Multiple-excited-state absorption of V2+ in low-field crystals:: An ab initio model-potential embedded-cluster study

In this paper we present an ab initio model-potential embedded-cluster study of the spin doublet excited states of V2+-doped KMgF3, KZnF3, and CsCaF3 fluoroperovskites, which includes intracluster electron correlation and quantum-mechanical lattice effects. The discrepancies of the calculated ground-state absorptions to the spin doublets with available experiments are systematic and amount to some 1000 cm(-1), leaving room for improvement of the treatment of valence electron correlation. The calculated excited-state absorption spectra originating in E-2(g) and T-2(1g) show that the broad E-2(g), T-2(1g)-->(2)A(1g) absorption bands considerably overlap the potential T-4(2g)-->(4)A(2g) laser emission, thus establishing a mechanism for laser loss that had not been considered so far in these low-field materials. The results obtained in these V2+-doped fluoroperovskites, together with those in other d(3)-doped low-field crystals, point out that the observed excited-state absorption spectra may correspond to either a single absorbing excited state or to the superposition of electronic transitions originating in all the different stable excited states lying below the energies used for pumping. Each of these excited states becomes a channel for absorptions that may result in multiple laser loss mechanisms. The separate study of each of these channels appears to be feasible using ab initio embedded-cluster methods. The conditions for the occurrence of single (T-4(2g)) versus multiple (T-4(2g), E-2(g), T-2(1g)) excited-state absorption, and, therefore, single versus multiple loss mechanisms, are discussed in this paper in terms of the energy barrier to E-2(g), T-2(1g)-->T-4(2g) nonradiative decay. [S0163-1829(98)08019-9].