SIMBAD references

The profile of the diffuse interstellar band at 6196 Å has been shown by high-resolution spectroscopy to be a smooth, bell-shaped curve that lacks the rotational structure predicted by the current theory that the bands are rovibronic transitions of carbon-rich molecules in the vapour phase. Although λ6196 is one of the narrowest of the diffuse interstellar bands, the explanation appears to be that some molecular broadening process is acting to wash out the rotational structure, but none of the processes previously discussed in connection with the bands is appropriate to this case. The zero-point vibrational isotope shift is therefore studied here; it arises because the zero-point vibrational energies of a carbon-rich molecule in the ground and excited electronic states of a transition both decrease as ¹² C atoms are substituted by ¹³ C, and any difference between the amounts by which they decrease causes a shift of the origin of the corresponding rovibronic band. The extent of the shift is found to be a few tenths of a wavenumber in molecules of the size presently under consideration for the interstellar bands, and a simple model yields theoretical profiles that provide a satisfactory fit to the spectrum of λ6196, the best fit being found when the parameters are given values appropriate to a carrier consisting of some 50 to 100 carbon atoms. The zero-point shift is unavoidable in carbon-rich molecules that are not isotopically pure and may set a fundamental limit to the clarity with which fine detail is discernible in the spectrum of every diffuse interstellar band.