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We present Spitzer IRS spectra (R ∼ 600, 10 -38 µm) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude and spanning ±0.3 • in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the "Bubble" lying along the same line-ofsight as the molecular cloud G0.11−0.11, and the diffuse interstellar gas along the line-of-sight at higher Galactic latitudes. From measurements of the, and H 2 S(0), S(1), and S(2) lines we determine the gas excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ∼ 1.6 times that of the Orion Nebula. The main source of excitation is photoionization, with the Arches Cluster ionizing the Arched Filaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O +3 and destroy dust grains, releasing iron into the gas phase (Fe/H∼ 1.3 × 10 −6 in the Arched Filaments and Fe/H∼ 8.8 × 10 −6 in the Bubble). The shock effects are particularly noticeable in the center of the Bubble, but O +3 is present in all positions. We suggest that the shocks are due to the winds from the Quintuplet Cluster Wolf-Rayet stars. On the other hand, the H 2 line ratios can be explained with multi-component models of warm molecular gas in photodissociation regions without the need for H 2 production in shocks.

Abstract. The wavelength and flux calibration, and the inorbit performance of the Infrared Space Observatory LongWavelength Spectrometer (LWS) are described. The LWS calibration is mostly complete and the instrument's performance in orbit is largely as expected before launch. The effects of ionising radiation on the detectors, and the techniques used to minimise them are outlined. The overall sensitivity figures achieved in practice are summarised. The standard processing of LWS data is described.Send offprint requests to: B.M. Swinyard

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