We report far-IR spectroscopy in [O Ⅲ] 52 and 88 μm, [N Ⅲ] 57 μm, and [O Ⅰ] 63 μm of the "sickle" (G0.18 -0.04) in the Galactic center radio arc. We also mapped the entire eastern region of the arched filaments, including the "banana" (G0.10+0.02) in [O Ⅲ] 88 μm. These observations and the recent detection of three near-IR He Ⅰ/H Ⅰ emission-line stars near the "sickle" strongly support the concept of a combined H Ⅱ/photodissociation region (PDR). G0.18-0.04 is a low-density H Ⅱ region with n_e ≈ 600 cm~(-3) that is most likely excited by stars of type earlier than O8 located in the Quintuplet cluster. Ionization of the arched filaments and the "sickle" by fast shocks or the "critical ionization mechanism" can be ruled out. We derive a fractional abundance ratio for [N]/[O] = 0.30-0.35 consistent with observations of other H Ⅱ regions in the vicinity of the Galactic center. Unusually wide [O Ⅲ] 88 μm emission lines of widths FWHM = (70-110) km s~(-1) have been detected in the mapped region of G0.18-0.04. The large intrinsic line widths may be related to the interaction of the plasma with strong magnetic fields that are known to be present in the radio arc.
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