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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7358fig1.eps}\end{figure}$ Figure 1: Normalised (second order Chebyshev polynomial), telluric-corrected HIRES spectra of the region around 8037 Å. Spectra have been Doppler shifted to place the mean K I wavelength at rest (using $\lambda _{\rm K {\sc i}\ rest}=7698.9645$ Å (Morton 2003). $\lambda$ Cyg is shown as an unreddened standard. Fitted continua are shown as dashed lines. The 2.74 K CH₂CN^- $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ origin band absorption spectrum is plotted at the top, calculated assuming a single cloud at rest with Doppler b=1 km s^-1 and convolved to the resolving power R=42 500 of the HIRES spectra. The $K_{\rm a}''=0$ transitions lie between 8035 and 8040 Å and at this temperature the $K_{\rm a}''=1$ transitions at around 8025 and 8046 Å are almost invisibly weak. The absorption feature at around 8026 Å is an unrelated diffuse interstellar band.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{7358fig2.eps}\end{figure}$ Figure 2: Comparison of computed and observed (averaged) spectra in the 8037 A region. The bottom trace shows the co-added, normalised, telluric-corrected HIRES spectrum around $\lambda$ 8037 observed towards the stars listed in Table 1. Spectra were Doppler-shifted to place the weighted-mean K I wavelength at rest before co-addition. The $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ , { $K_{\rm a}=2\longleftarrow 1$ , $K_{\rm a}=1\longleftarrow 0$ , $K_{\rm a}=0\longleftarrow 1$ } modelled absorption spectrum is shown, and includes the transitions arising from both $K_{\rm a}''=0$ (para) and $K_{\rm a}''=1$ (ortho) CH₂CN^-. The middle spectrum was calculated assuming a thermal Boltzmann distribution of $K_{\rm a}$ level populations. The upper spectrum shows the result taking the $K_{\rm a}''$ populations to be determined solely by a 3:1 nuclear spin-statistical weight ortho:para ratio for odd:even $K_{\rm a}''$ levels. The interstellar CH₂CN^- distribution is modelled as a single cloud at rest (v=0 km s^-1) with Gaussian line-shape (Doppler b=18 km s^-1), convolved to the resolving power R=42 500 of the HIRES spectra. The respective peak absorption wavelengths of the three Doppler-broadened $K_{\rm a}$ sub-bands are 8024.8, 8037.7, and 8049.6 Å, shown as dotted lines and correspond to ortho, para and ortho CH₂CN^- respectively.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7358fig3.eps}\end{figure}$ Figure 3: Telluric-corrected, normalised DIB $\lambda$ 8037 spectra (thin black traces) and least-squares fitted CH₂CN^- $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ , $K_{\rm a}=1\longleftarrow 0$ models (thick red traces). Models employ a single Gaussian interstellar cloud. Radial velocity shift, Doppler width, central depth and an additive continuum offset were free parameters in the fits. The DIB rest wavelength was set at $\lambda _{0}=8037.78$ Å for the displayed velocity scale. Best-fitting model parameters are shown in Table 2 and the inferred CH₂CN^- interstellar velocity distributions plotted in Fig. 4 (lines labeled "Mdl'').
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7358fig4.ps}\end{figure}$ Figure 4: Each of the eight panels above shows LSR velocity-space apparent column density profiles for interstellar species for which data were available. Data for the K I 7699 Å, CH⁺ 3958 Å, Ca II 3968 Å and Ti II 3384 Å lines are shown. Hypothetical model CH₂CN^- velocity distributions (labeled "Mdl'') are shown at the top of each panel, calculated based on the fits shown in Fig. 3 (parameters given in Table 2) with the assumption that the narrow $\lambda$ 8037 DIB is caused by the origin band $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ transitions of CH₂CN^- at 2.74 K, and that the interstellar velocity distribution of the gas is Gaussian. The interstellar K I, CH⁺, Ca II and Ti II apparent column densities ( $N_{\rm a}$ ) were calculated from the normalised HIRES spectra (I/I₀, where I₀ is the continuum intensity) according to $N_{\rm a}\propto\ln(I_0/I)$ (see for example Savage & Sembach 1991). For the right-hand panel, H I column density profiles are from the Leiden/Argentine/Bonn (LAB) Survey of Galactic H I (Kalberla et al. 2005).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7358fig5.eps}\end{figure}$ Figure 5: 2.74 K CH₂CN^- $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ , $K_{\rm a}''=0$ transitions convolved with the HIRES PSF and the interstellar Ti II profiles (solid lines), and the K I profiles (dashed lines), observed towards HD 183143, HD 168112 and Cyg OB2 8a, overlaid on the normalised, telluric-corrected $\lambda$ 8037 DIB profiles (thin black histograms).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7358fig5.eps}\end{figure}$	Figure 5: 2.74 K CH₂CN^- $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ , $K_{\rm a}''=0$ transitions convolved with the HIRES PSF and the interstellar Ti II profiles (solid lines), and the K I profiles (dashed lines), observed towards HD 183143, HD 168112 and Cyg OB2 8a, overlaid on the normalised, telluric-corrected $\lambda$ 8037 DIB profiles (thin black histograms).
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