  We have used post-synthesis separation methods based on density gradient
ultracentrifugation and DNA-based ion-exchange chromatography to produce
aqueous suspensions strongly enriched in armchair nanotubes for spectroscopic
studies. Through resonant Raman spectroscopy of the radial breathing mode
phonons, we provide macroscopic and unambiguous evidence that density gradient
ultracentrifugation can enrich armchair nanotubes. Furthermore, using
conventional, optical absorption spectroscopy in the near-infrared and visible
range, we show that interband absorption in armchair nanotubes is strongly
excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine
that observation of the broad, lower frequency (G^{-}) feature is a result of
resonance with non-armchair "metallic" nanotubes. These findings regarding the
fundamental optical absorption and scattering processes in metallic carbon
nanotubes lay the foundation for further spectroscopic studies to probe
many-body physical phenomena in one dimension.
