  Progress on a method of barium tagging for the nEXO double beta decay
experiment is reported. Absorption and emission spectra for deposits of barium
atoms and ions in solid xenon matrices are presented. Excitation spectra for
prominent emission lines, temperature dependence and bleaching of the
fluorescence reveal the existence of different matrix sites. A regular series
of sharp lines observed in Ba$^+$ deposits is identified with some type of
barium hydride molecule. Lower limits for the fluorescence quantum efficiency
of the principal Ba emission transition are reported. Under current conditions,
an image of $\le10^4$ Ba atoms can be obtained. Prospects for imaging single Ba
atoms in solid xenon are discussed.
