Transmission Zeeman beat spectroscopy: Application to the 4f66s27Fground-level term of Sm i

A spectroscopic technique for the accurate determination of multipole relaxation rates in ground- or near-ground atomic levels is described. The technique is based on the detection of Zeeman quantum beats in transmission geometry and yields relaxation rates for the destruction of both orientation and alignment. Collisional depolarization cross sections are reported for the 4${\mathit{f}}^6$6${\mathit{s}}^2$ ${}_{}{}^7{}_{}{}^{}$ ${\mathit{F}}_{\mathit{J}}$ (J=1–6) ground-term levels in samarium i by various rare-gas perturbers. For the ${}_{}{}^7{}_{}{}^{}$ ${\mathit{F}}_1$ level the ratios of the cross sections for the destruction of orientation and alignment are unusually large (${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$≊1.6 for each of the rare-gas perturbers), whereas for the J=2–6 levels the reverse is true (${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$ in the range 0.39 to 0.46 for argon perturbers). These values of ${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$ are found to be in agreement with the predictions of a weak-collision model in which the collision interaction has an essentially pure quadrupole character. The observed cross sections for the ${}_{}{}^7{}_{}{}^{}$ ${\mathit{F}}_1$ level appear to be the first experimental realization of the simple theoretical result, ${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$=5/3, initially obtained by Omont [J. Phys. (Paris) 26, 26 (1965)].