  We report the existence of broad and weakly asymmetric features in the
high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of
defined chiral index. A significant variation in peak width (from 12 cm-1 to
110 cm-1) is observed as a function of the nanotube's chiral structure. When
the nanotubes are electrostatically gated, the peak widths decrease. The
broadness of the Raman features is understood as the consequence of coupling of
the phonon to electron-hole pairs, the strength of which varies with the
nanotube chiral index and the position of the Fermi energy.
