Temperature Dependence of Optical Absorption Lines from Iron Atoms in a Krypton Matrix

The optical absorption spectrum of iron atoms isolated in a krypton matrix contains two narrow lines at $\lambda =3004\mathrm{}\pm 0.5 \mathrm{and} 2969\pm 1.0$ Å. These lines are related to the two multiplet lines $3d^{6}4s{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}-3d^7\left(a{}_{}{}^4{}_{}{}^{}F\right)4p{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{\circ }$ and $3d^{6}4s^2{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}-3d^7\left(a{}_{}{}^4{}_{}{}^{}F\right)4p{}_{}{}^5{}_{}{}^{}D_3^{}{}_{}{}^{\circ }$. The linewidths (full width at half-maximum) at a matrix temperature of 4 K are 42 ± 1.5 and 38 ± 4 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. The lines are asymmetric (high-energy tail) and the linewidths increase with increasing temperature. From the line-shape analysis of the 3004 Å line at 4.2 and 20.5 K, the effective Debye temperature of solid krypton as probed by iron atoms is determined: The temperature dependence of linewidth yields ${\Theta }_D=60\mathrm{}\pm 5$ K; the asymmetry of the line gives ${\Theta }_D=53\mathrm{}\pm 6$ K.