Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl ions. The compound crystallizes in two different structures, the tetragonal TlSe type and the monoclinic type. Photoemission studies were carried out, using a synchrotron photon source, on TlS crystals with both the TlSe- and -type structures. Comparison of the spectra shows that the line shape of the Tl 5d core level is more asymmetric in the TlSe type than the type. The asymmetry reflects the density of states near the Fermi level of the crystals. It was also found that as the incident photon energy increases the intensities of the valence band peaks increase relative to those of the Tl 5d core levels. Comparing the intensity ratios with the calculated photoionization cross section ratios, the characters of the valence bands were discussed.

The laser oscillation frequency of a semiconductor laser becomes unstable under modulation. The stability of the frequency stabilization method using the atomic absorption line is degraded by the oscillation spectrum broadening caused by the sidebands under modulation. Therefore, we designed two methods, the synchronous detection method and the peak‐hold method, to improve the frequency stability under modulation. From the experiment using the Rb absorption line, the synchronous detection method improved the stability by 3 times compared with the old method. The peak‐hold method improved the stability by 10 times compared with the old method under modulation and by 2 times compared with the old method without modulation.