All Figures

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2737fig1.eps} \end{figure}$ Figure 1: Low lying rotational energy levels in the electronic and vibrational ground state of the ¹⁶OH molecule, adapted from Brown et al. (1982). The $\Lambda$ -doubling of each level is shown but the splitting has been exaggerated for clarity. The relative spacing of the levels in energy is only approximate. The J values for each level are shown and the wavelength of each transition is given in $\mu$ m. The transitions used in the analysis presented here are shown as thick arrows.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2737fig2.ps} \end{figure}$ Figure 2: The ¹⁶OH $^{2}\Pi _{1/2}$ - $^{2}\Pi _{3/2}$ J=3/2-3/2 transition at 53.26 $\mu$ m from non-prime data (black) and prime data (grey). Both datasets are binned at 1/4 of their respective resolution elements (non-prime: 61 km s^-1 and prime: 45 km s^-1). The gain in signal-to-noise in the non-prime data is a factor of $\sim$ 9.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2737fig3.ps}\hspace*{3mm} \includegraphics[width=7.7cm,clip]{2737fig4.ps} \end{figure}$ Figure 3: Data for the ¹⁶OH cross ladder ( $^{2}\Pi _{1/2}$ - $^{2}\Pi _{3/2}$ ) transitions, J=1/2-3/2 at 79 $\mu$ m ( left; binned at 1/4 resolution element) and J=3/2-3/2 at 53 $\mu$ m ( right; binned at $\sim$ 1/6 resolution element), plotted as line to continuum ratio. The errors shown represent the combined statistical and systematic uncertainty in each bin. The two $\Lambda$ -doublet components have been averaged for the 53 $\mu$ m line. The best fitting model accounting for the absorbing clouds in the line of sight is shown as the solid line (see text).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2737fig5.ps} \end{figure}$ Figure 4: Co-added data from the two $\Lambda$ -doublet components of the ¹⁸OH $^{2}\Pi_{3/2}~J=5/2$ -3/2 transition. The data were binned at 1/4 resolution element and the errors represent the combined statistical and systematic uncertainty in each bin. The best fitting model accounting for absorbing clouds in the line of sight is shown (see text).
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{2737fig6.ps} \end{figure}$ Figure 5: Values of ¹⁶OH/¹⁸OH from Table 1 plotted against distance from Galactic Centre (large circles with cross). The velocity of each component is indicated in km s^-1 next to each point. The compilation of values from Kahane (1994) is plotted for comparison, as well as the best fitting galactic gradient found by Wilson & Rood (1994).
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2737fig2.ps} \end{figure}$	Figure 2: The ¹⁶OH $^{2}\Pi _{1/2}$ - $^{2}\Pi _{3/2}$ J=3/2-3/2 transition at 53.26 $\mu$ m from non-prime data (black) and prime data (grey). Both datasets are binned at 1/4 of their respective resolution elements (non-prime: 61 km s^-1 and prime: 45 km s^-1). The gain in signal-to-noise in the non-prime data is a factor of $\sim$ 9.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{2737fig5.ps} \end{figure}$	Figure 4: Co-added data from the two $\Lambda$ -doublet components of the ¹⁸OH $^{2}\Pi_{3/2}~J=5/2$ -3/2 transition. The data were binned at 1/4 resolution element and the errors represent the combined statistical and systematic uncertainty in each bin. The best fitting model accounting for absorbing clouds in the line of sight is shown (see text).
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{2737fig6.ps} \end{figure}$	Figure 5: Values of ¹⁶OH/¹⁸OH from Table 1 plotted against distance from Galactic Centre (large circles with cross). The velocity of each component is indicated in km s^-1 next to each point. The compilation of values from Kahane (1994) is plotted for comparison, as well as the best fitting galactic gradient found by Wilson & Rood (1994).
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