Summary

This chapter presented results for the atomic fine structure lines of OI and CII. An algorithm was developed to search for useful non-prime observations of these lines and this was used to find 5 observations of the [OI] 63 $\mu$m line. These had higher signal to noise than both the prime L03 data and data recorded in the L04 mode. An analysis of the errors showed that if the data were treated as line to continuum ratio the large systematic errors present in the absolute flux became unimportant. These high quality data were used to model the 63 $\mu$m line shape using a fit to the CII line^E.2. The resulting prediction of the atomic oxygen in the CII layers showed that most of the absorption must be due to the interior molecular component of each cloud. The molecular layer was fitted using CO observations. These results showed that total OI column density along the line of sight in the outer two PDR layers was $8.3\times10^{18}$ cm$^{-2}$, compared to $2.2\times10^{19}$ cm$^{-2}$ in the molecular layers. The results of comparing the OI and CO column densities deduced for the molecular layers agree well with the standard PDR geometry and with observations of CI in this line of sight. This is in contrast to the analysis of OI L04 observations towards Sgr B2 by Lis et al. (2001) which showed that there was no significant contribution to the oxygen absorption from the CII/H$_{2}$ and CI/H$_{2}$ layers. The current analysis improves on the results of Lis et al. (2001) by using better data reduction and extra information about CII in the line of sight.