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% aims To compare information gained from radio and optical absorption lineprofiles from the diffuse interstellar medium along the same sightline %methods We compare new and existing 21cm HI and 3.4mm HCO+ profiles withprofiles of the optical tracers CaII, NaI, and KI from an unpublished thesis ofTappe (2004) %results The atoms traced optically are all heavily depletedcompared to a Solar abundance and only the integrated optical depths of HI andHCO+ correlate well with E(B-V). HCO+ is the species with by far the mostlimited kinematic distribution and the narrowest lines followed in order by KI,HI, NaI and CaII. CaII behaves separately in both column density and kinematicsbecause it samples broader-lined warmer gas. Tracers of the cold neutral mediumNaI, KI, HI and HCO+ share the same kinematic space statistically withoutcorrelating to nearly the same extent in abundance. N(NaI) and N(KI) arecorrelated, as are the integrated optical depths of HI and HCO+ but abundancecorrelations between optical and radio tracers are not seen. In the onlydirection with a measured CH+ profile, a 2 km/s velocity shift between CH+ andCH, usually interpreted as the sign of shocked gas, is mimicked in the shift ofHI relative to HCO+. CH and HCO+ appear in the ratios N(CH)/N(HCO+)=14.6 and21.1 along the two sightlines with optically-measured N(CH),compared with amean of 12 determined previously at radio and submillimeter wavelengths.

Comparing absorption from classic tracers of the diffuse interstellar medium at optical and radio wavelengths along the same sightlines