  The presence of H2D+ in dense cloud cores underlies ion-molecule reactions
that strongly enhance the deuterium fractionation of many molecular species. We
determine the H2D+ abundance in one starless core, Barnard 68, that has a
particularly well established physical, chemical, and dynamical structure. We
observed the ortho-H2D+ ground-state line 1_10-1_11, the N2H+ J=4-3 line, and
the H13CO+ 4-3 line with the APEX telescope. We report the probable detection
of the o-H2D+ line at an intensity Tmb=0.22 +- 0.08 K and exclusively thermal
line width, and find only upper limits to the N2H+ 4-3 and H13CO+ 4-3
intensities. Within the uncertainties in the chemical reaction rates and the
collisional excitation rates, chemical model calculations and excitation
simulations reproduce the observed intensities and that of o-H2D+ in
particular.
