ULTRAFINE STRUCTURE IN THE LAMBDA-5797 DIFFUSE INTERSTELLAR ABSORPTION-BAND

High-resolution and high-S/N observations of the strong and relatively narrow lambda 5797 diffuse interstellar absorption band, recorded wit h the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Tel escope and the 0.9 m coude feed of the Kitt Peak National Observatory, are reported. For selected lines of sight and using resolving powers of up to 600,000, a remarkable degree of intrinsic ultrafine structure is found within the band. There exists an almost exact correspondence between the structure in the spectra recorded toward mu Sgr, zeta Per , and zeta Oph, including one particularly narrow component with a wid th that is comparable to the widths of atomic and molecular absorption lines along these lines of sight. The results provide a new and extre mely exacting test against which theoretical or laboratory candidates can be assessed, and they point to a new approach to studies of diffus e band carrier distributions through their velocity signatures.