Noble-gas broadening rates for barium transitions involving the metastable 6s5d 3DJ levels.

Abstract

The collisional-broadening rate coefficients, for both helium and argon buffer gases, have been measured for transitions from the metastable 6s5d $^{3}$${\mathit{D}}_{\mathit{J}}$ levels in barium. A weak cw diode laser was used to populate the $^{3}$${\mathit{D}}_{\mathit{J}}$ manifold through optical pumping of the 6${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$\ensuremath{\rightarrow}6s6p $^{3}$${\mathit{P}}_{1}$ transition, followed by radiative decay into the $^{3}$${\mathit{D}}_{2,1}$ states. The absorption coefficient for the transition of interest was then measured by monitoring the transmission of a very weak cw probe laser beam as its frequency was scanned over the line shape. Such scans were carried out for several diode (pump) laser powers, and using the standard line-shape normalization integral, we were able to construct density-independent line shapes. Each spectral line shape consists of many hyperfine components due to the presence of five barium isotopes (including two with nonzero nuclear spin). Each component is affected by Doppler, pressure, and natural broadening and, therefore, can be represented by a Voigt function. Thus, the measured line shapes (recorded with 50, 100, 200, and 500 Torr of either He or Ar buffer gas) were fitted with a sum of Voigt functions in which the only free parameter was the pressure-broadening rate. Using a least-squares fit of the broadening rates versus buffer-gas number density, the broadening-rate coefficients ${\mathit{k}}_{\mathrm{br}}$ were found. In addition, collisional line shifts for these transitions were measured, and shift rate coefficients ${\mathit{k}}_{\mathrm{shift}}$ were determined.