2002ApJ...567..407S

The dependence of the 4430 Å diffuse interstellar band (DIB) on dust extinction or total gas column density indicates that this DIB, like the narrower features seen in the yellow-red portion of the spectrum, does not continue to grow in strength beyond a certain threshold. One possible interpretation is that the 4430 Å DIB saturates at high column densities, suggesting that this band might consist of unresolved narrow features (such as molecular rotational lines), individually saturated, that are blended at the resolving power of previous observations. In this study we use the Ultra High Resolution Facility on the Anglo-Australian Telescope to assess that question by seeking evidence for fine structure within the 4430 Å DIB at the very high spectral resolving power of Δλ/λ~10⁶. Our target stars include lines of sight where apparent rotational line structure has been observed in other DIBs. We find no evidence of fine structure within the 4430 Å DIB, suggesting that the broadening of this band is intrinsic. If so, the lack of fine structure may indicate a molecular transition followed by very rapid internal conversion or a molecular transition consisting of overlapping rotational lines. In either case, our results support the hypothesis that the apparent saturation of the band as a function of extinction is a result of the ``skin effect'', in which the strength of DIBs stops increasing with extinction because the DIBs form primarily in the outer boundaries of interstellar clouds.