The 3${\mathrm{}}^3$${\mathit{S}}_1$–3${\mathrm{}}^3$${\mathit{P}}_{0,1,2}$ laser lines at 844.5 nm of atomic oxygen are studied in a weak discharge cell with argon buffer gas, using the optogalvanic effect. Three Doppler-limited peaks are observed at the center frequencies of the three fine-structure transitions. The measurements of the spectral widths indicate that the atoms in the lower state (3${\mathrm{}}^3$${\mathit{S}}_1$) are at room temperature or ‘‘cold.’’ This excludes the previous assumption that all the triplet states are very ‘‘hot’’ and moreover is consistent with the long-range atom-ion interaction model [Kwon et al., Phys. Rev. A 42, 4408 (1990); Bjerre and Keiding, Phys. Rev. Lett. 56, 1459 (1986)]. Based on the results, one can understand the unexplained observations such as the anomalously broadened spectra associated with the 3${\mathrm{}}^3$P triplet state, the peculiar four-peak emission spectra of the laser lines, and the absence of the J=0→J=1 transition.

DOI:https://doi.org/10.1103/PhysRevA.52.R895