  Neutrinoless double beta decay is one of the most sensitive probes for new
physics beyond the Standard Model of particle physics. One of the isotopes
under investigation is $^{136}$Xe, which would double beta decay into
$^{136}$Ba. Detecting the single $^{136}$Ba daughter provides a sort of
ultimate tool in the discrimination against backgrounds. Previous work
demonstrated the ability to perform single atom imaging of Ba atoms in a
single-vacancy site of a solid xenon matrix. In this paper, the effort to
identify signal from individual barium atoms is extended to Ba atoms in a
hexa-vacancy site in the matrix and is achieved despite increased
photobleaching in this site. Abrupt fluorescence turn-off of a single Ba atom
is also observed. Significant recovery of fluorescence signal lost through
photobleaching is demonstrated upon annealing of Ba deposits in the Xe ice.
Following annealing, it is observed that Ba atoms in the hexa-vacancy site
exhibit antibleaching while Ba atoms in the tetra-vacancy site exhibit
bleaching. This may be evidence for a matrix site transfer upon laser
excitation. Our findings offer a path of continued research toward tagging of
Ba daughters in all significant sites in solid xenon.
