Far-Infrared Spin and Combination Resonance in Bismuth

Seven new absorption lines have been observed at 1.5°K by transmitting unpolarized far-infrared light through a thin, single crystal of bismuth in a magnetic field $B$. Both the propagation direction and $B$ were along the trigonal axis and perpendicular to the sample face. We used an interference spectrometer built by P. L. Richards covering frequencies between 13 and 85 ${\mathrm{cm}}^{-1\mathrm{}}$ together with a superconducting solenoid producing fields up to 50 kG. All seven lines lie well below the cyclotron frequency for either electrons or holes. As $B$ is increased, certain lines narrow and disappear one by one until at 50 kG only three remain. As $B$ is tilted a few degrees away from the trigonal axis some of these lines split with a strong angular dependence while others remain almost unaffected. We attribute these lines to spin and combination resonances of the conduction electrons. From the fields at which particular resonances disappear, energy-level assignments can be made. These assignments and the observed splittings are compared with calculated values based on published masses.