THE OPTICAL SPECTRA OF DILUTE AND CONCENTRATED CUBIC CRYSTALS CONTAINING THE $ReCl^{=}_6$ AND $ReBr_6^{=}$ IONS.

Work supported in part by the U. S. Air Force Office of Scientific Research. Paul B. Dorain: on leave from Brandeis University.Author Institution: Department of Physics, Yale University, New HavenThe optical spectra of the ions $ReCl^{=}_6$ and $ReBr{^=}_6$ in cubic crystals of the type $M_{2}ReX_{6}$ where M is $K^{+}, Cs^{+}$, or $(CH_{3})_{4}N+$ exhibit sharp lines at $T = 4.2^{\circ}K$ for the transitions among states arising from the $t_{2g}{^3}$ configuration. The diluted crystals of $ReCl{^{=}}_6$, in $K_{2}PtCl_{6}$ have a particularly simple vibrational structure of 124, 165, and $320 cm^{-1}$ which may be associated with the ungerade vibration modes of the complex. The pure electronic transition for the $^{4}A_{2}$ to $^{2}T_{2} T_{8}$ transition shows a small splitting of about $2 cm^{-1}$ for the dilute crystal of Re in $K_{2}PtCl_{6}$ which results from poor packing. In contrast the pure crystal of $K_{2}ReCl_{6}$ shows no splitting and is therefore a very good cubic lattice. Zeeman measurements on the dilute and concentrated crystals show that the concentrated crystal has a singlet ground state. From the observed splitting of the $^{2}T \Gamma_{7}$ state, the local antiferro-magnetic field of concentrated crystal is found to be 27.5 K gauss for $K_{2}ReCl_{6}$ and 45 K gauss for $K_{2}ReBr_{6}$. Associated with each vibration line is a spin wave absorption in the concentrated crystals. The u.v. spectra of $Re Cl{^=}_6$ in $Cs_{2}ZrCl_{6}$ shows four additional electronic transitions which are sharp. These lines together with those above permit accurate calculation of the Racah, crystal field and spin orbit parameters for this complex