldane or paramagnetic ground state

We compare the zero-field and magnetic field-dependent optical spectra of the Haldane chain compound NENB (Ni[en]$_2$NO$_2$BF$_4 $; en = C$_2$N$_2$H$_{8}$) and the paramagnetic compound, Ni(en) $_3$(ClO$_4$)$_2$,H$_2$O. Due to similar electronic coordination of Ni$^{2+}$, the two materials show similar zero-field $d-d$ electronic transitions, including a spin-forbidden (SF) transition at 1.58 eV, overlapping a broad spin-allowed band at 1.45 eV. The relatively greater intensity of the SF band in the Haldane compound suggests activation by a spin exchange mechanism, whereas a spin-orbit coupling origin is likely in the paramagnet. A second narrower SF spin flip transition appears in NENB at 1.66 eV. In both compounds, the SF excitations are sensitive to applied field $H$. In NENB, the SF intensity is suppressed by $H$, consistent with behavior of spin exchange-activated bands. In Ni(en)$_3$(ClO$_4$)$-2$,H$_2 $O, the SF field sensitivity appears to combine an energy shift and intensity decrease. Details of the $H$ dependence reflect the magnetic ground state of the material: the field sensitivity commences only above $H_C$ $\approx$ 10 T, in the Haldane compound, whereas the field-induced modifications begin immediately at $H$ = 0 T in the paramagnet.