Excited-state absorption spectra of V2+ in KMgF3 and MgF2

Abstract

We have measured the excited-state absorption spectra of ${\mathrm{V}}^{2+}$ in ${\mathrm{KMgF}}_3$ and ${\mathrm{MgF}}_2$. The observed absorption bands can be identified as being due to the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$→${\mathrm{}}^4$ ${\mathrm{T}}_1$a and ${}_{}{}^4{}_{}{}^{}\mathrm{T}_1^{}{}_{}{}^{}$b transitions. The position and shape of the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$→${\mathrm{}}^4$ ${\mathrm{T}}_1$b transition are determined primarily by the displacement in the $a_{1g}$ mode while the coupling of the orbital triplet states to $e_g$ distortions is found to have a substantial influence on the nature of the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$→${\mathrm{}}^4$ ${\mathrm{T}}_1$a transition. In fact, this transition is observed as a broad band rather than as a sharp line, as is predicted solely on the basis of the $a_{1g}$ coordinate. The ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$→${\mathrm{}}^4$ ${\mathrm{T}}_1$a transition is shifted up in energy relative to its predicted position and significantly overlaps the emission spectrum, thereby providing an explanation for the poor lasing efficiency reported for ${\mathrm{V}}^{2+}$ lasers.