Far-infrared magnetoabsorption and refractive index of $2H-\mathrm{Mo}{\mathrm{S}}_2$

The far-infrared transmission of $2H-\mathrm{Mo}{\mathrm{S}}_2$ has been studied in magnetic fields up to 13 T, at temperatures between 1.5 and 40 K, in the spectral wave-number range 5-250 ${\mathrm{cm}}^{-1\mathrm{}}$. A single resonance has been observed in the magnetoabsorption spectra of some natural crystals. The resonance energy has a weak magnetic field dependence, and a zero-field value corresponding to 6 ${\mathrm{cm}}^{-1\mathrm{}}$. Optical pumping in the exciton bands near 2 eV strengthens the absorption and shifts it slightly. Polarization, intensity, anisotropy, and other measurements indicate that a probable cause of the absorption is magnetic resonance of ${\mathrm{Fe}}^{2+}$ impurity ions occupying octahedral sites with trigonal distortion in the ${\mathrm{MoS}}_2$ lattice. Based on this model, optical pumping effects are interpreted as being due to conversion of ${\mathrm{Fe}}^{3+}$ to ${\mathrm{Fe}}^{2+}$ via charge transfer following exciton decay. A search over a wide range of spectral wave number, magnetic field, and temperature has failed to reveal any "light mass" features attributable to free-carrier cyclotron resonance, interband, or hydrogenic impurity absorption. The presence of such absorption was implied by recent reports of exciton-associated magneto-oscillatory resonance, and related theoretical discussions. Interference fringes present in the transmission spectra of thicker samples have been used to obtain the value $n_0^{\perp }=3.96\mathrm{}\pm 0.1$ for the normal component of the low-frequency refractive index at 4.5 K. Comparison with other values from the literature leads to a determination of the temperature coefficient, $\left(\frac{1}{n_0^{\perp }}\right)\frac{{\mathrm{dn}}_0^{\perp }}{\mathrm{dT}}\approx 2\times {10}^{-4\mathrm{}}$ ${\mathrm{K}}^{-1\mathrm{}}$. It is shown that electronic contributions apparently dominate the temperature dependence, whose sign and magnitude agree with that deduced from the known temperature dependence of the optical gap.