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We present an analysis of moderately high resolution optical spectra obtainedfor the sight line to CD-23 13777, an O9 supergiant that probes high velocityinterstellar gas associated with the supernova remnant W28. Absorptioncomponents at both high positive and high negative velocity are seen in theinterstellar Na I D and Ca II H and K lines toward CD-23 13777. The highvelocity components exhibit low Na I/Ca II ratios, suggesting efficient graindestruction by shock sputtering. High column densities of CH+, and high CH+/CHratios, for the components seen at lower velocity may be indicative of enhancedturbulence in the clouds interacting with W28. The highest positive andnegative velocities of the components seen in Na I and Ca II absorption towardCD-23 13777 imply that the velocity of the blast wave associated with W28 is atleast 150 km/s, a value that is significantly higher than most previousestimates. The line of sight to CD-23 13777 passes very close to a well-knownsite of interaction between the SNR and a molecular cloud to the northeast. Thenortheast molecular cloud exhibits broad molecular line emission, OH maseremission from numerous locations, and bright extended GeV and TeV gamma-rayemission. The sight line to CD-23 13777 is thus a unique and valuable probe ofthe interaction between W28 and dense molecular gas in its environs. Futureobservations at UV and visible wavelengths will help to better constrain theabundances, kinematics, and physical conditions in the shocked and quiescentgas along this line of sight.

High-velocity interstellar absorption associated with the supernova remnant W28