  ALMA HCO+ observations of the infrared dark cloud G0.253+0.016 located in the
Central Molecular Zone of the Galaxy are presented. The 89 GHz emission is
area-filling, optically thick, and sub-thermally excited. Two types of
filaments are seen in absorption against the HCO+ emission. Broad-line
absorption filaments (BLAs) have widths of less than a few arcseconds (0.07 -
0.14 pc), lengths of 30 to 50 arcseconds (1.2 - 1.8 pc), and absorption
profiles extending over a velocity range larger than 20 km/sec. The BLAs are
nearly parallel to the nearby G0.18 non-thermal filaments and may trace HCO+
molecules gyrating about highly ordered magnetic fields located in front of
G0.253+0.016 or edge-on sheets formed behind supersonic shocks propagating
orthogonal to our line-of-sight in the foreground. Narrow-line absorption
filaments (NLAs) have line-widths less than 20 km/sec. Some NLAs are also seen
in absorption in other species with high optical depth such as HCN and
occasionally in emission where the background is faint. The NLAs, which also
trace low-density, sub-thermally excited HCO+ molecules, are mostly seen on the
blueshifted side of the emission from G0.253+0.016. If associated with the
surface of G0.253+0.016, the kinematics of the NLAs indicate that the cloud
surface is expanding. The decompression of entrained, milli-Gauss magnetic
fields may be responsible for the re-expansion of the surface layers of
G0.253+0.016 as it recedes from the Galactic center following a close encounter
with Sgr A.
