  We present sensitive, high angular resolution molecular-line observations of
C18O and N2H+ toward the dark globule B68. We directly compare these data with
the near-infrared extinction measurements of Alves, Lada, & Lada (2001) to
derive the first evidence for the depletion of N2H+, and by inference N2, in a
pre-stellar dark cloud. We also find widespread C18O depletion throughout the
centrally condensed core of the B68 cloud. Specifically, we find the N2H+
emission to peak in a shell partially surrounding the peak of dust extinction.
Moreover, the N2H+ peaks inside the much larger C18O depletion hole and has a
smaller depletion zone, confirming theoretical predictions. These data are
analyzed through a direct coupling of time dependent chemical models to a
radiation transfer code. This analysis highlights the importance of
photodissociation at cloud edges and suggests that the CO abundance declines by
two orders of magnitude from edge to center. In contrast N2H+ declines in
abundance, at minimum, by at least a factor of two. Indeed it is entirely
possible that both N2H+ and N2 are completely absent from the central regions
of the B68 core. The depletion of N2H+, and its parent molecule N2, opens the
possibility that the centers of dense cores, prior to the formation of a star,
may evade detection by conventional methods of probing cores using molecular
emission. Under these conditions H2D+ may be the sole viable molecular probe of
the innermost regions of star forming cores.
